The risk level of several insecticides of various chemical classes was estimated for honeybee workers, Apis mellifera L. (Hymenoptera: Apidae). Lethal time calculation was used to risk assessment for honeybees. Bioassay tests were conducted with six insecticides [dinotefuran (neonicotinoid), methomyl (carbamate), profenofos (organophosphate), azadirachtin (botanicalbioinsecticide), spinosad (bioinsecticide - an extract of the fermentation broth of soil actinomycete) and chlorfluazuron (IGR)] on honeybee workers by the insecticide / food mixture technique, at seven concentrations as ratios of recommended field rate [F (ug a. i. mL-1)], for 15 days. Results revealed that dinotefuran was significantly the most toxic to bees, which gave the shortest median lethal times (LT50s), 4.4, 4.9, 5.8, 6.4 and 10.3 days at concentrations of 1F×10-2, 5F×10-3, 1F×10-3, 5F×10-4 and 1F×10-4, respectively. Moreover, it gave 100% bee mortality after one day exposure time, at two higher concentrations, (1F×10-1) and (5F×10-2). The toxicity order of the tested insecticides for honey bees (Based on LT95s) varied by the reducing in their concentrations, whereas it was: dinotefuran > methomyl > profenofos > azadirachtin > chlorfluazuron > spinosad, at the higher concentrations and this became azadirachtin > dinotefuran > profenofos > chlorfluazuron > methomyl > spinosad at the lowest concentrations. It was concluded that the interaction among insecticide concentration, exposure time and its chemical class plays a great role in the risk level on honeybee workers. Spinosad and chlorfluazuron were significantly less toxic in comparison to the other insecticides tested and they can be safely applied to crops. |
