Mosquito larvicide and adulticide impacts on western honey bee larvae (Apis mellifera) reared in vitro

• Honey bee larvae were exposed in vitro to mosquito control products. • Risk quotients for isolated active ingredients did not exceed EPA levels of concern when using residue concentrations measured in field-collected pollen and nectar. Dibrom® and MMII® were associated with higher chronic risk indicators in immature honey bees. • The results provide data that may inform future risk assessments for mosquito control products. Effective mosquito control is essential for human health in many areas globally. However, some mosquito adulticides and larvicides administered by pesticide applicators could have non-target impacts, including on western honey bees ( Apis mellifera ). By elucidating the potential acute effects of mosquito adulticides and larvicides on honey bees, mosquito control personnel can work to minimize non-target impacts of pesticide applications. In this study, we exposed honey bee larvae to one of four mosquito adulticide active ingredients (chlorpyrifos, naled, prallethrin, sumithrin), their commercial formulations (Mosquito Mist®, Dibrom®, Duet®), or one larvicide ( Bacillus thuringiensis israelensis - Bti). Honey bee larvae reared in vitro were exposed to pesticides in their diet on day three after grafting (acute exposure), or from days two to five after grafting (chronic exposure). We measured daily mortality and used these data to calculate the lethal dose that killed 50% of the population (LD 50 ) and the no-observed-adverse-effect-dose (NOAED) for each compound. We used the toxicity data to calculate risk quotients (RQs) using BeeRex. The RQs for the active ingredients suggest that the compounds are unlikely to affect larval mortality at the residue concentrations detected in hive matrices. However, under our in vitro conditions, the formulated mosquito adulticides Mosquito Mist®, Dibrom®, and Duet® caused the highest toxicity of the tested formulations to immature honey bees after chronic exposure, whereas only Dibrom® impacted immature honey bees after acute exposure. Bti demonstrated a higher risk than the other formulations to honey bee larvae only following acute exposure. The resulting data can be used to improve risk assessments for mosquito control products.