Document 6514845

30 SEEDS OF GOLD
July 19, 2014 SATURDAY NATION
pests
EASY PREY» CROPS ATTACKED BY THE FLIES INCLUDE CASSAVA, POTATOES, MELONS AND CABBAGES
PREVENTION
USING STICK CARDS
TO MONITOR FLIES
A farmer takes stock of her
vegetables. They are among
the crops whiteflies attack.
FILE | NATION
How to stop deadly fly that
could wipe out your crop
BY EGERTON TEAM
Leaving land fallow
and treating crops
with plant sprays
such as pepper
and neem will help
control whiteflies
[email protected]
W
hiteflies are tiny insects about a millimetre long,
moth-like in appearance with
a bright white powdery body
that can transmit viruses to
crops and cause damage.
The insects have a sucking
structure called stylet, which
they use to penetrate plant tissues and acquire nutrient rich
fluids (water, phytosynthates
and amino acids). During
feeding, they may suck fluids
from virus-infected plants and
transfer them to healthy ones.
This causes massive crop
damage. The crops become
chlorotic, the leaves curl and
in tomatoes, they lead to irregular ripening. Direct damage occurs when white flies
feed on plant phloem (rich
in sugar) and reduce plant
vigour.
Later, the accumulated
sugar that is secreted by the
whitefly as honeydew leads
to the development of sooty
mold (Ascomycete fungi),
which interferes with photosynthesis and may lower yield
quality.
An adult whitefly mobility
and feeding behaviour encourages transmission of viruses,
as acquisition and inoculation
occur during feeding.
Some of the crops easily affected by the flies are cassava,
sweet potatoes, melons, eggplants, tomatoes, alfalfa, broccoli, cabbages, cauliflowers,
cotton, cucumbers, squash,
peanut, and pepper.
There are two main species of whiteflies affecting
crops both in the field and in
greenhouses. These are Silverleaf (Bemisia Tabaci) and the
greenhouse whitefly (Trialeurodes vaporariorum).
The lifecycle of greenhouse
whitefly is between 30 and 65
days, while that of Silverleaf is
between 26 and 29 days. The
adult stage is the most damaging with a lifespan of 10 to 40
days, but warm conditions
favour all stages.
The insects have a high
reproduction capacity and a
short generation cycle. Thus,
farmers using insecticides
for control have to increase
the frequency of application to curb new populations
emerging from subsequent
generations of the pest. This
practice is associated with
development of resistance to
insecticides.
Control of whiteflies using
insecticides is costly. On average, one uses Sh20,000 per
acre if insecticides are applied
three times during the period
of growth.
Global warming has encouraged a dramatic increase in
whitefly populations worldwide and consequently, the
emergence of new groups of
whitefly transmitted plant
viruses. Some groups of viruses traditionally known to
be transmitted by aphids are
increasingly becoming whitefly-borne viruses.
The current upsurge of
whiteflies could be an important pointer to an impending
pest outbreak. There is a need
to step up surveillance of the
whitefly populations and distribution.
Control measures
Whiteflies are controlled
primarily through application
of biological agents, chemical
pesticides and use of traps
(sticky cards and trap crops).
Sticky cards and trap crops
are important in monitoring
and control of whiteflies both
inside and outside the greenhouse. Sticky traps are used
to detect and monitor population levels and are placed
at strategic positions such
as near doors, vents and
known hotspots in the
greenhouse at one trap per
every 100m2.
Natural enemies
This involves introduction of natural enemies
such as parasitoid wasps
(Encarsia Formosa), predatory mites, lacewings and
pathogenic fungi (Verticillium (Lecanicillium) Lecanii and Beauveria Bassiana
strain GHA). These agents
are supplied commercially
by various biological control companies and can be
obtained from local stockists. Biological control is
effective if used over time
since it allows suppression
of the targeted pest without
killing the non-target beneficial insect in the system.
However, compatibility with
use of pesticides is limited and
hardly has any economic benefits since the agents can easily be destroyed by synthetic
insecticides. Other safer approaches in management include use of insecticidal soaps
and oil spays, plant extracts
such as neem and pepper and
leaving the field fallow.
Adequate rotation
This is done by applying
insecticides selectively and
in rotation by Mode of Action (MoA) group to avoid
development of resistance.
Insecticides should also be applied only when needed and in
recommended rates. The basic
rule for adequate rotation of
insecticides by MoA groups is
Sticky cards are an important
part of an Integrated Pest Management (IPM) programme for field
and greenhouse growers. They
alert a farmer on the presence of
certain insects. The cards will trap
the adults of flying insects such as
flower thrips, whiteflies, shoreflies,
leafminers and winged aphids.
Remember that mites, mealybugs, scales, and wingless aphids
do not fly and thus will not be
caught on the sticky cards. Also,
immature stages of thrips, and
whiteflies will not stick on the
cards.
Sticky cards help detect early
pest infestations more effectively
than intensive plant sampling. By
using sticky cards, you can keep
track of insect population trends,
and make more informed and
timely pest management decisions.
However, they are only part of
an IPM programme that includes
visual monitoring and the use indicator plants.
In the greenhouse, additional
cards are placed near doors, vents,
and over insect-sensitive plant
species or cultivars. Some cards
have a grid system that makes it
easier to count the insects on the
card. These cards are also available
commercially in large agro-vets
in major towns but not commonly
sold in small towns.
Sticky tapes or ribbons can also
be used to trap pests. To use, place
one to four yellow sticky cards per
300 square metre field area. Use
clothespins and stakes to vertically
attach sticky cards just above (4
to 6 inches) the crop canopy. As
plants grow, move the sticky cards
upward on the stake. Farmers
should replace traps at least once a
week. Farmers can make their own
sticky trap by spreading petroleum
jelly or used motor oil on yellow
painted plywood, 6cmx15cm, and
place the traps near the plants
but faraway enough to prevent
the leaves from sticking to the
board. Traps, when hung, should
be positioned 60 to 70 cm above
the plants. Yellow or blue coloured
sticky cards are commercially available. Blue cards may be more attractive to insects.
- Prof Paul Kimurto, Crops Horticulture and Soils Department, Egerton University
to avoid treating consecutive
generations of the target pest
with insecticides in the same
group.
The treatment window to
consider in changing the MoA
group is based on the minimum duration of a single generation of whiteflies. Within
the same treatment window,
multiple applications of the
same class of insecticides
should be done.
A treatment window in Silverleaf whitefly, for example, is
30 consecutive days (derived
from the life cycle). Chloropyrifos (as an endocrine
disruptor) and Imidacloprid
(acetylcholine inhibitor) are
examples of different MoA.
John Nganga, Alice Kamau
and Lilian Jeptanui, Crops
Horticulture and Soils Department, Egerton University