Long-term risk assessment on noneffective and effective toxic doses of imidacloprid to honeybee workers

Abstract

Imidacloprid is the most widely used insecticide in agriculture. Its impact on honeybees has received worldwide attention. Foliar sprays are commonly and frequently used for piercing insect control, particularly on cotton in southern USA. To simulate field exposures of formulated imidacloprid (Advise ® 2Fl), we used a modified spray tower to treat honeybee workers and monitored five enzyme activities and survival for up to 52 days. Results indicated that spray treatments twice a week for 52 days with 0.001 and 1 mg/L and once a week for three weeks with 4.3 mg/L Advise showed no adverse effect on bee survival, where imidacloprid-treated bees could live as long as untreated bees. Concentration ≥80 mg/L significantly reduced bee survival, and substantial number of bees continued to die after 48-hr of post-treatment period which was commonly used for measuring insecticide toxicity. The body weight of imidacloprid-treated bees (at LC 20 and LC 50 ) was also significantly reduced. Enzymatic data showed that activities of detoxification enzymes esterase and glutathione S-transferase (GST), insecticide-target enzyme acetylcholinesterase (AChE) and honey enzyme invertase in imidacloprid-treated survivors were mostly similar to those found in untreated bees. The immunity-related phenoloxidase (PO) activity in imidacloprid-treated survivors was also mostly similar to that of untreated control, but higher PO activity was detected in bees treated with higher concentrations for 3 weeks. By using both bioassays and enzymatic assays, this study revealed long-term noneffective and effective concentrations of imidacloprid that may be useful for accurate assessment of toxicity risk of neonicotinoids to bees.