Our Bumblebees Are Disappearing: The Neonicotinoid Connection

Transcription

Our Bumblebees Are Disappearing: The Neonicotinoid Connection
Jacob Parsons
SOIL 325
3/07/17
Our Bumblebees Are Disappearing: The Neonicotinoid Connection
Pollination, that delicate act of facilitated reproduction, is the domain of many forces in
nature. Wind, mammal, bird, and invertebrate all contribute their share to the continuation of the
genetic lineage of the angiosperms. Of all these actors, none are as well known as the bee. It is,
after all, bees who humans have established partnership with through domestication to assist in
the growth of our crop plants, and to produce delicacies like honey. The domesticated honeybee
holds a place in the public’s hearts and minds, and while Apis millifera is no doubt essential to
agriculture as we know it, there are a plethora of wild bees that provide the service of pollination
to our crops as well. Many of these native bees are able to pollinate crops that honeybees cannot,
and are therefore essential (Moisset and Buchman, 2011). It is estimated that wild bees
contribute anywhere from $3.07 to $9.00 billion in pollination services every year in the U.S.
alone (Losey and Vaughan, 2006; Jepson and Hatfield, 2017). These wild bees, and the essential
pollination services that they provide, are threatened.
For the first time in history, a bee species has been slated for protection under the United
State’s Endangered Species Act. Since the late 1990s the Rusty Patched Bumblebee, Bombus
affinis, has disappeared from over 87% of its historic habitat (Jepson and Hatfield, 2017). This
should not be seen as an isolated incident. Habitat fragmentation, global climate change, and
industrial agricultural practices like pesticide application are working in concert to create
inhabitable conditions for our wild pollinators (Jepson and Hatfield, 2017; U.S. Fish and Wildlife
Service, 2017). There are 52 species of wild bumblebee in the United States, including B. affinis,
and they are seen as particularly important to the stability of agricultural production (Moisset and
Buchman, 2011). A particular group of pesticides, the neonicotinoids, will be explored in this
paper in relation to the effect that they have on wild bumblebee populations. Neonicotinoids
were chosen due to their continued use in the United States, despite a growing awareness that
this class of pesticide may be particularly harmful to pollinators. Bumblebees are often exposed
to sub-lethal levels of neonicotinoid pesticides throughout their daily life, and these sub-lethal
exposures have been shown to interfere with the ability to forage and reproduce, and thus
neonicotinoid application may be linked to the overall decline of wild bee populations.
Neonicotinoid pesticides emerged in the mid to late 1990s, and were quickly adopted
(Vermont Agency of Agriculture, 2015). Neonicotinoids were developed to replace a class of
pesticides known as organophosphates, and are generally seen as safer due to their relatively low
levels of avian and mammalian toxicity. The nicotinic acetylcholine receptors that
neonicotinoids bind to are affected at a far higher rate in insects than in vertebrates. The action of
neonicotinoids at these receptors causes the excitation of nerves and paralysis, leading to death at
high enough doses (Goulson, 2013; Vermont Agency of Agriculture, 2015). As their name
suggests, neonicotinoids were inspired by and created to be similar to nicotine, a natural
pesticide. Today their use is extensive, and between 2010 and 2012 it was estimated that they
were used on 133 million acres of U.S. farmland (Mitchell, 2014). Neonicotinoids are primarily
used on commodity crops, and they are the most widely used class of pesticide in corn, soybean,
wheat, cotton, and sorghum production in the U.S. (Mitchell, 2014) The most commonly used
neonicotinoid is imidacloprid, whose popularity has skyrocketed it to the position of the second
most commonly used agrochemical in the world (Goulson, 2013). The extent of its use on U.S.
farmland alone is evidenced in figure one.
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Fig. 1. Most conservative USGS estimate of 2014 agricultural use of Imidacloprid in the U.S. (U.S. Geological Survey, 2017)
In addition to reduced vertebrate toxicity in relation to other pesticides, the flexibility of
application method for neonicotinoids has been responsible for their astronomical rise (Goulson,
2013). The most popular way to apply neonicotinoids is as a seed coat (Goulson, 2013; Mitchell,
2014). Neonicotinoid seed coats are convenient for farmers, because they allow for extended
crop protection without the need for repeated applications over the course of the growing season.
This is due to the systemic nature of neonicotinoids. From the moment that the cotyledons and
radicle burst through their seed coat, or even before depending on the crop, neonicotinoids enter
a plant’s system. There they remain for a period of time, distributed through the plant’s vascular
system to leaf, root, shoot, and stem, ready to target any insect that may be unfortunate enough to
feed upon the crop (Stevens and Jenkins, 2014; Vermont Agency of Agriculture, 2015; Hopwood
et al., 2016).
Bumblebees can come in contact with neonicotinoids in a variety of ways. The pollen and
nectar that they collect from crop plants can contain neonicotinoid residues, and they can also be
exposed via the guttation fluid of crops like corn (Stevens and Jenkins, 2014; Hopwood et al.,
2016; Hooven et al., 2016). As shown in figure two, when seeds are being planted they can
knock against one another and equipment causing contaminated dust to be released (Hopwood et
al., 2016; Stoddard et al., 2016). The agents used to aid in the flow of seeds from planter to field
are also often contaminated, and are released into the air at the same time (Hopwood et al.,
2016). From there the pesticide alights upon unintended vegetation, including non-crop
flowering plants that bumblebees depend upon as a food source (Stevens and Jenkins, 2014;
Hopwood et al., 2016). Even if these flowers are not yet blooming, or their seeds even
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germinated, the residues can still be absorbed in the future. Neonicotinoids are persistent in the
soil, and quite water soluble. If they do not leach into rivers, streams, or groundwater, where
some have been shown to retain their ability to be absorbed by plants, they are available for
uptake for a period of time by whatever may grow in the newly contaminated soil (Hopwood et
al., 2016). Imidacloprid has a documented half-life of 40-997 days, depending on the soil type
and environmental conditions, and has been shown to remain persistent in water supplies
(Hopwood et al., 2016). The lifecycle of bumblebees further increases their risk of exposure. As
ground nesters they may come into direct contact with neonicotinoid pesticides if they happen to
nest in or near agricultural fields, which they often do (Hopwood et al., 2016).
Fig. 2. Neonicotinoid-rich dust being generated by machinery in an Ontario Corn Field (Montgomery, 2013)
All of the exposure pathways described above can result in sub-lethal levels of exposure
in bumblebees. Some research has even suggested bumblebees may show pollination preference
for neonicotinoid-laced crops, although the compounds themselves are tasteless (Kessler et al.,
2015). Scientists are unsure of the reason behind this, but it may be due to the effect of
neonicotinoids on the perceived reward that some bees get from neonicotinoid-laced food source
(Kessler et al., 2015). The sensitivity to exposure varies between species, but research has
suggested that bumblebees may be more affected than domesticated honeybees (Cresswell et al.,
2012). Repeated contact results in bioaccumulation, and three days of field-realistic levels of
imidacloprid exposure have been shown to be enough to result in levels detectable in
bumblebee’s brains (Moffat et al., 2015). It has been shown that the bees become more sensitive
to the neurotransmitter acetylcholine following this accumulation, causing rapid degradation of
mitochondria in the neural cells of the brain (Moffat et al., 2015). This may be one of the reasons
for the behavioral abnormalities that have been documented following neonicotinoid exposure.
The efficiency of pollen and nectar collection by a bumblebee is important to the
colony’s overall growth and the survival of the queen. This efficiency can be measured both in a
bee’s ability to make many trips in one day, and how much pollen is collected from each bout of
foraging activity. Declines in foraging efficiency and reproduction in bumblebees after fieldrealistic levels of exposure to imidacloprid have been corroborated by many studies (Mommaerts
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et al., 2010; Whitehorn et al., 2012; Laycock et al., 2012; Gill et al., 2012; Gill and Raine, 2014).
Imidacloprid exposure has been shown to decrease the amount of pollen gathered by worker
bees, increase their total foraging time, and significantly reduce the total number of workers
returning to the hive (Gill et al., 2012). Because an adequate amount of nutrition is essential for
the maintenance and further expansion of any population, the declining efficiency of foraging
may be part of the reason that rates of bumblebee reproduction decline upon exposure to
imidacloprid (Whitehorn et al., 2012; Laycock et al., 2012; Hopwood et al., 2016). The rates of
exposure that lead to this decline in fecundity can be quite low. Exposure levels of 1 ppb have
been shown to reduce the ability of worker bees to reproduce by one third (Laycock et al., 2012).
Production of new queens has also been shown to be affected by imidacloprid, reducing by as
much as 85% when compared to colonies that were not exposed (Whitehorn et al., 2012).
Bumblebees follow an annual lifecycle, and the queens are the only individuals that overwinter.
It is up to the queen to begin the colony again once spring comes. This already vulnerable
ecological role is further compromised when overall levels of queens are low.
Fig. 3. Overall colony weight gain over an eight week period. Each line represents the trend of the mean weight gain of 25
Bombus terrestris colonies. The short dash line represents the control, the solid line a field-realistic “low” dose of imidacloprid
of 6 g kg-1 in pollen and .7g kg-1in sugar water, and the long dash line a dose of imidacloprid double that of the “low dose”.
Colonies only received this treatment for the first two weeks of the study, after which they were allowed to forage
independently (Whitehorn et al., 2012).
The problem of neonicotinoid-related bumblebee decline can seem daunting. Its massive
scale and widespread impact make it hard to know where to begin. Inspiration can be found by
looking to the European Commission, who in 2013 implemented a continent-wide ban on the use
of neonicotinoids (Carrington, 2013). Historically, Europe historically has had more progressive
agricultural policy than that of the United States, where a ban may garner less institutional
support. There is precedent for a ban in the U.S. though, in 2016, Maryland enacted a partial ban
that eliminates consumer use for this class of chemicals, and Minnesota enacted a variety of
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measures that increase the regulations surrounding neonicotinoid use for both consumers and
farmers (Geiling, 2016; Charles, 2016). In the meantime, it is not enough to continue business as
usual and hope for a ban. There are a variety of measures that can be taken by farmers, citizens,
and policy makers that can help reduce the availability of these chemicals and increase the health
of bumblebee populations nationwide.
Drawing inspiration from Minnesota, a nationwide “verification of need” program could
drastically reduce the rates at which neonicotinoids are used. This type of program would require
farmers to prove through a variety of means that neonicotinoid treated seeds were biologically
necessary to confront a proven pest pressure (Minnesota Department of Agriculture, 2016). A
similar program, just implemented in Ontario, Canada, is projected to reduce the planting of
treated seeds by up to 80% (Charles, 2016). Another regulatory step that has the potential to
make a drastic impact on the rate of neonicotinoids used would be the creation of a Treated Seed
Program. Currently, treated seeds are not classified as pesticides under U.S. law, and are
therefore not subject to regulation by the EPA in the same way that other classes of pesticides are
(Minnesota Department of Agriculture, 2016). There are two approaches that could be taken to
solve this problem: a nationwide push for the classification of treated seeds as pesticides, or a
state by state implementation of Treated Seed Programs. Because of the sprawling nature of U.S.
commodity agriculture, a nationwide re-classification seems necessary, but a state-by-state
regulatory shift could be a more realistic way to approach the problem. Minnesota’s proposed
Treated Seed Program would give the state the right to regulate treated seeds, thereby filling in
the gap left by the EPA. If a sufficient number of states adopted this model it could create a
demand for untreated commodity seed that could shift the trends of use even in states that did not
regulate treated seeds. Citizens can help by letting their legislators know that pollinator
conservation is a priority, ceasing the use of neonicotinoid pesticides in their home landscapes,
and helping augment the fragmented urban landscape by increasing forage stock via the planting
of flowering species that provide pollen and nectar.
The trend towards using neonicotinoids as a prophylaxis rather than a direct response to
pest pressure is a stark contrast to the more sustainable application of pesticides as prescribed by
the principles of integrated pest management (IPM). Prophylactic application is seen as a way to
ensure that crops are never harmed, and it is marketed as an almost necessary method to keep
yields high. However, research in both Brazil and North America has shown that prophylactic
applications of imidacloprid have had no significant affect on soybean yields (Goulson, 2013).
Communicating this information to farmers through educational programs that encourage an
IPM approach to managing pest pressure could help reduce the rates of neonicotinoid treated
seed planting. It is also important to communicate the importance of the creation of pollinator
habitat to the survival of bumblebees (Hooven et al., 2016). It is necessary to maintain the
integrity of existing forage and to increase the amount that is available to bumblebees. Increasing
access to information that explains the impact of neonicotinoid application on bumblebee food
supplies and habitat outside of crop fields may encourage farmers to better steward this habitat.
A tax incentive that encourages farmers to create uncontaminated pollinator habitat could also
help improve the quality and quantity of spaces for bumblebees to live and thrive.
It is going to take a concerted effort by citizens, farmers, and policy makers to tackle this
predicament. A ban on neonicotinoids and a collective move towards a reduced pesticide
ecological agriculture is a goal to strive for. This would be a compromise between farmers and
the natural species that support their ability to produce the food that we depend upon, now and
into the future. However, ceasing neonicotinoid use is not the end. We must also work toward an
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increase in pollinator habitat and cultural practices that reduce greenhouse gas output (Jepson
and Hatfield, 2017). The three-pronged threat of pesticides, habitat degradation, and climate
change has to be tackled from all sides if we are to create sustainable food systems that support
pollinator health. This predicament is one of the greatest of our time, and unless we find a
solution agriculture as we know it will be forever changed.
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