Toxicity and biochemical changes in the honey bee Apis mellifera exposed to four insecticides under laboratory conditions

The toxicity and biochemical changes in honey bees (Apis mellifera) treated with four insecticides—acetamiprid, dinotefuran, pymetrozine, and pyridalyl—were evaluated under controlled laboratory conditions. Foraging bees were exposed to different dosages of tested insecticides by oral feeding at different dosages recommended by the manufacturers for agricultural crops in Egypt (0.01-, 0.02-, 0.04-, 0.1-, and onefold). Moreover, the acute toxicity of these insecticides was evaluated by topical application on the thorax of foragers to calculate the LD50 values. The specific activities of acetylcholinesterase (AChE), carboxylesterase, glutathione S-transferase (GST), and polyphenol oxidase (PPO) were measured in different tissues of surviving foragers after 24 h of treatment to explore the possible mode of action of insecticides and honey bees' strategies for detoxification and tolerance. The results indicated that regardless of how the bees were exposed to insecticides, dinotefuran was extremely toxic to adult A. mellifera (topical LD50 = 0.0006 μg/bee and oral feeding LC50 = 1.29 mg/L). Pyridalyl showed moderate toxicity compared to dinotefuran at the recommended application rate; however, acetamiprid and pymetrozine were relatively less toxic to bees (<25 % mortality at the recommended application rates). Data showed that tested insecticides varied in their influence on AChE, carboxylesterase, GST, and PPO activities that were highly correlated to their toxicity against A. mellifera. The biochemical analysis of carboxylesterase and GST showed that these enzymes detoxified the low doses of acetamiprid, pymetrozine, and pyridalyl, but not dinotefuran. Overall, our results are valuable not only in evaluating the toxicity of common insecticides onto honey bees, but also in highlighting the validity of enzymes activities as proper indicators for exposure to agrochemicals.