Field-level evaluation of honey bee exposure and risk from pesticides used in maize production

Maize seeds are coated with systemic fungicides and insecticides to control pests in maize production. The risk assessments are not well covered for their effects on non-target arthropods, e.g., honey bees, in real farm conditions. The effects of seed-applied thiamethoxam, cyantraniliprole, and a fludioxonil + metalaxyl-M mixture were investigated on honey bees using complementary field, semi-field, and laboratory experiments. Pesticide Residues were analyzed in guttation fluid, honey, pollen, and dead bees, and acute toxicity bioassays were conducted with freshly collected guttation droplets from treated maize plants. The mortality rate was higher than in the control across all pesticide treatments in the semi-field, and it was highest with thiamethoxam, followed by cyantraniliprole and the fungicide mixture. Field experiments showed lower overall effects, but cyantraniliprole and the fungicide mixture still caused significantly increased mortality. Thiamethoxam reached very high concentrations in guttation fluid (2364-2565 µg L⁻¹) soon after plant emergence, resulting in rapid mortality of over 80% within 4 h. Cyantraniliprole-contaminated guttation droplets also caused acute toxicity, with mortality often exceeding 60-90% within 24 to 72 h. Guttation droplets are a critical, yet underappreciated, exposure route for seed-coated pesticides and can deliver lethal doses to honeybees in maize fields. Incorporating guttation exposure into pesticide risk assessment schemes and strengthening integrated pest management strategies are essential to reduce non-target impacts while maintaining effective pest control.