Effects of imidacloprid and common plant metabolites of imidacloprid in the honeybee: toxicological and biochemical considerations

Honeybees foraging in crop plants seed-dressed with imidacloprid may be exposed to imidacloprid and imidacloprid plant metabolites. Metabolism studies on a large variety of crop plants were reviewed to identify plant metabolites which have a potential toxicological relevance to honeybees. Three different bioassays were conducted to characterize the pharmacological and toxicological profile of imidacloprid and these potentially relevant plant metabolites in the honeybee. The nicotinic acetylcholine receptor (nAChR) was identified as the molecular target of [ 3 H]imidacloprid and some of the tested plant metabolites in honeybee head membrane preparations. IC50-values for the displacement of [ 3 H]imidacloprid of 2.9, 0.45, 24, 6600, >100000, and >100000 nM were recorded for imidacloprid and the plant metabolites olefine, 5-OH-imidacloprid, 4,5-OHimidacloprid, urea metabolite and 6-chloronicotinic acid (6-CNA), respectively. These values indicate a potential toxicological relevance only for the olefine and the 5-hydroxy metabolite. Whole cell voltage clamp electrophysiology revealed that imidacloprid, the olefine and the 5-hydroxy metabolite act agonistically on the nAChR of neurons isolated from the antennal lobe of Apis mellifera. All other metabolites did not induce inward currents at test concentrations up to 3 mM. As for the parent compound, the electrophysiology data of the active metabolites revealed Hill coefficients of approximately 1, thus indicating a single binding site responsible for an activation of the receptor and no direct cooperativity or allosteric interaction with a second binding site, respectively. In standard toxicity assays following the EPPO guideline 170 LD50 values between 40 and 104 ng/bee were determined for the oral and contact toxicity of imidacloprid to honeybees. No indications were found for significant differences in sensitivity to imidacloprid between honeybees from different apiaries. The acute oral toxicity of the potentially relevant plant metabolites of imidacloprid to honeybees were well correlated with their receptor binding affinity and receptor activation potential. Only the olefine and the 5-hydroxy metabolite were identified as toxicologically relevant. The results from the current studies suggest that residue investigations aiming to define the field exposure of honeybees to imidacloprid applied as seed dressing should not only include a detection method sensitive for the parent compound but should also be sensitive to the presence of the olefine and 5-hydroxy-imidacloprid.