The Pesticide Dilemma

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Salad vac. This machine controls certain insect
pests without the use of chemical pesticides. Photographed in Salinas, California.
The Pesticide Dilemma
Learning Objectives
After you have studied this chapter you should be able to
1. Define pesticide, distinguish among various types of
pesticides such as insecticides and herbicides and describe
the major groups of insecticides and herbicides.
2. Relate the benefits of pesticides.
3. Summarize the problems associated with pesticide use,
including development of genetic resistance; creation of
imbalances in the ecosystem; persistence, bioaccumulation, and biological magnification; and mobility in the
environment.
4. Discuss pesticide risks to human health, including
short-term effects, long-term effects, pesticides as
endocrine disrupters, and risks to children.
5. Describe alternative ways to control pests, including cultivation methods, biological controls, reproductive controls,
pheromones and hormones, genetic controls, quarantine,
integrated pest management, and irradiating foods.
6. Briefly summarize the three U.S. laws that regulate pesticides: the Food, Drug, and Cosmetics Act; the Federal
Insecticides, Fungicide, and Rodenticide Act; and the Food
Quality Protection Act.
7. Describe the purpose of the Stockholm Convention on
Persistent Organic Pollutants.
516
P
icture a giant vacuum cleaner slowly moving over
rows of strawberry or vegetable crops and sucking
insects off the plants. Such a machine, given names like
“salad vac” and “bug vac,” was invented by an entomologist (a biologist who studies insects) as a substitute for
the chemical poisons we call pesticides. Each vacuuming
eliminates the need for one application of pesticide.
More than forty California growers use the farmsized vacuum cleaner to remove and kill lygus bugs,
leafhoppers, Colorado potato beetles, and other insect
pests from their strawberries and other crops. Some beneficial insects are also removed and killed by the bug vac,
but fewer than would be killed if chemical insecticides
were sprayed. Increasing public concern about pesticide
residues on food has caused farmers to look seriously at
other ways to control pests—even by vacuuming insects.
In this chapter we examine the types and uses of
pesticides, their benefits, and their disadvantages. Pesticides have saved millions of lives by killing insects that
carry disease and by increasing the amount of food we
grow. Modern agriculture depends on pesticides to produce blemish-free fruits and vegetables at a reasonable
cost to farmers (and therefore to consumers).
However, pesticides also cause environmental and
health problems, and it appears that in many cases their
harmful effects outweigh their benefits. Pesticides rarely
affect the pest species alone, and the balance of nature,
such as predator–prey relationships, is upset. Certain
pesticides concentrate at higher levels of the food chain.
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Humans who apply and work with pesticides may be at
risk for pesticide poisoning (short term) and cancer (long
term), and people who eat traces of pesticide on food are
concerned about the long-term effects. In this chapter we
also consider some alternatives to pesticides and discuss
the pesticide laws that are supposed to protect our health
and the environment.
WHAT IS A PESTICIDE?
Any organism that interferes in some way with human
welfare or activities is called a pest. Some weeds, insects,
rodents, bacteria, fungi, nematodes (microscopic worms),
and other pest organisms compete with humans for food;
other pests cause or spread disease. The definition of pest
is subjective; a mosquito may be a pest to you, but it is not
a pest to the bat or bird that eats it. People try to control
pests, usually by reducing the size of the pest population.
Toxic chemicals called pesticides are the most common
way of doing this, particularly in agriculture. Pesticides
can be grouped by their target organisms—that is, by the
pests they are supposed to eliminate. Thus, insecticides
kill insects, herbicides kill plants, fungicides kill fungi,
and rodenticides kill rodents such as rats and mice.
Agriculture is the sector that uses the most pesticides
worldwide—approximately 85% of the estimated 2.6
million metric tons (2.9 million tons) used each year.
Highly developed countries use about three fourths of all
pesticides, but pesticide use is increasing most rapidly in
developing countries.
The “Perfect” Pesticide
The ideal pesticide would be a narrow-spectrum pesticide that would kill only the organism for which it was
intended and not harm any other species. The perfect pesticide would also be readily broken down, either by natural
chemical decomposition or by biological organisms, into
safe materials such as water, carbon dioxide, and oxygen.
The ideal pesticide would stay exactly where it was put and
would not move around in the environment.
Unfortunately, there is no such thing as an ideal pesticide. Most pesticides are broad-spectrum pesticides,
which kill a variety of organisms, including some that are
beneficial, in addition to the target pest. Some pesticides
do not degrade readily or else break down into compounds that are as dangerous as, if not more dangerous
than, the original pesticide. And most pesticides move
around a great deal throughout the environment.
First-Generation and Second-Generation
Pesticides
Before the 1940s, pesticides were of two main types,
inorganic compounds (also called minerals) and organic
Figure 22.1 Pesticide derived from plants. Chrysanthemum
flowers, shown here as they are harvested in Rwanda, are the
source of the insecticide pyrethrin. Botanicals are chemicals
from plants that can be used as pesticides.
compounds. Inorganic compounds that contain lead,
mercury, and arsenic are extremely toxic to pests but are
not used much today, in part because of their chemical
stability in the environment. Natural processes do not
degrade inorganic compounds, which therefore persist
and accumulate in the soil and water. This accumulation
poses a threat to humans and other organisms, which,
like the target pests, are susceptible to poisoning by inorganic compounds.
Plants, which have been fighting pests longer than
humans, have developed several natural organic compounds that are poisonous, particularly to insects. Such
plant-derived pesticides are called botanicals. Examples
of botanicals include nicotine from tobacco, pyrethrin
from chrysanthemum flowers (Figure 22.1), and
rotenone from roots of the derris plant, all of which are
used to kill insects. Botanicals are easily degraded by
microorganisms and therefore do not persist for long in
the environment. However, they are highly toxic to
aquatic organisms and to bees (beneficial insects that pollinate crops).
Synthetic botanicals are human-made insecticides
produced by chemically modifying the structure of natural botanicals. An important group of synthetic botanicals
are the pyrethroids, which are chemically similar to
pyrethrin. Pyrethroids do not persist in the environment;
they are slightly toxic to mammals and bees but very toxic
to fishes. Allethrin is an example of a pyrethroid.
In the 1940s a large number of synthetic organic
pesticides began to be produced. Earlier pesticides,
both inorganic compounds and botanicals, are called
first-generation pesticides to distinguish them from
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THE PESTICIDE DILEMMA
Figure 22.2 Early use of DDT.
Pesticides such as DDT
(dichlordiphenyltrichloroethane), shown as it was
sprayed to control mosquitoes
at New York’s Jones Beach State
Park in 1945, were used in ways
that would be unacceptable now.
The sign on the truck reads, in
part, “D.D.T. Powerful insecticide. Harmless to humans.” The
harmful environmental effects
of DDT were not known until
many years later.
the vast array of synthetic poisons in use today, called
second-generation pesticides. The insect-killing ability of dichlorodiphenyltrichloroethane (DDT), the
first of the second-generation pesticides, was recognized in 1939 (Figure 22.2). There are currently about
20,000 registered commercial pesticide products, consisting of combinations of about 675 active chemical
ingredients. Table 22.1, which summarizes the characteristics of many of the pesticides discussed in the chapter, provides a useful overview of the varied effects
different pesticides have on mammals, including
humans, and on the environment.
The Major Groups of Insecticides
Insecticides, the largest category of pesticides, are usually
classified into groups based on chemical structure. Three
of the most important groups of second-generation
insecticides are the chlorinated hydrocarbons,
organophosphates, and carbamates.
DDT is an example of a chlorinated hydrocarbon,
an organic compound containing chlorine. After DDT’s
insecticidal properties were recognized, many more chlorinated hydrocarbons were synthesized as pesticides.
Generally speaking, chlorinated hydrocarbons are broadspectrum insecticides. Most are slow to degrade and
therefore persist in the environment (even inside organisms) for many months or even years. They were widely
used from the 1940s until the 1960s, but since then many
have been banned or their use has largely been restricted,
mainly because of problems associated with their persistence in the environment and impacts on humans and
wildlife. Three chlorinated hydrocarbons still in use in
the United States are endosulfan, lindane, and methoxychlor. Many people first became aware of the problems
with pesticides in 1963, when Rachel Carson published
her book Silent Spring (see Chapter 3).
Organophosphates, organic compounds that contain phosphorus, were developed during World War II as
an outgrowth of German research on nerve gas.
Organophosphates are more poisonous than other types
of insecticides, and many are highly toxic to birds, bees,
and aquatic organisms. The toxicity of many
organophosphates in mammals, including humans, is
comparable to that of some of our most dangerous poisons—arsenic, strychnine, and cyanide. Organophosphates do not persist in the environment as long as
chlorinated hydrocarbons do. As a result, organophosphates have generally replaced the chlorinated hydrocarbons in large-scale uses such as agriculture, although
many are not widely available to consumers because of
their high level of toxicity. Methamidophos, dimethoate,
and malathion are three examples of organophosphates.
Carbamates, the third group of insecticides, are
broad-spectrum insecticides derived from carbamic acid.
Carbamates are generally not as toxic to mammals as the
organophosphates, although they still show broad, nontarget toxicity. Two common carbamates are carbaryl and
aldicarb.
The Major Kinds of Herbicides
Chemicals that kill or inhibit the growth of unwanted
vegetation such as weeds in crops or lawns are called herbicides. Like insecticides, herbicides can be classified into
groups on the basis of chemical structure, but this
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method is cumbersome because there are at least 12 different chemical groups that are used as herbicides. It is
easier to group herbicides according to how they act and
what they kill. Selective herbicides kill only certain
types of plants, whereas nonselective herbicides kill all
vegetation. Selective herbicides can be further classified
according to the types of plants they affect. Broad-leaf
herbicides kill plants with broad leaves but do not kill
grasses; grass herbicides kill grasses but are safe for
most other plants.
Two common herbicides with similar structures are
2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5trichlorophenoxyacetic acid (2,4,5-T). Both were developed in the United States in the 1940s. These broad-leaf
herbicides are similar in structure to a natural growth
hormone in plants and therefore disrupt the plants’ natural growth processes; they kill plants such as dandelions
but do not harm grasses. You may recall from Chapter 18
that many of the world’s important crops, such as wheat,
corn, and rice, are cereal grains, which are grasses. Both
2,4-D and 2,4,5-T can be used to kill weeds that compete
with these crops, although 2,4,5-T is no longer used in
the United States. The Environmental Protection
Agency (EPA) banned most uses of 2,4,5-T in 1979
because of possible harmful side effects to humans that
became apparent after its use in the Vietnam War.
CASE·IN·POINT
The Use of Herbicides in the
Vietnam War
One of the controversial aspects of the Vietnam War was
the defoliation program carried on by the United States
in South Vietnam. From 1962 to 1971, the United States
sprayed more than 12 million gallons of herbicides over
about 4.5 million acres of South Vietnam to kill vegetation around military bases and to expose hiding places
and destroy crops planted by the Vietcong and North
Vietnamese troops (Figure 22.3). The three mixtures of
herbicides used were designated Agent White, Agent
Blue, and Agent Orange.
The negative impacts of these herbicides on the
environment are still being felt today. It is estimated that
14% of the ecologically important mangrove forests of
South Vietnam were destroyed. Shrubs and wild grasses
have replaced the vast tracts of denuded forestlands, and
the forests may take many decades to return. Approximately 30% of the nation’s commercially valuable hardwood forests were killed, and bamboo and weedy grasses
have replaced them.
In addition to the ecological damage, the herbicide
sprays appear to have caused health problems in the Vietnamese people and in some members of the U.S. military
who were exposed to them in the Vietnamese jungles.
Agent Orange, a mixture of two herbicides (2,4-D and
2,4,5-T), also contained minute amounts of dioxins, a
group of mildly to very toxic chemical compounds
formed during the manufacture of these herbicides. The
519
dioxins in Agent Orange were reportedly 1,000 times
more toxic than were those in domestic herbicides. (The
contamination of Agent Orange by dioxins was discovered after the war.)
High doses of dioxins have been shown to cause
birth defects in animals. According to the Vietnamese
government, about 500,000 Vietnamese people have died
or contracted serious illnesses as a result of exposure of
themselves, their parents, or their grandparents to Agent
Orange. Several researchers have noted that the number
of birth defects, stillbirths, female reproductive disorders, and certain adult soft-tissue cancers increased dramatically in Vietnam following the war, particularly in
areas where the herbicide was heavily sprayed or where it
was stored and subsequently leaked into the soil and bodies of water. Although medical researchers caution that it
is very difficult to prove a direct cause-and-effect relationship between Agent Orange and these medical problems, statistical evidence suggests that Agent Orange is at
least partly responsible.
The level of dioxin in the breast milk of Vietnamese
mothers has been measured at 1,800 parts per trillion. In
comparison, the breast milk of U.S. mothers contains an
average of 4 parts per trillion of dioxin. The high level of
dioxin in human milk indicates that dioxin has contaminated Vietnam’s food chain, including fish. Vietnamese
people who eat the most fish have the highest levels of
dioxins in their blood.
As a group, U.S. veterans who were exposed to high
levels of Agent Orange have more health problems than
do other veteran groups. The U.S. Department of Veterans Affairs currently recognizes 10 medical conditions
linked to dioxin exposure. These include a variety of soft-
Figure 22.3 Herbicide spraying during the Vietnam War.
This historic photo, taken in 1966, shows a forested area near a
South Vietnamese highway.
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Table 22.1
THE PESTICIDE DILEMMA
Characteristics of Selected Pesticides
Type
Example
Insecticides
Chlorinated
hydrocarbons
Organophosphates
Toxicity to Mammals/
Regulatory Status*
Ecological Effects
Persistence/
Bioaccumulation
Endosulfan
Highly toxic; restricteduse pesticide**
Highly toxic to birds and
fishes
Moderately persistent;
bioaccumulates
Lindane
Moderately toxic; some
formulations are
restricted use
Highly toxic to fishes and
aquatic invertebrates;
thins egg shells of birds
Highly persistent;
bioaccumulates
Methoxychlor
Slightly toxic;
general-use pesticide
Slightly toxic to birds; highly
toxic to fishes and aquatic
invertebrates
Relatively low persistence;
does not bioaccumulate
to any significant degree
Dichlorodiphenyl- Moderately to slightly
Chronic exposure in birds
Highly persistent;
trichloroethane
toxic; banned from use in
may cause eggshell thinning
bioaccumulates
(DDT)
the United States in
and embryo death; highly
significantly
1972
toxic to many aquatic
invertebrates and fish species
Methamidophos
Highly toxic; restrictedHighly toxic to birds, bees,
Relatively low
use pesticide
and aquatic organisms
persistence; does not
bioaccumulate
Moderately to highly toxic to
birds, fishes, aquatic
invertebrates, and bees
Highly toxic to bees and
aquatic invertebrates;
moderately toxic to birds;
range of toxicities for fishes
Relatively low
persistence; does not
bioaccumulate
Relatively low persistence; bioaccumulation demonstrated in
brown shrimp
Highly toxic; restricteduse pesticide
Highly toxic to birds;
moderately toxic to fishes;
not toxic to bees
Relatively low
persistence; does not
bioaccumulate
Carbaryl
Slightly, moderately, or
highly toxic, depending
on formulation;
general-use pesticide
Moderately toxic to aquatic
organisms; highly toxic to
bees and other beneficial
insects
Relatively low
persistence;
bioaccumulates in
acidic waters
Botanicals
Pyrethrin-I,
Pyrethrin-II
Highly toxic to aquatic
organisms and bees; slightly
toxic to birds; more toxic in
acidic waters
Relatively low
persistence; does not
bioaccumulate
Synthetic
botanicals
(pyrethroids)
Allethrin
Slightly, moderately, or
highly toxic,
depending on
formulation; most are
general-use pesticides
Slightly toxic; generaluse pesticide
Toxic to fishes; slightly toxic
to bees; not toxic to birds
Data not available
Insecticides
Carbamates
Dimethoate
Moderately toxic;
general-use pesticide
Malathion
Slightly toxic; generaluse pesticide
Aldicarb
(continues)
tissue cancers, skin diseases, urological disorders, and
birth defects. However, the U.S. government has not
agreed to compensate Vietnamese people with similar
health problems.
health officials fight their own war against the ravages of
human diseases transmitted by insects. One of the most
effective weapons in the arsenals of farmers and health
officials is the pesticide.
Disease Control
BENEFITS OF PESTICIDES
Each day a war is waged as farmers, struggling to produce
bountiful crops, battle insects and weeds. Similarly,
Insects transmit several devastating human diseases.
Fleas and lice carry the microorganism that causes typhus
in humans. Malaria, which is also caused by a microor-
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BENEFITS OF PESTICIDES
Table 22.1
521
(Continued)
Type
Herbicides
Triazine
Example
Cyanazine
Atrazine
Toxicity to Mammals/
Regulatory Status*
Ecological Effects
Persistence/
Bioaccumulation
Moderately toxic; causes
a variety of birth
defects in rats;
restricted-use
pesticide
Slightly to moderately
toxic; restricted-use
pesticide (because it
has the potential to
contaminate
groundwater)
Slightly to moderately toxic
to birds and aquatic
organisms; not toxic to
bees
Low to moderate
persistence; has been
found in groundwater
Not toxic to birds or bees;
slightly toxic to aquatic
organisms
Highly persistent in soil;
soluble in water and a
common river
contaminant in
agricultural areas;
does not
bioaccumulate
Phenoxy
compound
2,4-D
Slightly to moderately
toxic; causes birth
defects in rats at high
doses; general-use
pesticide
Slightly to moderately toxic to
birds; some formulations
are highly toxic to fishes
Low persistence in soil
and water; does not
bioaccumulate
Thiocarbamate
Butylate
Slightly toxic; generaluse pesticide
Moderately toxic to fishes; not
toxic to birds, bees, or other
wildlife
Analine
Alachlor
Slightly toxic; potential
to cause cancer in rats;
restricted-use
pesticide
Moderately toxic to fishes;
slightly toxic to some birds
and aquatic invertebrates;
not toxic to bees
Low to moderate
persistence in soil;
low to moderate
bioaccumulation in
fishes
Low persistence; does
not bioaccumulate
Dinitroanaline
compound
Trifluralin
Slightly toxic; generaluse pesticide
Not toxic to birds or bees;
highly toxic to fishes and
aquatic invertebrates
Moderate to high
persistence in soil;
moderate
bioaccumulation
Fungicides
Phthalimide
Captan
Very slightly toxic;
general-use pesticide
Not toxic to birds or bees;
highly toxic to fishes
Low persistence; low to
moderate
bioaccumulation
Fumigants
—
Methyl bromide
Highly toxic gas;
restricted-use
pesticide
Potential to destroy ozone;
moderately toxic to aquatic
organisms; not toxic to bees
Moderately persistent in
soil; evaporates into
atmosphere; does not
bioaccumulate
* The information in this table does not replace or supersede the information on the pesticide product or other regulatory requirements.
** Restricted use pesticides may be purchased and used only by certified applicators.
ganism, is transmitted to millions of humans each year by
female Anopheles mosquitoes (Figure 22.4). According to
the World Health Organization (WHO), approximately
300 million to 500 million people currently suffer from
malaria, and as many as 2.7 million people, mostly children in developing countries, die from the disease each
year. Because there are only a few antimalarial drugs
available to treat malaria, the focus of controlling this
disease is on killing the mosquitos that carry it.
Pesticides, particularly DDT, have helped control
the population of mosquitoes, thereby reducing the incidence of malaria. Consider Sri Lanka. In the early 1950s,
more than 2 million cases of malaria were reported in Sri
Lanka each year. When spraying of DDT was initiated to
control mosquitoes, malaria cases dropped to almost
zero. When DDT spraying was discontinued in 1964,
malaria reappeared almost immediately. By 1968, its
annual incidence had increased to greater than 1 million
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THE PESTICIDE DILEMMA
Figure 22.4 Location of malaria.
Insecticides sprayed to control
mosquitoes in these locations
have saved millions of lives.
60°N
NORTH
AMERICA
30°N
60°N
ASIA
EUROPE
ATLANTIC
30°N
OCEAN
PACIFIC
OCEAN
AFRICA
0°
0°
INDIAN
OCEAN
30°S
SOUTH
AMERICA
AUSTRALIA
0
0
60°S
cases per year. Despite the negative effects of DDT on
the environment and organisms, the Sri Lankan government decided to begin spraying DDT once again in
1968. Today, DDT is still used in at least 20 tropical
countries to control mosquitoes.
Crop Protection
Although exact assessments are difficult to make, it is
widely estimated that more than one third of the world’s
crops are eaten or destroyed by pests. Given our expanding population and world hunger, it is easy to see why
control of agricultural pests is desirable.
Pesticides reduce the amount of a crop that is lost
through competition with weeds, consumption by
insects, and diseases caused by plant pathogens
(microorganisms, such as fungi and bacteria, that cause
disease). Although many insect species are beneficial
from a human viewpoint (two examples are honeybees,
which pollinate crops, and ladybugs, which prey on cropeating insects), a large number are considered pests. Of
these, about 200 species have the potential to cause large
economic losses in agriculture. For example, the Colorado potato beetle is one of many insects that voraciously consume the leaves of the potato plant, reducing
the plant’s ability to produce large tubers for harvest (see
Figure 18.11).
Serious agricultural losses are minimized in the
United States and other highly developed nations primarily by the heavy application of pesticides. Pesticide use is
usually justified economically, in that farmers save an
estimated $3 to $5 in crops for every $1 that they invest
in pesticides. In developing countries where pesticides
are not used in appreciable amounts, the losses due to
agricultural pests can be considerable.
1500
30°S
3000 Miles
1500 3000 Kilometers
Malaria has been eradicated or never existed
Malaria poses a limited risk
Malaria is common
60°S
Why are agricultural pests found in such great numbers in our fields? Part of the reason is that agriculture is
usually a monoculture; that is, only one variety of one
crop species is grown on large tracts of land. The cultivated field thus represents a very simple ecosystem. In
contrast, forests, wetlands, and other natural ecosystems
are extremely complex and contain many different
species, including predators and parasites that control
pest populations and plant species that are not used by
pests for food. A monoculture reduces the dangers and
accidents that might befall a pest as it searches for food. A
Colorado potato beetle in a forest would have a hard time
finding anything to eat, but a 500-acre potato field is like
a big banquet table set just for the pest. It eats, prospers,
and reproduces. In the absence of many natural predators
and in the presence of plenty of food, the population
thrives and grows, and more of the crop becomes damaged.
PROBLEMS ASSOCIATED WITH
PESTICIDE USE
Although pesticides have their benefits, they are accompanied by several problems. For one thing, many pest
species evolve a resistance to pesticides after repeated
exposure to them. Also, pesticides affect numerous
species in addition to the target pests, generating imbalances in the ecosystem (including agricultural fields)
and posing a threat to human health. And, as mentioned
earlier, the ability of some pesticides to resist degradation and to readily move around in the environment
causes even more problems for humans and other
organisms.
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P R O B L E M S A S S O C I AT E D W I T H P E S T I C I D E U S E
Evolution of Genetic Resistance
The prolonged use of a particular pesticide can cause a
pest population to develop genetic resistance to the pesticide. Genetic resistance is any inherited characteristic
that decreases the effect of a pesticide on a pest.
In the 50 years during which pesticides have been
widely used, at least 520 species of insects and mites have
evolved genetic resistance to certain pesticides (Figure
22.5). Many pests now have multiple resistance to several
pesticides, and at least 17 species, such as diamondback
moths and palm thrips, are resistant to all major classes of
insecticides that farmers are legally allowed to use on
them. Insects are not the only pests to evolve genetic
resistance; at least 84 weed species are currently resistant
to certain herbicides. Some weeds, such as annual ryegrass and canary grass, are resistant to all available herbicides.
How does genetic resistance to pesticides occur?
Every time a pesticide is used to control a pest, some survive. The survivors, because of certain genes they already
possess, are genetically resistant to the pesticide, and they
pass on this trait to future generations. Thus,
evolution—any cumulative genetic change in a population of organisms—occurs, and subsequent pest populations contain larger percentages of pesticide-resistant
pests than before.
Insects and other pests are constantly evolving. The
short generation times (the period between the birth of
Number of resistant insect and mite species
600
500
400
300
200
100
523
one generation and that of another) and large populations that are characteristic of most pests favor rapid evolution, which allows the pest population to quickly adapt
to the pesticides used against it. As a result, an insecticide
that kills most of an insect population becomes less effective after prolonged use because the survivors and their
offspring are genetically resistant.
Manufacturers of chemical pesticides have often
responded to genetic resistance by recommending that
the pesticide be applied more frequently or in larger
doses. Alternatively, they recommend switching to a new,
often more expensive, pesticide. These responses result
in a predicament that has come to be known as the pesticide treadmill, in which the cost of applying pesticides
increases while their effectiveness decreases. Over time,
the pesticide treadmill results in increased pesticide use,
higher production costs, and declines in crop yields.
Resistance Management Resistance management is
a relatively new approach to dealing with genetic resistance. It involves efforts to delay the evolution of genetic
resistance in insect pests or weeds so that the period of
time in which a pesticide is useful is maximized. Strategies of resistance management vary depending on the
pest species involved.
One strategy of resistance management for insect
pests is to maintain a nearby “refuge” of untreated plants
where the insect pest can avoid being exposed to the
insecticide. Those insects that live and grow in the refuge
remain susceptible to the insecticide. When susceptible
insects migrate into the area being treated with insecticide, they mate with, and thus mix genes with, the genetically resistant population. This interbreeding delays the
development of genetic resistance in the population as a
whole.
Avoiding repeated use of the same herbicide on the
same field is one strategy of resistance management that
slows the development of weed resistance to herbicides.
After herbicides are applied, the field should be scouted
to see if any weed plants survived the herbicide application. These weeds are resistant to the herbicide, and they
should be removed from the field before they flower.
Cultural methods that prolong the usefulness of herbicides include planting seed that is certified to be free of
weed seeds and mechanically pulling weeds.
Imbalances in the Ecosystem
0
1950
1960
1970 1980
Year
1990
2000
Figure 22.5 Genetic resistance. There has been a dramatic
increase in the number of insect species exhibiting genetic
resistance to insecticides. More than 520 insect and mite
species have evolved resistance to insecticides.
One of the worst problems associated with pesticide use
is that pesticides affect species other than the pests for
which they are intended. Beneficial insects are killed as
effectively as pest insects. In a study of the effects of
spraying the insecticide dieldrin to kill Japanese beetles,
scientists found a large number of dead animals in the
treated area, such as various birds, rabbits, ground squirrels, cats, and beneficial insects. (Use of dieldrin in the
United States has since been banned.) Pesticides do not
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Figure 22.6 Insect pests, insect predators,
Insect pest
Insect predator or parasite
Population
and pesticides. Natural population fluctuations
are controlled by a variety of factors, including
the presence of predators and parasites. When
pesticides are applied, the predators and parasites of the pest species are also affected. The
effect on predator/parasite populations disrupts
the normal interactions between species and
can cause a huge increase in the population of
the pest species a short time after the pesticide
is applied.
Natural population fluctuations
Pesticide
application
Time
have to kill organisms to harm them. Quite often the
stress of carrying pesticides in its body makes an organism more vulnerable to predators, diseases, or other
stressors in its environment.
Because the natural enemies of pests often starve or
migrate in search of food after pesticide has been sprayed
in an area, pesticides are indirectly responsible for a large
reduction in the populations of these natural enemies.
Pesticides also kill natural enemies directly, because
predators consume a lot of the pesticide by consuming
the pests. After a brief period, the pest population
rebounds and gets larger than ever, partly because no
natural predators are left to keep its numbers in check
(Figure 22.6).
Despite a 33-fold increase in pesticide use in the
United States since the 1940s, crop losses due to insects,
diseases, and weeds have not changed appreciably (Table
22.2). Increasing genetic resistance to pesticides and the
destruction of the natural enemies of pests provide a partial explanation. Changes in agricultural practices are also
to blame; for example, crop rotation, a proven way of
controlling certain pests, is not practiced as much today
as it was several decades ago (see Chapter 14).
Creation of New Pests In some instances, the use of a
pesticide has resulted in a pest problem that did not exist
before. Creation of new pests—that is, turning minor
pest organisms into major pests—is possible because the
Table 22.2
Period
1989–1999
1974
1951–1960
1942–1951
Percentage of Crops Lost Annually to Pests in
the United States
Insects
Diseases
13.0
13.0
12.9
7.1
12.0
12.0
12.2
10.5
Weeds
12.0
8.0
8.5
13.8
natural predators, parasites, and competitors of a certain
pest may be largely killed by a pesticide, allowing the
pest’s population to rebound. The use of DDT to control
certain insect pests on lemon trees was documented as
causing an outbreak of a scale insect (a sucking insect that
attacks plants) that had not been a problem before spraying (Figure 22.7). In a similar manner, the European red
mite became an important pest on apple trees in the
northeastern United States, and beet armyworms became
an important pest on cotton, both after the introduction
of pesticides.
Persistence, Bioaccumulation, and Biological
Magnification
The effects of DDT on many bird species first demonstrated certain problems of chlorinated hydrocarbon pesticide use. Falcons, pelicans, bald eagles, ospreys, and
many other birds are very sensitive to traces of DDT in
their tissues. A substantial body of scientific evidence
indicates that one of the effects of DDT on these birds is
that they lay eggs with extremely thin, fragile shells that
usually break during incubation, causing the chicks’
deaths. After 1972, the year DDT was banned in the
United States, the reproductive success of many birds
improved (Figure 22.8).
The impact of DDT on birds is the result of three
characteristics of DDT: its persistence, bioaccumulation,
and biological magnification. Some pesticides, particularly chlorinated hydrocarbons, are extremely stable in
the environment and may take many years to be broken
down into less toxic forms. The persistence of synthetic
pesticides is a result of their novel (not found in nature)
chemical structures. Natural decomposers such as bacteria have not yet evolved ways to degrade synthetic pesticides, so they accumulate in the environment and in the
food web.
When a pesticide is not metabolized (broken down)
or excreted by an organism, it is simply stored, usually in
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P R O B L E M S A S S O C I AT E D W I T H P E S T I C I D E U S E
California red scale density
Photo
to come
1,400
1,300
1,200
1,100
1,000
900
800
700
600
500
400
(a)
DDT treated trees
250
200
150
Untreated control trees
100
50
20
0
Economic injury level
1965
1966
1967
Year
1968
1969
(b)
Figure 22.8 Effect of DDT on birds. A com-
DDE residue in eagle eggs
1.3
130
1.0
100
0.7
70
0.4
40
Mean number of young
per breeding area
DDT ban
0.1
10
1966
1968
1970
1972
1974
Year
1976
1978
1980
DDE (ppm, dry weight)*
Mean number of young per breeding area
Figure 22.7 Pesticide use and new pest species. An infestation of red scale insects on
lemons occurred after DDT (dichlorodiphenyltrichloroethane) was sprayed to control a different pest. Prior to DDT treatment, red scale did not cause significant economic injury to citrus
crops. (a) Red scale on green (unripened) citrus fruit. (b) A comparison of red scale populations on DDT-treated trees (blue line) and untreated trees under biological control (red line).
parison of the number of successful bald
eagle offspring with the level of DDT
(dichlorodiphenyltrichloroethane) residues in
their eggs. Note that reproductive success
improved after DDT levels decreased. (DDE
= dichlorodiphenyldichloroethylene.)
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fatty tissues. Over time, the organism may accumulate
high concentrations of the pesticide. The buildup of a
persistent pesticide in an organism’s body is known as
bioaccumulation or bioconcentration.
Organisms at higher levels on food webs tend to have
greater concentrations of bioaccumulated pesticide stored
in their bodies than those lower on food webs. The
increase in pesticide concentrations as the pesticide passes
through successive levels of the food web is known as biological magnification or biological amplification.1
As an example of the concentrating characteristic of
persistent pesticides, consider a food chain studied in a
Long Island salt marsh that was sprayed with DDT over
a period of years for mosquito control: algae and plankton → shrimp → American eel → Atlantic needlefish →
ring-billed gull (Figure 22.9). The concentration of
DDT in water was extremely dilute, on the order of
0.00005 parts per million (ppm). The algae and other
plankton contained a greater concentration of DDT, 0.04
ppm. Each shrimp grazing on the plankton concentrated
the pesticides in its tissues to 0.16 ppm. Eels that ate
shrimp laced with pesticide had a pesticide level of 0.28
ppm, and needlefish that ate eels contained 2.07 ppm of
DDT. The top carnivores, ring-billed gulls, had a DDT
level of 75.5 ppm from eating contaminated fishes.
Although this example involves a bird at the top of the
food chain, it is important to recognize that all top carnivores, from fishes to humans, are at risk from biological
magnification. Because of this risk, currently approved
pesticides have been tested to ensure they do not persist
and accumulate in the environment.
Mobility in the Environment
Another problem associated with pesticides is that they
do not stay where they are applied but tend to move
through the soil, water, and air, sometimes long distances
(Figure 22.10). Pesticides that are applied to agricultural
lands and then wash into rivers and streams when it rains
can harm fishes. If the pesticide level in their aquatic
ecosystem is high enough, the fishes may be killed. If the
level is sub-lethal (that is, not enough to kill the fishes),
the fishes may still suffer from undesirable effects such as
bone degeneration. These effects may decrease their
competitiveness and increase their chances of being
preyed upon.
Pesticide mobility is also a problem for humans. In
1994 the Environmental Working Group (EWG), a private environmental organization, analyzed herbicides in
drinking water by evaluating 20,000 water tests performed by state and federal government inspectors.
1 Other toxic substances besides pesticides may exhibit bioaccumulation and biological magnification, including radioactive isotopes, heavy
metals such as mercury, flame retardants known as polybrominated
diphenyl ethers (PBDEs), and industrial chemicals such as PCBs (polychlorinated biphenyls); see Chapters 11, 21, and 23.
AMOUNT OF
DDT IN TISSUE
TROPHIC
LEVEL
Tertiary
consumer
(ring-billed gull)
75.5 ppm
Secondary
consumer
(Atlantic needlefish)
2.07 ppm
0.28 ppm
1,510,000
times
increase
Secondary
consumer
(American eel)
0.16 ppm
Primary
consumer
(shrimp)
0.04 ppm
Producer and
primary consumer
(Algae and other plankton)
0.00005 ppm
Water
Figure 22.9 Biological magnification of DDT in a Long Island
salt marsh. Note how the level of DDT (dichlorodiphenyltrichloroethane), expressed as parts per million, increased in
the tissues of various organisms as DDT moved through the
food chain from producers to consumers. The ring-billed gull
at the top of the food chain had approximately 1.5 million
times more DDT in its tissues than the concentration of DDT
in the water.
Their study revealed that 14.1 million U.S. residents
drink water that contains traces of five widely used herbicides. Because these herbicides are often used on corn
and soybeans, the study focused on the Midwestern states
where these crops are commonly grown. The study concluded that 3.5 million people living in the Midwest face
a slightly elevated cancer risk because of their exposure to
the herbicides. The EPA recently reduced the use of five
of the herbicides (alachlor, metachlor, atrazine, cyanazine, and simazine).
From 1996 to 1998 the EWG conducted a different
study in California that revealed pesticide mobility in the
atmosphere. They collected nearly 100 air samples in
several California counties and submitted them to a certified laboratory for analysis. Almost two thirds of the samples contained small amounts of pesticides that had
drifted from farm fields.
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INTENDED PATHWAY
FOR PESTICIDE
Aerial spraying
of pesticide,
evaporation
Gravitational settling
Air
and precipitation
Agricultural
soil
527
Target pest
Crop
Harvest
Animals
Precipitation
ACTUAL PATHWAYS
OF PESTICIDE IN
THE ENVIRONMENT
Erosion,
leaching
Food
Especially
Humans
Fresh water
groundwater
Runoff
and
seepage
Food
Ocean
Figure 22.10 Intended versus actual pathways of pesticides. The intended pathway of
pesticides in the environment is shown at the top of the figure, while the actual pathways
are shown at the bottom.
RISKS OF PESTICIDES TO
HUMAN HEALTH
Exposure to pesticides, which is greater than most people
realize and often occurs without their knowledge, can
also damage human health. Pesticide poisoning caused by
short-term exposure to high levels of pesticides can result
in harm to organs and even death, whereas long-term
exposure to lower levels of pesticides can cause cancer.
There is also concern that exposure to trace amounts of
certain pesticides can disrupt the human endocrine (hormone) system. Children are considered to be at greater
risk from exposure to household pesticides than adults.
Short-Term Effects of Pesticides
Pesticides poison approximately 67,000 people in the
United States each year. Most of these are farm workers
or others whose occupations involve daily contact with
large quantities of pesticides. A person with a mild case of
pesticide poisoning may exhibit symptoms such as nausea, vomiting, and headaches. More serious cases, particularly organophosphate poisonings, may result in
permanent damage to the nervous system and other body
organs. Although the number is low in the United States,
people do die from overexposure to pesticides. Almost
any pesticide can kill a human if the dose is large enough.
The WHO estimates that globally more than 3 million people are poisoned by pesticides each year; of these,
about 220,000 die. The incidence of pesticide poisoning
is highest in developing countries, in part because they
often use dangerous pesticides that have been banned or
greatly restricted by highly developed nations. Also, pesticide users in developing nations often are not trained in
the safe handling and storage of pesticides, and safety
regulations are generally more lax there.
CASE·IN·POINT
The Bhopal Disaster
In December 1984, the world’s worst industrial accident
occurred at a Union Carbide pesticide plant in the Indian
city of Bhopal. As much as 36 metric tons (40 tons) of
methyl isocyanate (MIC) gas, which is used to produce
carbamate pesticides, erupted from an underground storage tank after water leaked in and caused an explosive
chemical reaction. Some of the MIC, which is itself
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highly toxic, converted in the atmosphere to hydrogen
cyanide, which is even more deadly. The toxic cloud settled over 78 km2 (30 mi2), exposing up to 600,000 people.
According to official counts, about 2,500 people
were killed outright from exposure to the deadly gas, and
at least 2,500 have died since then. An international
group of medical specialists (the International Medical
Commission on Bhopal) estimated in 1996 that between
50,000 and 60,000 survivors had serious respiratory, ophthalmic, intestinal, reproductive, and neurological problems. Young women who were exposed to the gas were
unable to marry because it was widely assumed they were
sterile. Many also suffer from psychological disorders,
such as post-traumatic stress and pathological guilt over
being unable to save loved ones.
Union Carbide agreed in 1989 to pay $470 million in
compensation and spend more than $100 million to build
a hospital for victims. The Indian government is still disbursing the $470 million paid by Union Carbide. Most
people who have been compensated have received $500
each.
Meanwhile, cleanup of contaminated land and
groundwater in the vicinity of the former pesticide plant
remains to be addressed. Tests conducted in 1999 by
Greenpeace International and several Bhopal-based
organizations revealed contamination of land and drinking water supplies with heavy metals and persistent
organochlorine compounds. People living in communities surrounding the former plant site may be at risk from
these hazardous chemicals, particularly in their drinking
water.
Long-Term Effects of Pesticides
Many studies of farm workers and workers in pesticide
factories, both of whom are exposed to low levels of pesticides over many years, show an association between
cancer and long-term exposure to pesticides. A type of
lymphoma (a cancer of the lymph system) has been associated with the herbicide 2,4-D. Other pesticides have
been linked to a variety of cancers, such as leukemia and
cancers of the brain, lungs, and testicles. Although the
issue of whether certain pesticides cause breast cancer
remains unresolved, researchers have noted a correlation
between a high level of one or more pesticides in the
breast’s fatty tissue and cancer.
Long-term exposure to at least one pesticide may
have resulted in sterility in thousands of farm workers on
banana and pineapple plantations. More than 26,000
workers in 12 countries sued their employers or pesticide
manufacturers, and most of the defendants have paid settlements but not admitted liability.
A 2001 study that compared records of pesticide
applications in 1984 in California’s Central Valley with
state health records for the same area showed that pregnant women who live near pesticide applications have
higher rates of miscarriage. Other studies have indicated
that the children of agricultural workers are at greater
risk for birth defects, particularly stunted limbs. There is
also evidence that exposure to pesticides may compromise the body’s ability to fight infections.
Long-term exposure to pesticides may increase the
risk of Parkinson’s disease, which afflicts about 1 million
people in the United States. When researchers injected
rats with repeated doses of rotenone, a plant-derived pesticide, the rats developed the symptoms of Parkinson’s
disease, including difficulty in walking, tremors, and
abnormal protein deposits in their brains. Many pesticides have chemical structures similar to rotenone, which
suggests that other pesticides may increase the risk of
Parkinson’s disease as well. It is important to remember
that this connection between pesticide exposure and
Parkinson’s disease is very tentative and will require additional research to prove or disprove. To date, no study
has connected Parkinson’s disease in humans with exposure to a specific pesticide.
Pesticides as Endocrine Disrupters
In the 1990s, many scientific articles were published that
linked certain pesticides and other persistent toxic chemicals with reproductive problems in animals (Table 22.3;
see Figure 1.5 and Chapter 1 discussion on endocrine
disrupters). River otters exposed to synthetic chemical
pollutants were found to have abnormally small penises.
Female sea gulls in Southern California exhibited behavioral aberrations by pairing with one another rather than
with males during the mating season. In many cases scientists have been able to reproduce the same abnormal
symptoms in the laboratory, providing support that the
defects are caused by certain persistent chemicals.
However, it was the suggestion in the 1996 book, Our
Stolen Future, by Theo Colburn, Dianne Dumanoski, and
John Myers, that persistent toxic chemicals in the environment are disrupting human hormone systems that ignited a
barrage of media attention. Theo Colburn, a senior scientist at the World Wildlife Fund, hypothesized that ubiquitous chemicals in the environment are linked to disturbing
Table 22.3
Some Pesticides That Are Known Endocrine
Disrupters*
Pesticide
General Information
Atrazine
Chlordane
Herbicide; still used
Insecticide; banned in United
States in 1988
Insecticide; banned in United
States in 1972
Insecticide; still used
Insecticide; banned in United
States in 1977
Insecticide; still used
DDT (dichlorodiphenyltrichloroethane)
Endosulfan
Kepone
Methoxychlor
* Based on experimental research with laboratory animals.
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trends in human health. These include increases in breast
and testicular cancer, increases in male birth defects, and
decreases in sperm counts (see Chapter 1).
Although a few studies suggest that certain chemicals
in the environment may affect human health, a direct
cause-and-effect relationship between these chemicals
and adverse effects on the human population remains to
be established. Some scientists think the potential danger
is so great, however, that persistent chemicals such as
DDT should be internationally banned immediately.
Other scientists are more cautious in their assessment of
the danger but still think the problem should not be disregarded.
Our Stolen Future ignited public concern and triggered scientific investigations by universities, governments, and industries on how synthetic chemicals
interfere with the actions of human hormones. It will
take years before these studies can tell us if these chemicals are acting as endocrine disrupters in humans. Meanwhile, an international effort is currently underway to
ban all production and use of nine pesticides suspected of
being endocrine disrupters (see section on the global ban
of persistent organic pollutants later in this chapter).
stated that infants and children may not be adequately
protected.
Recent research supports an emerging hypothesis
that exposure to pesticides may affect the development of
intelligence and motor skills of infants, toddlers, and
preschoolers. One study, published in Environmental
Health Perspectives in 1998, compared two groups of rural
Yaqui Indian preschoolers. These two groups, both of
which live in northwestern Mexico, shared similar
genetic backgrounds, diets, water mineral contents, cultural patterns, and social behaviors. The main difference
between the two groups was their exposure to pesticides:
One group lived in a farming community where pesticides were used frequently (45 times per crop cycle) and
the other in an adjacent nonagricultural area where pesticides were rarely used. When asked to draw a person,
most of the 17 children from the low-pesticide area drew
recognizable stick figures, whereas most of the 34 children from the high-pesticide area drew meaningless lines
and circles (Figure 22.11). Additional tests of simple
mental and physical skills revealed similar striking differences between the two groups of children.
Drawings of a person
(by 4-year-olds)
Pesticides and Children
In recent years, there has been increased attention to the
health effects of household pesticides on children
because it appears that household pesticides are a greater
threat to children than to adults. For one thing, children
tend to play on floors and lawns, where they are exposed
to greater concentrations of pesticide residues. Also, children may be more sensitive to pesticides because their
bodies are still developing. Several preliminary studies
suggest that exposure to household pesticides may cause
brain cancer and leukemia in children, but more research
must be done before any firm conclusions can be made.
The EPA estimates that 84% of U.S. homes use pesticide products, such as pest strips, bait boxes, bug
bombs, flea collars, pesticide pet shampoos, aerosols, liquids, and dusts. Several thousand different household
pesticides are manufactured, and these contain over 300
active ingredients and more than 2,500 inert ingredients
(discussed later in the chapter). Poison control centers in
the United States annually receive more than 130,000
reports of exposure and possible poisoning from household pesticides. More than half of these incidents involve
children.
There is also concern about the ingestion by children of pesticide residues on food. The National
Research Council published a 3-year study in 1993 called
Pesticides in the Diets of Infants and Children. It called for
additional research on how pesticide residues on food
affect the young because it is not known if infants and
children are more, or less, susceptible than adults. Also,
because current pesticide regulations are intended to
protect the health of the general population, the report
529
Foothills
(a)
54 mo.
female
Valley
(b)
54 mo.
female
Figure 22.11 Effect of pesticide exposure on preschoolers. A
study in Sonora, Mexico, found that Yaqui Indian preschoolers
varied in their motor skills on the basis of the degree of pesticide exposure. The children were asked to draw a person. Two
representative pieces of art are shown, both drawn by 41/2year-old girls. (a) Most preschoolers who received little pesticide exposure were able to draw a recognizable stick figure.
(b) Most preschoolers who lived in an area where agricultural
pesticides were widely used could only draw meaningless lines
and circles.
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ALTERNATIVES TO PESTICIDES
Given the many problems associated with pesticides, it is
clear that they are not the final solution to pest control.
Fortunately, pesticides are not the only weapons in our
arsenal. Alternative ways to control pests include cultivation methods, biological controls, reproductive controls,
pheromones and hormones, genetic controls, quarantine,
and irradiation. A combination of these methods in agriculture, often including a limited use of pesticides as a last
resort, is known as integrated pest management (IPM).
IPM is the most effective way to control pests. (Also see
the Chapter 18 introduction for a discussion of the growing popularity of organic foods, which are grown in the
absence of pesticides and commercial fertilizers.)
Using Cultivation Methods to Control Pests
Sometimes agricultural practices can be altered in such a
way that a pest is adversely affected or discouraged from
causing damage. Although some practices, such as the
insect vacuum mentioned at the beginning of the chapter,
are relatively new, other cultivation methods that discourage pests have been practiced for centuries.
One way to reduce damage by crop pests is by interplanting mixtures of plants, such as by alternating rows of
different plants. When corn was interplanted with
molasses grass in an experiment in Kenya, only about 5%
of the corn crop was damaged, as compared with about
39% damage in the control field of corn. Molasses grass
attracts wasps that lay their eggs inside stem borers,
insects that destroy the stems of corn plants.
A technique that has been used with success in
alfalfa crops is strip cutting, in which only one segment of
the crop is harvested at a time. The unharvested portion
of the crop provides an undisturbed habitat for natural
predators and parasites of the pest species. The same
type of benefit is derived from keeping strips of
unplowed plants (that is, wild plants, including weeds)
along the margins of fields. A German study found that
pest mortality was about 50% higher in oilseed rape
fields with margin strips of other plants, as compared to
fields without margins. The margins provided a refuge
for three parasites of the pollen beetle, which is a significant pest on oilseed rape plants.
The proper timing of planting, fertilizing, and irrigating promotes healthy, vigorous plants that are more
able to resist pests because they are not being stressed by
other environmental factors. The rotation of crops also
helps control pests. When corn is not planted in the same
field for 2 years in a row, the corn rootworm is effectively
controlled.
Biological Controls
Biological controls involve the use of naturally occurring disease organisms, parasites, or predators to con-
trol pests. As an example, suppose that an insect species
is accidentally introduced into a country where it was
not found previously, and becomes a pest. It might be
possible to control this pest by going to its native country and identifying an organism that is an exclusive
predator or parasite of the pest species. That predator
or parasite, if successfully introduced, may be able to
lower the population of the pest species so that it is no
longer a problem.
Cottony-cushion scale provides an example of successfully using one organism to control another. The cottony-cushion scale is a small insect that sucks the sap from
the branches and bark of many fruit trees, including citrus
trees (Figure 22.12). It is native to Australia but was accidentally introduced to the United States in the 1880s. A
U.S. entomologist went to Australia and returned with
several possible biological control agents. One, the vedalia
beetle, was found to be very effective in controlling scale,
which it eats voraciously and exclusively. Within 2 years of
its introduction, the vedalia beetle had significantly
reduced the cottony-cushion scale in citrus orchards.
Today both the cottony-cushion scale and the vedalia beetle are present in very low numbers, and the scale is not
considered an economically important pest.
More than 300 species have been introduced as biological control agents to North America. The Agricultural Research Service of the U.S. Department of
Agriculture (USDA) is currently investigating possible
biological controls for about a dozen other insect and
weed pests. Although some examples of biological control are quite spectacular, finding an effective parasite or
predator is very difficult. And just because a parasite or
predator has been identified does not mean it can become
successfully established in a new environment. Slight
variations in environmental conditions such as temperature and moisture can alter the effectiveness of the biological control organism in its new habitat.
Care must also be taken to ensure that the introduced control agent does not attack unintended hosts
and become a pest itself. To guard against this when the
control agent is an insect, scientists put the insects in
cages with samples of important crops, ornamentals, and
native plants to determine if the insects will eat the plants
when they are starving.
Despite such tests, organisms introduced as biological controls sometimes cause unintended problems in
their new environment, and once they are introduced,
they cannot be recalled. A weevil was introduced in 1968
to control the Eurasian musk thistle, a noxious weed that
arrived in North America in the mid-19th century. Since
then, the weevil has expanded its host range to include a
North American thistle that is listed as a threatened
species. The weevils significantly reduce seed production
in the native thistles.
Insects are not the only biological control agents.
Bacteria and viruses that harm insect pests have also been
used successfully as biological controls. Bacillus popilliae,
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531
(b)
(a)
Figure 22.12 Biological control of cottony-cushion scale. (a) The cottony-cushion scale is an
insect that attacks the stems and bark of several important crops. The white, ridged mass on
top of each female contains up to 800 eggs. The males are reddish brown. (b) A vedalia beetle
larva feeds on cottony-cushion scale. Both adults and larvae of the vedalia beetle control the
cottony-cushion scale.
which causes milky spore disease in insects, can be
applied as a dust on the ground to control the larval
(immature) stage of Japanese beetles. The common soil
bacterium Bacillus thuringiensis, or Bt, produces a natural
pesticide that is toxic to some insects, such as the cabbage
looper, a green caterpillar that damages many vegetable
crops, and the corn earworm. When eaten by insect larvae, Bt toxin damages the intestinal tract, killing the
young insect. The toxin does not persist in the environment and is not known to harm mammals, birds, or other
noninsect species.
ENVIROBRIEF
A Lethal “Bug Juice”
The cassava hornworm is a serious pest of cassava, a root crop that
supports 500 million of the world’s poorest people. An innovative,
low-tech pesticide for use against the hornworm was developed
through studies conducted by plant and agricultural scientists at
Cornell University and in Colombia and Brazil:
■
When 12 hornworms infected with a particular virus lethal to that
species are mixed with water in a blender, the resulting “milkshake” is a potent insecticide against other hornworms.
■
This size batch of homemade pesticide effectively treats 2.5
acres.
■
When sprayed on cassava plant leaves, the blended pesticide is
60% to 100% effective at killing hornworms, which eat the virus
and die a few days later.
■
The unusual milkshake is harmless to other insects, other animals, and humans.
Pheromones and Hormones
Pheromones are natural substances produced by animals
to stimulate a response in other members of the same
species. Pheromones are commonly called sexual attractants because they are often produced to attract members
of the opposite sex for mating. Each insect species produces its own specific pheromone, so once the chemical
structure is known, it is possible to make use of
pheromones to control individual pest species.
Pheromones have been successfully used to lure insects
such as Japanese beetles to traps, where they are killed
(Figure 22.13). Alternatively, pheromones can be
released into the atmosphere to confuse insects so that
they cannot locate mates.
Insect hormones are natural chemicals produced by
insects to regulate their own growth and metamorphosis,
which is the process by which an insect’s body changes
through several stages to an adult. Specific hormones
must be present at certain times in the life cycle of the
insect; if they are present at the wrong time, the insect
develops abnormally and dies. Many such insect hormones have been identified, and synthetic hormones with
similar structures have been made. Entomologists are
actively pursuing the possibility of using these substances
to control insect pests.
A synthetic version of the insect hormone ecdysone,
which causes molting, was the first hormone to be
approved for use. Known as MIMIC, the hormone triggers abnormal molting in insect larvae (caterpillars) of
moths and butterflies. Since MIMIC also affects some
beneficial insects, its use has risks.
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sterilization is discontinued, the pest population
rebounds to a high level in a few generations (which, you
will recall, are very short). The procedure is also expensive, as it requires the rearing and sterilization of large
numbers of insects in a laboratory or production facility.
During the 1990 Mediterranean fruit fly (medfly) outbreak in California, as many as 400 million sterile male
medflies were released each week. Such extraordinary
measures are taken because the medfly is so destructive.
The adult medfly lays its eggs on 250 different fruits and
vegetables. When the eggs hatch, the maggots feed on
the fruits and vegetables and turn them into a disgusting
mush.
Genetic Controls
Figure 22.13 Pheromone traps. A Japanese beetle trap uses a
sexual attractant to lure Japanese beetles, which fall into the
bag and die. Such insect traps draw large numbers of Japanese
beetles.
Reproductive Controls
Like biological controls, reproductive controls of pests
involve the use of organisms. Instead of using another
species to reduce the pest population, however, reproductive control strategies suppress pests by sterilizing
some of its members. In the sterile male technique,
large numbers of males are sterilized in a laboratory, usually with radiation or chemicals. Males are sterilized
rather than females because male insects of species
selected for this type of control mate several to many
times, but females of that species mate only once. Thus,
releasing a single sterile male may prevent successful
reproduction by several females, whereas releasing a single sterile female would prevent successful reproduction
by only that female.
The sterilized males are released into the wild, where
they reduce the reproductive potential of the pest population by mating with normal females, who then lay eggs
that never hatch. As a result, of course, the population of
the next generation is much smaller.
One disadvantage of the sterile male technique is
that it must be carried out continually to be effective. If
Traditional selective breeding has been used to develop
many varieties of crops that are genetically resistant to
disease organisms or insects. Traditional breeding of crop
plants typically involves identifying individual plants that
are in an area where the pest is common but that do not
appear to be damaged by the pest. These individuals are
then crossed with standard crop varieties in an effort to
produce a pest-resistant version. It may take as long as 10
to 20 years to develop a resistant crop variety, but the
benefits are usually worth the time and expense.
Although traditional selective breeding has resulted
in many disease-resistant crops and decreased the use of
pesticides, there are potential problems. Fungi, bacteria,
and other plant pathogens evolve rapidly. As a result, they
can quickly adapt to the disease-resistant host plant,
meaning that the new pathogen strains can cause disease
in the formerly disease-resistant plant variety. Plant
breeders, then, are in a continual race to keep one step
ahead of plant pathogens.
Genetic engineering offers great promise in breeding pest-resistant plants more quickly (see Chapter 18).
For example, a gene from the soil bacterium Bt (already
discussed in the section on biological controls) has been
introduced into several plants, such as cotton. Caterpillars that eat cotton leaves from these genetically modified (GM) plants die or exhibit stunted growth.
CASE·IN·POINT
Bt, Its Potential and Problems
Bt has been marketed since the 1950s, but it was not sold
on a large scale until recently, mainly because there are
many different varieties of the Bt bacterium, and each
variety produces a slightly different protein toxin. Each is
toxic to only a small group of insects. For example, the Bt
variety that works against corn borers would not be effective against Colorado potato beetles. As a result, Bt was
not economically competitive against chemical pesticides, each of which could kill many different kinds of
pests on many different crops.
Genetic engineers have greatly increased the potential of Bt’s toxin as a natural pesticide by modifying the
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A LT E R N AT I V E S T O P E S T I C I D E S
gene coding for the toxin so that it affects a wider range
of insect pests. They then inserted the Bt gene that codes
for the toxin into at least 18 crop species, including corn,
potato, and cotton. Bt corn, one of the first genetically
modified (GM) crops, has been engineered to produce a
continuous supply of toxin, which provides a natural
defense against insects such as the European corn-borer.
Likewise, Bt tomato and Bt cotton are more resistant to
pests such as the tomato pinworm and the cotton bollworm. Early ecological risk assessment studies, performed before EPA approved their use, indicated that
genetically modified crops are essentially like their more
conventional counterparts that are produced by selective
breeding. Genetically modified crops do not become
invasive pests or persist in the environment longer than
crops that have not been genetically modified. Significantly fewer pesticide applications are needed for GM
crops with the Bt gene.
The future of the Bt toxin as an effective substitute
for chemical pesticides is not completely secure, however. Beginning in the late 1980s, several farmers began
to notice that chemically applied Bt was not working as
well against the diamondback moth as it had in the past.
All of the farmers who reported this reduction in effectiveness had used Bt frequently and in large amounts on
their fields. Also, in 1996, many farmers growing Bt cotton reported that their crop succumbed to the cotton
bollworm, which Bt is supposed to kill. It appears that
certain insects, such as the diamondback moth and possibly the cotton bollworm, may have developed resistance
to this natural toxin in much the same way that they
develop resistance to chemical pesticides. If Bt continues
to be used in greater and greater amounts, it is likely that
more insect pests will develop genetic resistance to it,
greatly reducing Bt’s potential as a natural pesticide. Scientists are studying resistance management strategies to
curb pest resistance to genetically engineered crops that
make Bt.
Unintended Ecological Risks of Genetically Modified
Crops One of the concerns about growing GM crops is
that they might do some unintentional harm to the environment (see Chapter 18 discussion of genetic engineering). In 1999 biologists from Cornell University
published results that suggested that GM crops containing the Bt gene might harm the monarch butterfly. The
primary food of monarch larvae (caterpillars) is milkweed, a weed that often grows along the margins of cornfields. Corn is a wind-pollinated plant that disperses its
pollen up to 60 m (197 ft). Corn pollen lands on other
plants near cornfields and can be eaten by the organisms
that consume these plants. The biologists conducted a
laboratory experiment in which they fed monarch larvae
milkweed leaves dusted with Bt corn pollen. In a few days
these larvae ate less, grew more slowly, and had higher
mortality rates than control larvae fed milkweed leaves
without Bt corn pollen.
533
Supporters of GM crops point out that the chemical
pesticides that were sprayed before Bt was introduced
into corn would certainly have killed many more nontarget organisms, including large numbers of monarch larvae. Moreover, because the researchers did not perform
field studies, we cannot assume that the monarch population is at risk from the increasing acreage of Bt corn
being planted in the United States. (In 2000, 6.2 million
hectares, or 15.3 million acres, of Bt corn were planted in
the United States; this represents about 20% of the U.S.
corn crop.) A 1999 study suggests that monarchs in the
wild are not at significant risk from Bt corn.
Quarantine
Governments attempt to prevent the importation of foreign pests and diseases by practicing quarantine, or
restriction of the importation of exotic plant and animal
material that might harbor pests. If a foreign pest is accidentally introduced, quarantine of the area where it is
detected helps prevent its spread. If a foreign pest is
detected on a farm, the farmer may be required to
destroy the entire crop.
Quarantine is an effective, although not foolproof,
means of control. The USDA has blocked the accidental
importation of medflies on more than 100 separate occasions. On the few occasions when quarantine failed and
these insects successfully passed into the United States,
millions of dollars’ worth of crop damage has been
incurred in addition to the millions spent to eradicate the
pests. Eradication efforts include the use of helicopters to
spray the insecticide malathion over hundreds of square
kilometers and the rearing and releasing of millions of
sterile males to breed the medfly out of existence.
Many experts think that the repeated finds of medflies in California indicate that, rather than being accidentally introduced each year, the medfly has become
established in the state. If so, there are potentially disastrous consequences for California’s $18 billion agricultural economy. Other countries could stop importing
California produce or require expensive inspections and
treatments of every shipment in order to prevent the
importation of the medfly into their countries.
Integrated Pest Management
Many pests cannot be controlled effectively with a single
technique; a combination of control methods is often
more effective. Integrated pest management (IPM)
combines the use of a variety of biological, cultivation,
and pesticide controls that are tailored to the conditions
and crops of an individual farm (Figure 22.14). Biological
and genetic controls, including GM crops designed to
resist pests, are used as much as possible, and conventional pesticides are used sparingly and only when other
methods fail (see “Meeting the Challenge: Reducing
Agricultural Pesticide Use by 50% in the United
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Resistant
crop
varieties
Cultivation
practices
IPM
tools
Natural
enemies
Judicious
use of
pesticides
Pheromone
traps
Figure 22.14 Tools of integrated pest management (IPM).
Cultivation practices are agricultural practices, such as crop
rotation, that reduce the pest populations for a particular crop
from one season to the next. Where available, resistant crop
varieties are grown to protect against important pests. IPM
encourages conditions for natural enemies of pests.
Pheromone traps keep pest populations at acceptable levels
because they lure and destroy male insects, thereby reducing
the insects’ rate of reproduction. Pesticides are applied only
after available alternatives have been tried and the pest population is large enough to cause economic damage.
States”). When pesticide methods are required, the least
toxic pesticides that are effective are applied in the lowest
possible quantities. Thus, IPM allows the farmer to control pests with a minimum of environmental disturbance
and often at a minimal cost.
In order to be effective, IPM requires a thorough
knowledge of the life cycles, feeding habits, travel, and
nesting habits of the pests as well as all of their interactions with their hosts and other organisms. The timing of
planting, cultivation, and treatments with biological controls is critical and is determined by carefully monitoring
of the concentration of pests. Integrated pest management also optimizes natural controls by using agricultural
techniques that discourage pests. Integrated pest management is an important part of sustainable agriculture
(see Chapter 18).
There are two fundamental premises associated with
IPM. First, IPM is the management rather than the eradication of pests. Farmers who have adopted the principles
of IPM allow a low level of pests in their fields and accept
a certain amount of economic damage caused by the
pests. These farmers do not spray pesticides at the first
sign of a pest. Instead, they periodically sample the pest
population in the field to determine when the pest population reaches an economic injury threshold at which the
benefit of taking action (such as the judicious use of pesticides) exceeds the cost of that action.
Second, IPM requires that farmers be educated so
they can adopt the strategies that will work best in their
particular situations. Managing pests is much more
complex than trying to eradicate them. The farmer
must know what pests to expect on each crop, and what
steps can be taken to minimize their effects. The farmer
must also know what beneficial species will assist in
controlling the pests, and how these beneficial species
can be encouraged.
Cotton, which is attacked by many insect pests, has
responded well to IPM. Cotton has the heaviest insecticide application of any crop: Although only about 1% of
agricultural land in the United States is used for this
crop, cotton accounts for almost 50% of all the insecticides used in agriculture! By applying simple techniques
such as planting a strip of alfalfa adjacent to the cotton
field, the need for chemical pesticides is lessened. Lygus
bugs, a significant pest of cotton, move from the cotton
field to the strip of alfalfa, which they prefer as a food.
Thus, less damage is done to the cotton plants.
Adoption of IPM by U.S. farmers has steadily
increased since the 1960s, but the overall proportion of
ENVIROBRIEF
Organic Cotton
Although organic foods are doing well in the United States, consumers have traditionally been unwilling to pay the premium price
placed on clothes made from pesticide- and fertilizer-free cotton.
Now, however, the organic cotton industry, backed by several of the
country’s largest garment manufacturers, is slowly making gains.
Patagonia, Inc., which is the most enthusiastic backer of organic
cotton, shifted its entire cotton line to organic fibers in 1996. More
recently, organic cotton farmers made an unusual deal with LeviStrauss, Nike, and the Gap. These companies agreed to buy organic
cotton and mix it with ordinary cotton. Shoppers cannot buy allorganic clothing, but the clothing companies can tout their environmental commitment, and organic cotton farmers have gained a
tremendous foothold in the industry. Growing interest in personal
care products made from organic cotton (cotton balls, ear swabs,
tampons, etc.) has also boosted the U.S. industry, which planted an
estimated 11,500 acres of organic cotton across seven states in
2001.
These changes couldn’t come soon enough, as runoff from cotton farming is the most significant source of water pollution in some
states, and globally, more insecticides are used to grow cotton than
any other crop. The European Union is financing a $12.7 million effort
to grow environment-friendly cotton in Asia, including China, India,
and Pakistan, which together produce 46% of the world’s cotton.
Farmers there will be trained in integrated pest management techniques. The project’s goals are to cut pesticide use by half, while
increasing cotton production.
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535
MEETING THE CHALLENGE
Reducing Agricultural Pesticide Use by
50% in the United States
Pesticides have benefited farmers (by increasing agri-
cultural productivity) and consumers (by lowering food
prices). But pesticide use has had its price—not necessarily in economic terms, for it is difficult to assign
monetary values to many of its effects—but in terms of
health problems and damage to agricultural and natural
ecosystems. Society is increasingly concerned about
pesticide use.
Is it feasible to ban all pesticides known to cause
cancer in laboratory animals? Probably not, at least not
now. In many cases, substitute pesticides either do not
exist, are less effective, or are considerably more expensive. A pesticide ban would increase food prices,
although estimates on the magnitude of that increase
vary considerably from one study to another. A pesticide ban would also cause considerable economic hardship for certain growers, although other farmers might
benefit. For example, some insects are more troublesome in certain areas than in others. A farmer growing a
crop in an area where an insect was very harmful might
not be able to afford the crop losses that would occur
without the use of a banned pesticide. Growers in areas
where the insect was less of a problem could then
increase their production of that crop, benefiting financially from the first farmer’s loss.
Since it is impractical to ban large numbers of pesticides right now, is there another way to provide
greater protection to the environment and human
health without reducing crop yields? Governments in
Sweden, Denmark, the Netherlands, and the Canadian
Province of Ontario think so. Sweden achieved a 50%
reduction in pesticide use in 1992 and is now on a second program to reduce pesticide use by another 50%.
Denmark, the Netherlands, and the Province of
Ontario also implemented similar programs to reduce
farmers using IPM is still small. One reason that IPM is
not more widespread is that the knowledge required to
use pesticides is relatively simple compared to the complex, sophisticated knowledge needed to use IPM.
Integrated pest management has been most successful in controlling insect pests. Scientists are now
trying to develop IPM techniques to effectively control
weeds with a minimal use of herbicides. There is also a
widely recognized need to develop IPM techniques to
reduce pesticide use in urban and suburban environments.
pesticide use by 50% during the 1990s. As of 1999,
Denmark had achieved this goal, and the Netherlands
and Ontario were still working toward it. Strategies to
reduce pesticide use include removing subsidies that
encourage pesticide use, applying pesticide only when
needed, using improved application equipment, and
adopting integrated pest management (IPM) practices.
Too often pesticides are applied unnecessarily to
prevent a possible buildup of pests. Calendar spraying
is the regular use of pesticides regardless of whether
pests are a problem or not. Pesticide use can be
decreased by continually monitoring pests so that pesticides are applied only when pests become a problem;
this technique is known as scout-and-spray. A 1991
study at Cornell University determined that a monitoring program might reduce the use of insecticides on
cotton by 20% for example.
The use of aircraft to apply pesticide is extremely
wasteful because 50% to 75% of the pesticide does not
reach the target area, but instead drifts in air currents
until it settles on soil or water (recall Figure 22.10).
Pesticides applied on land with traditional methods also
drift in air currents. Advances in the design of equipment for applying pesticides could reduce pesticide use
considerably. A recently developed rope-wick applicator reduces herbicide use on soybean fields by approximately 90%.
Pesticide use can also be reduced considerably
through alternative pest control strategies. The widespread adoption of IPM would make it feasible, based
on current available technologies, for the United States
to reduce pesticide use by 50% within a 5- to 10-year
period, assuming that congress passed the necessary
legislation (similar to what Sweden and Denmark
passed). The economic results of such a reduction
would benefit farmers and consumers.
CASE·IN·POINT
Integrated Pest Management
in Asia
Beginning in the 1950s, rice farmers in many Asian countries applied pesticides on their paddies several times
during each growing season. They were encouraged to
do so by both scientists and chemical pesticide companies. In the 1980s and 1990s, however, a quiet revolution
against the indiscriminate use of pesticides occurred. The
International Rice Research Institute (IRRI), the world’s
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THE PESTICIDE DILEMMA
largest scientific establishment devoted to rice cultivation, promoted this revolution.
The IRRI has gone into different rice-growing areas
in Asia and asked local farmers to conduct a group experiment. Some farmers were asked to not spray any pesticide
during the first 40 days of the growing season. These farmers, who received training in IPM, were also told to use the
more traditional cultivation practices that were abandoned
when pesticides were introduced. Other farmers were
asked to spray pesticides on their rice crops as usual,
including the two, three, or more “preventative” treatments they usually apply during the early part of the growing season; this group served as the experiment’s control.
When the crops were harvested, the rice yields on treated
and untreated fields were compared. Many farmers were
astonished to learn that untreated fields had yields as large
as or larger than those of treated fields (Figure 22.15).
The prevailing agricultural philosophy—to wipe out
pests while their populations are low by repeatedly applying pesticides throughout the growing season—is expensive, damaging to the environment, and, as the rice farmers
discovered, ineffective. Farmers reported seeing many
more natural enemies of rice pests, such as spiders, frogs,
and carnivorous beetles, in the untreated fields than in
fields that were regularly sprayed with pesticides. Predator
populations, which were normally held in check by pesticides, had a chance to grow when paddies were untreated.
The Indonesian government helped encourage the
widespread adoption of IPM, in part by phasing out pesticide subsidies. (Many governments subsidize pesticide use
on the assumption that pesticide use boosts crop production. These subsidies, which make pesticides cheaper and
more available, encourage the inefficient use or overuse of
pesticides and discourage the adoption of alternative pest
control measures.) Between 1985 and 1990, Indonesian
pesticide subsidies declined from $141 million U.S. dollars
to zero, according to the World Bank. Pesticide production and importation declined by 58%, whereas rice production rose by 14% during that time period.
use in Indonesia, 1972 to 1990. The decline
in pesticide use in the late 1980s and early
1990s did not cause a decrease in rice yields.
Instead, rice production increased during the
4 years following the new policy. Indonesia
was the first Asian country to widely embrace
integrated pest management (IPM). It phased
out pesticide subsidies, banned the use of
dozens of pesticides on rice, and trained more
than 200,000 farmers in IPM techniques.
It is possible to prevent insects and other pests from damaging harvested food without using pesticides. The food
can be harvested and then exposed to ionizing radiation
(usually gamma rays from cobalt 60), which kills many
microorganisms, such as Salmonella, a bacterium that
causes food poisoning. Numerous countries, such as
Canada, much of Western Europe, Japan, Russia, and
Israel, extend the shelf lives of foods with irradiation.
The U.S. Food and Drug Administration (FDA)
approved this process for fruits and vegetables as well as
fresh poultry in 1986, and the first irradiated food was
sold in the United States in 1992. In 2000, the USDA
approved the irradiation of additional raw meats, such as
ground beef, steaks, and pork chops, to eliminate bacterial contamination.
The irradiation of foods is somewhat controversial.
Some consumers are concerned because they fear that
irradiated food is radioactive. This is not true. In the
same way that you do not become radioactive when your
teeth are x-rayed, food does not become radioactive
when it is irradiated. Therefore, you are not exposed to
radiation when you eat irradiated foods. Nevertheless,
many U.S. consumers refuse to purchase food that is
labeled as having been irradiated.
Critics of irradiation are concerned because irradiation forms traces of certain chemicals called free radicals, some of which have been demonstrated to be
carcinogenic in laboratory animals. They also point out
that we do not know the long-term effects of eating
irradiated foods. Proponents of irradiation argue that
free radicals normally occur in food and are also produced by cooking processes such as frying and broiling.
They assert that more than 1,000 investigations of irradiated foods, conducted worldwide for more than 3
decades, have demonstrated that it is safe. Furthermore,
irradiation lessens the need for pesticides and food
additives.
60
Pesticide production (x 10,000 tons)
Figure 22.15 Rice production and pesticide
Irradiating Foods
32
50
40
28
Pesticides
24
Rice
30
20
20
16
10
12
0
1975
1980
Year
1985
1990
Milled rice production (million tons)
536
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LAWS CONTROLLING PESTICIDE USE
LAWS CONTROLLING PESTICIDE USE
The federal government has passed several laws to regulate pesticides in the interest of protecting human health
and the environment. These include the Food, Drug, and
Cosmetics Act; the Federal Insecticide, Fungicide, and
Rodenticide Act; and the Food Quality Protection Act.
The EPA and, to a lesser extent, the FDA and the USDA
oversee the implementation of these laws.
Food, Drug, and Cosmetics Act
The Food, Drug, and Cosmetics Act (FDCA), passed
in 1938, recognized the need to regulate pesticides found
in food but did not provide a means of regulation. The
FDCA was made more effective in 1954 with the passage
of the Pesticide Chemicals Amendment. This amendment, also called the Miller Amendment, required the
establishment of acceptable and unacceptable levels of
pesticides in food.
An amended FDCA, passed in 1958, contained an
important section known as the Delaney Clause, which
stated that no substance capable of causing cancer in test
animals or in humans would be permitted in processed
food. Processed foods are prepared in some way, such as
frozen, canned, dehydrated, or preserved, before being
sold. The Delaney Clause recognized that pesticides tend
to concentrate in condensed processed foods, such as
tomato paste and applesauce.
The Delaney Clause, although commendable for its
intent, had two inconsistencies. First, it did not cover
pesticides on raw foods such as fresh fruits and vegetables, milk, meats, fish, and poultry. As an example of this
double standard, residues of a particular pesticide might
be permitted on fresh tomatoes but not in tomato
ketchup. Second, because the EPA lacked sufficient data
on the cancer-causing risks of pesticides that have been
used for a long time, the Delaney Clause applied only to
pesticides that were registered after strict tests were put
into effect in 1978. This situation gave rise to one of the
paradoxes of the Delaney Clause. There were cases in
which a newer pesticide that posed minimal risk was
banned because of the Delaney Clause, but an older pesticide, which the newer one was to have replaced, was
still used despite the fact that it was many times more
dangerous.
When the Delaney Clause was passed, the technologies for detecting pesticide residues could only reveal
high levels of contamination. Modern scientific techniques are so sensitive, however, that it is almost impossible for any processed food to meet the Delaney standard.
As a result, the EPA found it difficult to enforce the strict
standard required by the Delaney Clause. In 1988 the
EPA began granting exceptions that permitted a “negligible risk” of one case of cancer in 70 years for every 1 million people. However, the EPA was taken to court
because of its failure to follow the Delaney Clause as cur-
537
rently written, and the U.S. courts decided in 1994 that
no exceptions could be granted unless Congress modified
the Delaney Clause. (One of the key provisions of the
1996 Food Quality Protection Act, discussed shortly, did
just that.)
Federal Insecticide, Fungicide and
Rodenticide Act
The Federal Insecticide, Fungicide, and Rodenticide
Act (FIFRA) was originally passed in 1947 to regulate
the effectiveness of pesticides—that is, to prevent people
from buying pesticides that did not work. FIFRA has
been amended over the years to require testing and registration of the active ingredients of pesticides. Also, any
pesticide that does not meet the tolerance standards
established by the FDCA must be denied registration by
FIFRA.
In 1972 the EPA was given the authority to regulate
pesticide use under the terms of the FDCA and FIFRA.
Since that time, the EPA has banned or restricted the use
of many chlorinated hydrocarbons. In 1972 the EPA
banned DDT for almost all uses. Aldrin and dieldrin
were outlawed in 1974 after more than 80% of all dairy
products, fish, meat, poultry, and fruits were found to
contain residues of these insecticides. The banning of
kepone occurred in 1977 and of chlordane and heptachlor in 1988.
A 2-year study by the National Research Council
concluded in 1987 that U.S. laws regarding pesticide
residues in food were not adequate to protect the public
from cancer-causing pesticides (Table 22.4). It included
several recommendations that were made into law—an
amended FIFRA—in 1988. The 1988 law required
Table 22.4
Worst-Case Estimates of Risk of Cancer from
Pesticide Residues on Food
Food
Cancer Risk*
Tomatoes
Beef
Potatoes
Oranges
Lettuce
Apples
Peaches
Pork
Wheat
Soybeans
8.75 × 10–4†
6.49 × 10–4
5.21 × 10–4
3.76 × 10–4
3.44 × 10–4
3.23 × 10–4
3.23 × 10–4
2.67 × 10–4
1.92 × 10–4
1.28 × 10–4
* Please note that these figures are worst-case estimates. Four assumptions were made in arriving at these values: (1) the entire U.S. crop (of
tomatoes, for example) is treated (2) with all pesticides that are registered for use on that crop; (3) the pesticides are applied the maximum
number of times (4) at the maximum rate, or amount, each time.
† As an example of how to interpret these figures, tomatoes are estimated to cause an average of 8.75 deaths from cancer for every 10,000
people.
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reregistration of older pesticides, which subjected them
to the same toxicity tests that new pesticides face.
The 1988 law had its critics. Although it was stricter
than previous legislation, it represented a compromise
between agricultural interests, including pesticide manufacturers, and those opposed to all uses of pesticides. The
new law did not address a very important issue, the contamination of groundwater by pesticides. It also failed to
address the establishment of standards for pesticide
residues on foods and the safety of farm workers who are
exposed to high levels of pesticides.
FIFRA also did not require pesticide companies to
disclose the inert ingredients in their formulations. Many
pesticide products contain as much as 99% inert ingredients, which are not supposed to have active properties
against the target organism. The National Coalition for
Alternatives to Pesticides has examined the EPA’s chemical ingredient database and determined that 394 of the
more than 2,500 chemicals listed as “inert” were once
listed as active ingredients. Many inert ingredients are
generally recognized as safe (examples include pine oil,
ethanol, silicone, and water). Others are known to be
toxic (examples include asbestos, benzene, formaldehyde,
lead, and cadmium). In fact, more than 200 inert ingredients are classified as hazardous air and water pollutants,
and 21 are known or suspected to cause cancer.
Food Quality Protection Act
The Food Quality Protection Act of 1996 amended
both the FDCA and FIFRA. It revised the Delaney
Clause by establishing identical pesticide residue limits—
those that pose a negligible risk—for both raw produce
and processed foods. The law requires that the increased
susceptibility of infants and children to pesticides be considered when establishing pesticide residue limits for
some 9,700 pesticide uses on specific crops. The pesticide
limits are established for all health risks, not just cancer.
For example, the EPA must develop a program to test
pesticides for endocrine-disrupting properties. The EPA
has until 2006 to reevaluate pesticide limits, which must
include the cumulative effect of all exposures, from
household bug sprays to traces in drinking water and
food. Another key provision of the Food Quality Protection Act is that it reduces the time it takes to ban a pesticide considered dangerous, from 10 years to 14 months.
THE MANUFACTURE AND USE
OF BANNED PESTICIDES
Some U.S. companies manufacture pesticides that have
been banned or heavily restricted in the United States
and export them to developing countries, particularly in
Asia, Africa, and Latin America. International trade of
banned or restricted pesticides is notoriously difficult to
monitor. The Foundation for Advancements in Science
and Education began documenting the extent of the
trade in hazardous pesticides in 1991 by examining customs records from U.S. ports. The latest available data
indicate that during 1995 and 1996, U.S. companies
exported 9,524 metric tons (10,504 tons) of pesticides
forbidden from use in the United States. This amount
includes pesticides that are considered too dangerous to
use in the United States as well as pesticides that have
never been evaluated by the EPA. Other highly developed nations also export banned pesticides.
The U.N. Food and Agriculture Organization (FAO)
is attempting to help developing nations become more
aware of dangerous pesticides. It established a “red alert”
list of more than 50 pesticides that have been banned in
five or more countries. The FAO further requires that the
manufacturers of these pesticides inform importing countries about why such pesticides have been banned. The
United States supports these international guidelines and
exports banned pesticides only with the informed consent
of the importing country. However, many foreign farmers
never receive these guidelines or any training on the safe
storage and application of pesticides.
Another concern relating to banned pesticides is that
unwanted stockpiles of leftover, deteriorated pesticides
are accumulating, particularly in developing countries.
The United Nations estimates there are more than
100,000 tons of these obsolete pesticides in developing
countries. They are often stored in drums at waste sites
in the countryside because developing countries have few
or no hazardous waste disposal facilities. Over time,
chemicals leach from such waste sites into the soil, and
from there they get into waterways and groundwater (see
Chapter 23).
The Importation of Food Tainted with
Banned Pesticides
The fact that many dangerous pesticides are no longer
being used in the United States is no guarantee that
traces of those pesticides are not in our food. Although
many pesticides have been restricted or banned in the
United States, they are widely used in other parts of the
world. Much of our food—some 1.2 million shipments
annually—is imported from other countries, particularly
those in Latin America. Some produce contains traces of
banned pesticides such as DDT, dieldrin, chlordane, and
heptachlor.
It is not known how much of the food coming into the
United States is tainted with pesticides. The FDA monitors toxic residues on incoming fruits and vegetables, but it
is able to inspect only about 1% of the food shipments that
enter the United States each year. In addition, the General
Accounting Office reports that some food importers illegally sell food after the FDA has found it to be tainted with
pesticides. When caught, these companies face fines that
are not severe enough to discourage such practice.
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C H A N G I N G AT T I T U D E S
539
The Global Ban of Persistent Organic
Pollutants
The Stockholm Convention on Persistent Organic
Pollutants, which was adopted in 2001, is an important
international treaty that seeks to protect human health
and the environment from the 12 most toxic chemicals
on Earth (Table 22.5). Classified as persistent organic
pollutants, or POPs, these chemicals, 9 of which are
pesticides, bioaccumulate in organisms and can travel
thousands of kilometers through air and water to contaminate sites far removed from their source (see discussion of long-distance transport of air pollution in
Chapter 19). Some POPs also disrupt the endocrine system, cause cancer, or adversely affect the developmental
processes of organisms.
The Stockholm Convention requires countries to
develop plans to eliminate the production and use of
intentionally produced POPs. A notable exception to this
requirement is that DDT can still be produced and used
to control mosquitoes that carry the malaria pathogen in
countries where no affordable alternatives exist. (DDT is
inexpensive, and many of these countries cannot afford
safer alternatives.) The treaty also specifies that efforts
should be undertaken to eliminate, where feasible, the
unintentional production of industrial by-products such
as dioxins and furans.
CHANGING ATTITUDES
Heavy pesticide use can be attributed partly to the consumer, who has come to expect perfect, unblemished
produce. There is no question that pesticides help farm-
Table 22.5
Persistent Organic Pollutants:
The “Dirty Dozen”
Persistent Organic Pollutant
Aldrin
Chlordane
DDT (dichlorodiphenyltrichloroethane)
Dieldrin
Endrin
Heptachlor
Hexachlorobenzene
Mirex™
Toxaphene™
PCBs (polychlorinated biphenyls)
Dioxins
Furans (dibenzofurans)
Use
Insecticide
Insecticide
Insecticide
Insecticide
Rodenticide and
insecticide
Fungicide
Insecticide; fire retardant
Insecticide
Insecticide
Industrial chemical
By-product of certain
manufacturing
processes
By-product of certain
manufacturing
processes
Figure 22.16 Organically grown produce in a supermarket.
If consumers are willing to pay extra for food that is grown in
the absence of pesticides, then farmers will grow more pesticide-free crops.
ers grow crops that are more visually appealing. However, consumers must ask themselves, would they rather
buy apples that are smaller and have an occasional blemish or worm but are pesticide-free, or apples that are free
from all imperfections but contain traces of pesticides?
Until consumers change their attitudes and demand produce that is grown without pesticides, farmers will continue to use pesticides (Figure 22.16).
Many farmers are exploring alternatives to pesticides
on their own because they come into contact with pesticides on a regular basis. These farmers are aware of the
dangers and problems associated with pesticide use and
are concerned for their own safety and that of their families. They do not want to inhale pesticide or let it settle
on their skin when they are applying it. They do not want
to drink water or eat food with traces of pesticide any
more than the typical consumer does.
Pesticide Risk Assessment
Although the effects of long-term exposure to low levels
of carcinogenic pesticides should be of concern to all
informed consumers, panic, which is often fueled by sensational news reports, is not justified. It is important to
keep a balanced perspective when considering pesticides.
The threat of cancer from consuming pesticide residues
on our food is quite small compared to the threat of cancer from smoking cigarettes or from overexposure to
ultraviolet radiation from the sun (see Chapter 2 discussion of risk assessment).
However, it is difficult for consumers to make
informed decisions about the risks of pesticide residues on
food, because we have no way of knowing what kinds of
pesticides have been used on the foods we eat. A requirement that all foods list such chemicals would go a long way
toward helping us assess risks, as well as discouraging
heavy pesticide use.
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THE PESTICIDE DILEMMA
SUMMARY WITH SELECTED KEY TERMS
I. Pesticides are toxic chemicals that are used to kill pests such
as insects (insecticides), weeds (herbicides), fungi (fungicides), and rodents (rodenticides).
A. The ideal pesticide would be a narrow-spectrum pesticide
that kills only the target organism.
B. Most pesticides are broad-spectrum pesticides, which kill
a variety of organisms besides the target organism.
II. Insecticides are classified into groups based on their chemical structure.
A. Chlorinated hydrocarbons are organic compounds containing chlorine. Dichlorodiphenyltrichloroethane, or
(DDT) is a broad-spectrum chlorinated hydrocarbon that is
highly persistent in the environment; its use has been
banned in the United States and many other countries.
B. Organophosphates are organic compounds that contain
phosphorus. As a group, they are highly toxic to many
organisms, although they do not persist in the environment
as long as chlorinated hydrocarbons do.
C. Carbamates are derived from carbamic acid. As a group,
they show broad, nontarget toxicity, although they have a
low persistence in the environment.
III. Herbicides may be classified as either selective or nonselective.
A. Selective herbicides kill only certain types of plants. Selective herbicides include broad-leaf herbicides, which kill
plants with broad leaves but do not kill grasses, and grass
herbicides, which kill grasses but are safe for most other
plants.
B. Nonselective herbicides kill all vegetation.
IV. Pesticides provide important benefits to humans.
A. Pesticides help prevent diseases such as malaria that are
transmitted by insects.
B. Pesticides reduce crop losses from pests, thereby increasing
agricultural productivity.
1. Pesticides reduce competition with weeds, consumption
by insects, and diseases caused by plant pathogens such
as certain fungi and bacteria.
2. Agricultural fields are monocultures that provide abundant food for pest organisms. Many natural predators are
absent from monocultures.
V. Pesticide use has caused environmental problems.
A. Pests have evolved genetic resistance, inherited characteristics that decrease the effect of the pesticide on the pest.
Resistance management is a series of techniques used to
delay the evolution of genetic resistance in the population
so that the period in which an insecticide is useful can be
maximized.
B. Pesticides affect species other than those for which they are
intended, causing imbalances in ecosystems. In some
instances the use of a pesticide has resulted in a pest problem that did not exist before.
C. Other problems associated with some types of pesticides are
persistence, bioaccumulation, and biological magnification.
1. A pesticide that demonstrates persistence takes a long
time to be broken down into less toxic forms.
2. Bioaccumulation is the buildup of a persistent pesticide
in an organism’s body.
3. Biological magnification is an increase in pesticide concentration as the pesticide passes through successive levels of the food web.
D. Many pesticides move through the soil, water, and air,
sometimes for long distances.
VI. Some serious health problems are associated with pesticide
use.
A. Humans may be poisoned by exposure to a large amount of
pesticide.
B. Lower levels of many pesticides may pose a long-term
threat of cancer.
C. Certain persistent pesticides may interfere with the actions
of natural hormones.
D. Pesticides are a greater threat to children than to adults. For
example, pesticides may affect the development of intelligence and motor skills in infants and young children.
VII. Alternative methods exist to reduce the need for pesticides in agriculture.
A. Cultivation techniques such as strip cutting, interplanting,
and crop rotation are effective in controlling pests.
B. Biological controls involve the use of naturally occurring
disease organisms, parasites, or predators to control pests.
C. Reproductive controls include reducing the pest population
by sterilizing some of its members (the sterile male technique).
D. Pheromones, natural substances produced by animals to
stimulate a response in other members of the same species,
can be used to lure insects to traps or to confuse insects so
they cannot locate mates.
E. Insect hormones are natural substances produced by
insects to regulate their own growth and metamorphosis; a
hormone that is present at the wrong time in an insect’s life
cycle disrupts its normal development.
F. Genetic control of pests involves producing varieties of
crops and livestock animals that are genetically resistant to
pests. Some genetically modified (GM) crops contain the
Bacillus thuringiensis (Bt) gene that codes for a toxin used
against insect pests.
G. Quarantine involves restricting the importation of exotic
plant and animal material that might harbor pests. If a foreign pest is accidentally introduced, quarantine of the area
where it is detected helps prevent its spread.
H. Integrated pest management (IPM) is a combination of
agricultural methods that emphasizes biological controls
and cultivation methods along with the judicious use of pesticides.
1. IPM is the management rather than the eradication of
pests.
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THINKING ABOUT THE ENVIRONMENT
2. IPM requires that farmers be educated so they can adopt
the strategies that will work best in a given pest situation.
I. Irradiation of food can be used to control pests after food has
been harvested.
VIII. Several U.S. laws regulate pesticides in the interest of
protecting human health and the well-being of the environment.
A. The Food, Drug, and Cosmetics Act (FDCA), as originally passed in 1938, recognized the need to regulate pesticides in food but did not provide a means of regulation.
1. The Miller Amendment, passed in 1954, required the
establishment of acceptable and unacceptable levels of
pesticides in food.
2. The Delaney Clause, passed in 1958, stated that no substance capable of causing cancer in laboratory animals or
541
in humans would be permitted in processed food.
a. The Delaney Clause overlooked pesticides on raw
foods.
b. It applied only to pesticides that were registered after
strict tests were put into effect in 1978.
B. The Federal Insecticide, Fungicide, and Rodenticide
Act (FIFRA), originally passed in 1947, has been amended
over the years to require testing and registration of the
active ingredients of pesticides. The 1988 version required
the reregistration of older pesticides, which subjected them
to the same toxicity tests that new pesticides face.
C. The Food Quality Protection Act of 1996 amended both
the FDCA and FIFRA. It revised the Delaney Clause by
establishing identical pesticide residue limits—those that
pose a negligible risk—for both raw and processed foods.
THINKING ABOUT THE ENVIRONMENT
1. What is a pest? What is a pesticide?
2. Distinguish among insecticides, herbicides, fungicides, and
rodenticides.
3. Explain the difference between narrow-spectrum and
broad-spectrum pesticides.
4. Describe the general characteristics of each of the following groups of insecticides: chlorinated hydrocarbons,
organophosphates, and carbamates.
5. Explain two benefits of pesticide use.
6. What is the dilemma referred to in the title of this chapter?
7. Overall, do you think the benefits of pesticide use outweigh its disadvantages? Give at least two reasons for your
answer.
8. Sometimes pesticide use can increase the damage done by
pests. Explain.
9. How is the buildup of insect resistance to pesticides similar to the increase in bacterial resistance to antibiotics?
10. Distinguish among persistence, bioaccumulation, and biological magnification.
11. How do pesticides move around in the environment?
12. What is the pesticide treadmill?
13. Describe some of the effects of pesticides on human
health.
14. Describe the pesticide disaster that occurred at Bhopal,
India.
15. Biological control is often much more successful on a
small island than on a continent. Offer at least one reason
why this might be the case.
16. It is more effective to use the sterile male technique when
an insect population is small than when it is large.
Explain.
17. Define integrated pest management (IPM). List five tools of
IPM, and give an example of each.
18. Is a major goal of IPM to eradicate the pest species?
Explain your answer.
19. Which of the following uses of pesticides do you think are
most important? Which are least important? Explain your
views.
a. Keeping roadsides free of weeds
b. Controlling malaria
c. Controlling crop damage
d. Producing blemish-free fruits and vegetables
20. List three laws by which pesticides are regulated in the
United States. Summarize the goals of each law.
21. What is the Stockholm Convention on Persistent
Organic Pollutants?
22. Why is pesticide misuse increasingly viewed as a global
environmental problem?
*23. A water sample was measured and found to have 0.00005
ppm of DDT. The plankton living in the water were then
measured and found to have 800 times that amount of
DDT. What was the concentration of DDT, in ppm, in
the plankton?
*24. Worldwide, the pesticide industry sold $27.8 billion in
pesticides in 1994. Preliminary data indicate that sales in
1998 were $34 billion. Calculate the average percent
annual increase in pesticide sales from 1994 to 1998.
*Solutions to questions preceded by an asterisk appear in Appendix VII.
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THE PESTICIDE DILEMMA
TAKE A STAND
Visit our Web site at http://www.wiley.com/college/raven
(select Chapter 22 from the Table of Contents) for links to
more information about issues surrounding the genetic engineering of the Bt gene into crops. Consider the opposing views
of those who support and those who oppose the use of genetic
engineering for this purpose and debate the issue with your
classmates. You will find tools to help you organize your
research, analyze the data, think critically about the issues, and
construct a well-considered argument. Take a Stand activities
can be done individually or as part of a team, as oral presentations, written exercises, or Web-based (e-mail) assignments.
Additional on-line materials relating to this chapter, including
Student Quizzes, Activity Links, Useful Web Sites, Flash
Cards, and more, can also be found on our Web site.
SUGGESTED READING
Carson, R.L. Silent Spring. Boston: Houghton Mifflin, 1962.
The classic book that first alerted the public to the environmental dangers of pesticide use.
Colburn, T., D. Dumanoski, and J.P. Myers. Our Stolen
Future. New York: Dutton, 1996. Drawing on work from
many fields, this controversial book records the rapidly
unfolding story of commonly used chemicals that disrupt
the activities of hormones.
Dickie, P. “Use as Directed: How Much Do We Know About
Home Pest Control?” Living Planet (summer 2001). A quick
consumer course on home pesticide use.
Dreyfuss, R. “Apocalypse Still.” Mother Jones (February 1,
2000). A well-researched article on the effects on the Vietnamese people and land of Agent Orange use during the
Vietnam War.
Helmuth, L. “Pesticide Causes Parkinson’s in Rats.” Science,
Vol. 290 (November 10, 2000). This news article reports on
a study that tentatively implicates pesticide exposure to the
development of Parkinson’s disease.
Holcomb, B. “How Safe Is Your Dinner?” Good Housekeeping
(March 2000). This short article answers many of the questions consumers have about irradiated food.
Knight, J. “Alien Versus Predator.” Nature, Vol. 412 (July 12,
2001). The use of biological controls is gaining in popularity.
McGinn, A.P. “Malaria, Mosquitoes, and DDT.” WorldWatch,
Vol. 15, No. 3 (May–June 2002). The war against malaria
and the mosquitoes that transmit it is relentless.
Obrycki, J.J., J.E. Losey, O.R. Taylor, and L.C.H. Jesse.
“Transgenic Insecticidal Corn: Beyond Insecticidal Toxicity
and Ecological Complexity.” BioScience, Vol. 51, No. 5 (May
2001). Don’t let the title scare you away from this informative article on Bt crops.
Raloff, J. “The Case for DDT.” Science News, Vol. 158 (July 1,
2000). The use of the environmental pollutant DDT saves
lives.
Strong, D.R., and R.W. Pemberton. “Biological Control of
Invading Species—Risk and Reform.” Science, Vol. 288 (June
16, 2000). Biological controls can be used successfully
against invasive species, but they are not a panacea.
Yoon, C.K. “When Biological Control Gets Out of Control.”
New York Times (March 6, 2001). A fascinating account of a
biological control agent that is now eating many nontarget
species.