Various routes of formulated insecticide mixture whole-body acute contact toxicity to honey bees (Apis mellifera)

Honey bees, Apis mellifera (Linnaeus) are crucial pollinators, however large overwinter losses of managed hives and declines in feral colonies have occurred since 2006. Increased insecticide use and the introduction of new insecticide classes have been proposed as factors contributing to these declines. Previous studies have demonstrated contact toxicity of single active ingredient insecticides to honey bees. However, the field realistic contact toxicity of formulated (pre-mix) insecticide mixtures containing multiple active ingredients (each with a different mode of action) to honey bees has not been thoroughly assessed. Here, we simulate whole body contact to spray applications made at low label recommended field rates of thiamethoxam + lambda-cyhalothrin, imidacloprid + beta-cyfluthrin, chlorantraniliprole + lambda-cyhalothrin and methoxyfenozide + spinetoram to honey bees under three realistic scenarios in field crops: (1) continuous exposure, (2) spray-only exposure, and (3) residual exposure. A customized spray tower in a laboratory setting was used to apply insecticides at the lowest recommended dose in the Environmental Protection Agency regulated label of each insecticide. Acute toxicity was determined by screening resulting mortality up to 96-hours post treatment. Mortality varied significantly between treatments at all evaluation periods for all experiments except residual exposure. For continuous and spray-only exposure, thiamethoxam + lambda-cyhalothrin and imidacloprid + beta-cyfluthrin resulted in 80–90% mortality at 48-hours, while at 96-hours we observed close to complete mortality for all treatments. This difference in mortality onset between treatments highlights the importance of extended observation beyond the 48-hours recommended by the USDA-EPA. Our finding of overall high mortality demonstrates the importance of comparing both combined and individual active ingredients in determining insecticide risk to honey bees.