Acute contact toxicity of thiamethoxam to the Indian honey bee, Apis cerana: Insights from topical and residue bioassays

Abstract

Thiamethoxam, a widely used neonicotinoid insecticide, is highly effective against insect pests but poses risks to non-target pollinators such as Apis cerana. Acute contact toxicity of Thiamethoxam 25 WG to A. cerana workers was evaluated using topical and residue contact bioassays under controlled laboratory conditions. Topical application resulted in an LC50 of 1.76 ppm, while residue contact exposure yielded an LC50 of 2.45 ppm, indicating high susceptibility of bees even at low concentrations. Behavioral symptoms preceding mortality included tremors, wing paralysis, impaired mobility, and eventual immobility, reflecting neurotoxic effects consistent with nicotinic acetylcholine receptor agonism. The slightly higher LC50 observed in residue contact bioassays highlights the influence of exposure route on toxicity. These findings, in comparison with previous research, confirm that A. cerana is particularly sensitive to neonicotinoids and emphasize the need for pollinator-safe pesticide management, including limiting applications during flowering periods and adopting integrated pest management strategies.