Effect of Alachlor and Metolachlor on Toxicity of Chlorpyrifos and Major Detoxification Enzymes in the Aquatic Midge, Chironomus tentans (Diptera: Chironomidae)

The toxicities of two herbicides (alachlor and metolachlor) and an organophosphate insecticide (chlorpyrifos) were evaluated individually and with either herbicide in binary mixture with chlorpyrifos in fourth-instar larvae of the aquatic midge (Chironomus tentans). Alachlor alone up to 1,000 micrograms per liter of water (μg/L) did not exhibit significant toxicity, whereas metolachlor at 1,000 μg/L affected 58% of midges in 72-h bioassays. However, alachlor at 1,000 μg/L and metolachlor at 10, 100, and 1,000 μg/L enhanced the toxicity of chlorpyrifos to the midges. Furthermore, alachlor and metolachlor at 1,000 μg/L reduced acetylcholinesterase (AChE) activity by 34.3% and 27.6%, respectively, in the treated midges. Although alachlor at 1,000 μg/L did not significantly affect protein production in the treated midges, it reduced glutathione S-transferases (GST) total activities by 1.9- to 2.1-fold. In contrast, metolachlor at 1,000 μg/L reduced protein production by 3.2-fold, which was associated with a 2.8-fold reduction of cytochrome P450 O-deethylation total activity and 1.4- to 1.7-fold reductions of GST total activities in the treated midges. Such reduced total activities of the major detoxification enzymes in herbicide-treated midges might impede the metabolic detoxification of chlorpyrifos and, therefore, increase the susceptibility of the midges to chlorpyrifos. Thus, it is likely that coexistence of chlorpyrifos and these herbicides, particularly metolachlor, in surface waters may pose increased risks to midges in aquatic environments.