Previous studies have suggested select herbicides and fungicides can synergize organophosphate toxicity in insects, yet the combined effects of herbicides, fungicides and insecticides to honey bees is poorly understood. As chlorpyrifos is among the most commonly detected insecticide residues inside honey bee colonies, we aimed to evaluate the influence of commonly-encountered pesticides atrazine, chlorothalonil, and alachlor on the acute toxicity of chlorpyrifos. Exposure to atrazine, alachlor, or chlorothalonil individually did not cause acute toxicity to individual honey bees, yet atrazine (1 mg/bee), and alachor (4 mg/bee) in binary combinations with chlorpyrifos increased chlorpyrifos toxicity 2.8 and 2.5-fold, respectively. Reciprocally, exposure to atrazine and chlorothalonil (1 mg/bee) reduced the toxicity of chlorpyrifos-oxon, an oxidative metabolite of chlorpyrifos, by 2.3 and 1.3-fold, respectively. Assays measuring the activity of cytochrome P450, an important class of insecticide detoxification enzymes, indicate that atrazine and alachlor exposure significantly increase P450 activity in individual bees, while chlorothalonil exposure leads to decreased activity. Enhanced P450 activity suggests that the increases in chlorpyrifos toxicity (atrazine and alachlor) may be due to oxidative activation, while the decreased toxicity of chlorpyrifos oxon may result from enhanced detoxification. The decrease in chlorpyrifos oxon toxicity seen upon chlorothalonil exposure is similarly explained, as the increased P450 activity observed may lead to increased detoxification of the compound. Changes to acetylcholinesterase activity after exposure to these synergists is currently being determined, and will be presented. These results suggest that simultaneous exposure to chlorpyrifos and these herbicides/fungicides during the crop pollination may lead to enhanced toxicity, suggesting the potential for weakening or loss of honey bee colonies. The data we have collected will greatly add to our understanding of the risk posed by simultaneous exposure to combinations of agrochemicals, and may allow for management decisions that provide crop protection while reducing colony loss.

Synergistic effects of agrochemical exposure phosphorothioate insecticides in the Western honey bee, <i>Apis mellifera</i>

ACS Fall 2021

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Synergistic effects of agrochemical exposure phosphorothioate insecticides in the Western honey bee, Apis mellifera