Of Multiple Stressors, Pesticides Are the Most Harmful to Bees by Acting Synergistically to Increase Mortality

August 11, 2021 |Multiple stressors, including pesticides, parasites, and poor nutrition, act synergistically to increase the risk of bee mortality, according to a meta-analysis published in Nature. The findings are yet another indictment of the U.S. pesticide regulatory system’s ability to protect pollinators, as the authors note that their results, “. . . demonstrate that the regulatory process in its current form does not protect bees from the unwanted consequences of complex agrochemical exposure.” Scientists evaluate how combinations of multiple pesticides, parasites, and lack of floral resulted in bee death or subchronic effects that impacted overall fitness (reproductive ability, colony health, etc.), behavior, parasite load, or immune response. The effects of multiple stressors can be characterized as antagonistic when stressors cancel themselves out, additive when the impacts seen are what would be predicted when summing the individual effects, and synergistic when the effects are multiple times more harmful than what would be predicted additively. To conduct the analysis, researchers began with nearly 15,000 papers on bee health, and narrowed down their review to 90 studies that observed over 350 interactions between multiple stressors. “Our results show that although many classes of anthropogenic stressors may have additive effects on bee mortality and fitness proxies, exposure to combined agrochemicals can have synergistic effects that are more detrimental than would be predicted by independent risk assessments,” the study reads. [Siviter, Harry et al. Agrochemicals interact synergistically to increase bee mortality. Nature 596:389–392, 2021.]