Assessing the toxicity of pesticide inert ingredients and spray adjuvant principal functioning agents to honey bees ( <i>Apis mellifera</i> )

Commercial beekeepers transport colonies across the United States to provide pollination services for >100 crops, especially for almond production in California. In these agricultural settings, honey bees are exposed to adjuvant compounds that are either included as "inert ingredients" in pesticide formulations or added as a separate adjuvant product to pesticides to improve application characteristics. However, evidence suggests that some of these inert ingredients pose risks to bees. This study used a Potter spray tower to determine the 48-hr acute toxicities (lethal concentration 50) of 37 pesticide inert ingredients or adjuvant principal functioning agents. Additionally, the toxicity trends of 15 alcohol ethoxylates were determined for structural and physical metrics, including moles of ethoxylation, number of carbons in the alcohol group, and hydrophilic-lipophilic balance. Last, median lethal dose values for an organosilicone (Silwet Eco) and a nonionic surfactant (Makon 10) were determined when applied to different parts of the adult honey bee. The results show that 25 of the 37 tested inert ingredients demonstrated a significant 48-hr dose-response and that 15 of these 25 inert ingredients had a median lethal concentration value below the maximum concentration tested. For alcohol ethoxylates, moles of ethoxylation and number of carbons in the alcohol group were not significant predictors for toxicity, but hydrophilic-lipophilic balance did show a significant trend with toxicity. The body placement assay indicated that applications to the dorsal part of the head and ventral thorax were generally the most toxic but that the two adjuvants produced differing results. The findings from this study can guide the selections of adjuvants and pesticide inerts to reduce risks to honey bees.