Modeling Cumulative Toxicity in Honey Bee Colonies

Dylan Ricke
Graduate (MS)
Reed Johnson

Honey bees are regularly exposed to trace levels of various environmental pollutants, which persist in their colonies over prolonged periods of time. In contrast, most toxicological tests with honey bees, which are required during pesticide registration, last for just 2-4 days (acute toxicity assays). If a chemical's acute toxicity exceeds allowable levels, longer assays (up to 10 days) are required. Even so, there's no standard model in place for extrapolating the results of honey bee laboratory assays into longer and more realistic timescales of exposure (weeks or months). As a result, regulators may fail to account for chemicals cumulative toxicity when setting acceptable levels of honey bee exposure (limits of exposure) to pesticides and other chemicals. To address this issue, there is high demand for modeling approaches that can predict the cumulative effects of chemicals to honey bee colonies over time. In the last decade, the General Unified Thresholds Model of Survival (GUTS) has emerged as a state-of-the-art model of toxicity, capable of extrapolating data from laboratory trials over any timescale of interest. For the present study, I incorporated GUTS into an existing honey bee population model. Using the model, I ask whether regulatory limits of exposure to diverse chemicals appear to be protective of honey bee colonies over prolonged periods of time. I present preliminary results from simulations of colony exposure to the insecticide clothianidin, which is frequently applied as a seed-treatment of corn and soy. Although simulations using the EPA's limits of exposure showed only minor changes to the population sizes of simulated colonies, effects were more severe at the upper end of clothianidin concentrations reported in previous field studies from nectar and pollen. Altogether, this work demonstrates the utility of a novel honey bee population model for chemical risk assessment as well as basic research in the area of honey bee ecotoxicology.