Picky eater syndrome: The pesticide imidacloprid alters honey bee (Apis mellifera) sucrose response threshold and, potentially, colony health

In recent years, beekeepers have experienced high honey bee (Apis mellifera) loss, some reporting a loss of 30-90 percent of their hives. Unlike some other honey bee diseases, beekeepers report a sudden disappearance of adult bees and few dead bees inside or around the colony. Where have the bees gone? This phenomenon, known as colony collapse disorder (CCD), has been attributed to parasites carrying diseases, pathogens, management techniques, and pesticides. Using the proboscis extension reflex (PER) assay and assessing an individual’s sucrose response threshold (SRT), this study examines the sublethal effects of the pesticide, imidacloprid, on honey bee foraging behavior. Our results indicate that foragers treated with imidacloprid (0.126 ng/bee and 2.16 ng/bee) have a significantly elevated sucrose response threshold as compared to those not treated with the pesticide. The oral LD₅₀ value averages 22 ng/bee. Thus, nectar foragers exposed even to minute quantities of imidacloprid alter their sucrose response threshold and will feed at only higher concentration sugar solutions. Bees with low SRTs typically become water and pollen foragers. Pollen foragers whose SRTs are pesticide-elevated should switch to become nectar foragers, potentially resulting in a colony-wide nutritional imbalance that could lead to reduced brood production and a dwindling population. Such a protein deficit could create a nutritional imbalance that reduces colony health.