Laboratory bioassays conducted to determine the contact honey bee toxicity of commercial neonicotinoid insecticides showed that the nitro-substituted compounds were the most toxic to the honey bee with LD50 values of 18 ng/bee for imidacloprid and 30 ng for thiamethoxam. The cyano-substituted neonicotinoids exhibited a much lower toxicity with LD50 values for acetamiprid and thiacloprid of 7.1 and 14.6 µg/bee, respectively. However, piperonyl butoxide and propiconazole increased honey bee toxicity of acetamiprid 6.0- and 105-fold and thiacloprid 154- and 559-fold, respectively, but had a minimal effect on imidacloprid (1.70 and 1.52-fold, respectively). A broad survey of pesticide residues conducted on samples from North American apiaries during the 2007–08 growing seasons revealed the presence of 121 different pesticides and metabolites within wax, pollen, bee and associated hive samples, including acetamiprid, thiacloprid, imidacloprid, thiamethoxam, piperonyl butoxide and propiconazole. Thus, under practical circumstances, acetamiprid and thiacloprid can be as toxic to honey bees as imidacloprid and thiamethoxam.
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