Mechanism of Malathion and Parathion Resistance in the Two-Spotted Spider Mite, Tetranychus urticae1

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Abstract
Comparative studies on the breakdown activities in organophosphate-resistant and -susceptible spider mite, Tetranychus urticae Koch, strains, against radioactive malation and parathion and non-radioactive malaoxon and para-oxon revealed that the resistant Blauvelt strain had a superior ability to detoxify malathion, parathion, and malaoxon as compared with the susceptible Niagara strain. In the case of malathion breakdown, the highest interstrain difference was found in the amount of carboxyesterase products, but a substantial difference in the phosphatase activity was also observed. These results were in accordance with the experiments in vivo with radioactive malathion and parathion. A second OP-resistant spider mite strain, characterized by another protection mechanism, also showed a high rate of malathion and parathion breakdown. The total uptake of malathion by the Blauvelt strain did not significantly differ from that by the Niagara strain. The rate of ChE inhibition in vivo in Niagara mites was much higher than in Blauvelt mites. Furthermore, homogenates of the Blauvelt strain were found to hydrolyse β-naphthyl benzoate at a higher rate than Niagara homogenates. The unique features of the malathion-resistance in the Blauvelt strain were compared with data already published on malathion-resistance in mosquitoes and house flies (Musca domestica L.).