Behavioral disruption in honey bees (Apis mellifera) exposed to isolated and combined insecticides

The global decline of pollinators threatens ecosystem stability and food security. Bees are particularly affected by agricultural insecticides, with neonicotinoids and pyrethroids being among the most widely used classes. While their individual sublethal effects are documented, the consequences of combined exposures remain poorly understood. This study evaluated the isolated and combined effects of field-realistic oral doses of Imidacloprid (0.5 ng/bee) and Deltamethrin (2 ng/bee) on Africanized honey bees (Apis mellifera) of southern Brazil. We quantified grooming, wing fanning, feeding and locomotion (walking time, distance and speed) under laboratory conditions. Imidacloprid caused a strong elevation in grooming behavior (8.64-fold) and reduced both walking distance (-308 cm) and speed (-1.04 cm·s⁻¹). Deltamethrin increased grooming to a lesser extent (2.96-fold) without affecting any locomotion capability. Combined exposure resulted in non-additive effects, with grooming activity reaching levels similar to those of Imidacloprid alone (7.79-fold). However, unlike Imidacloprid, the mixture caused no locomotor deficits, which contrasts with the additive or synergistic interactions commonly reported for insecticide mixtures. No treatment affected syrup consumption or mortality over a 24 h period. These results demonstrate that neonicotinoids and pyrethroids disrupt honey bee behavior through distinct pathways and that mixture effects cannot be predicted from single-compound responses. Incorporating behavioral endpoints and mixture toxicology into risk assessments is essential for improving pesticide-pollinator dynamics and protection strategies.