Assessment of Sublethal Effects of Imidacloprid on Honey Bee and Colony Health Final report submitted to the North American Pollinator Protection Campaign

Colony collapse disorder (CCD) has caused much concern among beekeepers nationwide. While the specific causes are still unknown, many believe that honey bees have reached a tipping point wherein the colony can no longer protect itself from multiple stresses. One possible stressor is exposure to pesticides used within the hive as well as those used on plants that bees visit for nectar and pollen. Of the potential exposure to pesticides outside the hive, the sublethal effects of the neonicotinoid insecticides on honey bees have been the focus of intensive discussion and research studies in recent years. Many beekeepers and scientists believe that this class of pesticides is a primary stress factor responsible for CCD, although there is no conclusive scientific evidence to directly link neonicotinoids with the disorder. In this study, we examined the potential sublethal effects of imidacloprid, which is a widely used neonicotinoid applied as a seed treatment, foliar spray, and soil drench on many fruit, vegetable, and field crops pollinated by bees. As a systemic insecticide, imidacloprid accumulates primarily in the vegetative parts of plants and much less in fruiting structures. However, crop residue studies have detected imidacloprid at levels of 2-5 ppb in pollen and >1.5 ppb in nectar of seed-treated corn, sunflowers and rape (review in Maus et al. 2003, Bonmatin et al. 2004). Recent in-hive surveys of pesticide residues have also found imidacloprid in hive wax and pollen loads (Chauzat et al. 2006; Stoner and Eitzer 2008; Frazier et al. 2008) but significantly less frequent in samples and at much lower concentrations than other pesticides, especially acaricides used for mite control (i.e. coumaphos, fluvalinate). In a 2007 study funded by the Foundation for the Preservation of Honey Bees, we also found imidacloprid residues at levels as high as 123 ppb in the staminate and pistillate flowers of seedless watermelon treated at planting with the low labeled rate. However, no detectable levels were found in samples of flower rinsate containing pollen and nectar, suggesting that the potential exposure to residues is probably very low if imidacloprid is applied only at planting. Nevertheless, it is conceivable that higher residues of imidacloprid may be present in pollen and nectar of crops if the insecticide is applied closer to bloom. Although published and unpublished results of sublethal effects on honey bees are conflicting, sublethal exposure to imidacloprid residues may lead to subtle physiological and behavioral abnormalities in honey bees, including decreased queen fecundity, as well as increased susceptibility to other stresses, such as diseases and environmental factors. These effects may be more disruptive to overall colony health than the direct effects on foragers. Most studies to date suggest that imidacloprid can cause disorientation and associative learning problems in honey bees at exposure levels above 20 ppb (review in Maus et al. 2003, Decourtye et al. 2004). These studies measured sublethal effects on bee larvae or worker bees by exposing