Toxicity of neonicotinoid insecticides on different honey bee genotypes

Toxicity effects of the neonicotinoid insecticides clothianidin, imidacloprid, and thiamethoxam were tested in the laboratory on different honey bee (Apis mellifera L.) genotypes belonging to the following subspecies: Apis mellifera mellifera L., Apis mellifera ligustica Spinola, and Apis mellifera carnica Pollmann. Oral and indirect contact trials were carried out on adult worker honey bees for each pesticide, using commercial formulations. The acute oral toxicity (AOT) LD50 and the acute indirect contact toxicity (ICT) LC50 were calculated. Mean AOT LD50 values at 24 hours (clothianidin 3.53 ng/honey bee; imidacloprid 118.74 ng/honey bee; thiamethoxam 4.40 ng/honey bee), 48 hours (clothianidin 3.35 ng/honey bee; imidacloprid 90.09 ng/honey bee; thiamethoxam 4.27 ng/honey bee), and 72 hours (clothianidin 3.28 ng/honey bee; imidacloprid 69.68 ng/honey bee; thiamethoxam 4.16 ng/honey bee) from test start were of the same order of magnitude of those reported in the literature for all three neonicotinoids. Statistically significant differences emerged in a few instances between groups of honey bees coming from the different hives tested for clothianidin, between the groups of honey bees coming from the single A. m. mellifera hive and the four A. m. ligustica hives tested for imidacloprid, and more extensively between the two A. m. carnica, the single A. m. mellifera, and the six A. m. ligustica groups of honey bees tested for thiamethoxam. ICT LC50 values were obtained for a reduced number of hives: the single A. m. mellifera and two A. m. ligustica hives for clothianidin, the single A. m. mellifera and one A. m. ligustica hive for imidacloprid, the single A. m. mellifera, three A. m. ligustica hives, and one A. m. carnica hive for thiamethoxam. Nevertheless statistically significant differences were observed for clothianidin and thiamethoxam, but not for imidacloprid. The results confirm that genetic differences in the response to pesticide toxic action exist in the honey bee, but they do not constitute the key factor involved in the uneven results observed in toxicity tests. In any case, the LD50 or other similar toxicity indexes should not be determined on a single colony.