The declines of honey bees and other pollinators have been at the forefront of recent scientific publications and popular press. Accumulating evidence suggests that no single stressor alone is responsible for declines. Rather, it is probably a combination of abiotic and biotic factors acting in synchrony, to have a negative impact on pollinator populations.
Pesticides are stressors that have received considerable attention, and among these no single class has received more recent attention than the neonicotinoids. These insecticides are acutely toxic to honey bees, environmentally persistent and mobile in the environment. They are also among the most widely used insecticide classes in grain and oilseed crops: in the United States, 36.6 million hectares of maize and 33.8 million hectares of soybeans were planted in 2014, with 79–100% and 34–44% of these seeds, respectively, treated before planting with a formulation of neonicotinoid insecticides (typically thiamethoxam or its metabolite clothianidin).
Although it is known that neonicotinoids can and do move from crop fields during planting, as dust, and afterwards in ground and surface water, the extent of this contamination in the environment throughout the foraging season of honey bees has only recently begun to be quantified therefore, this presents a relatively unexplored route for pollinator exposure to this pesticide class.
Moreover, the presence of other, potentially synergistic, pesticides encountered in honey bee food resources has seldom been examined and most published work in this area tends to focus exclusively on the occurrence of neonicotinoid insecticides in pollen or nectar resources; however, exposures of mobile insects such as pollinators to a single pesticide rarely occur in field situations. Given the vast acreages devoted to these crops and concerns about worldwide pollinator decline, we initiated this work to describe both how (that is, plant species) and when (that is, time in season), pollen-foraging honey bees are exposed to a range of pesticides in agricultural landscapes, with an eye towards clarifying potential high-risk compounds and identifying common combinations of pesticides encountered in field environments.
Pollen collected by honey bees was collected, identified and screened for agricultural pesticides over a period of 16 weeks in 2011. We initiated this study after all planting of treated seeds in our study was completed, to minimize dust from pneumatic planters as a direct source of pesticide residues.
Two Langstroth hives were placed at each of three sites: (1) an open meadow with wildflowers, woody shrubs and trees present (non-agricultural site), (2) the border of a maize field planted with seeds treated with the neonicotinoid clothianidin and three fungicides, and (3) the border of a maize field planted with seeds that received no pesticide treatment.
Honey bee colonies remained at each site for the length of the growing season and foraging bees had free access to pollen from all crop and non-crop species in the vicinity. Analysis of forager-collected pollen revealed that in all cases honey bees foraged primarily on non-cultivated plants and residues of multiple pesticides were found throughout the season. Although a variety of agricultural pesticides were found at all sites, the contaminants likely to provide the greatest hazard to honey bees in our study were non-agricultural pyrethroid insecticides targeting nuisance pests such as mosquitoes.