A resistant strain of Phytoseiulus persimilis selected by methidathion pressure for several years metabolizes the [14C]methidathion faster than does the corresponding susceptible strain. The metabolism is for the main part glutathione dependent and gives the methidathion conjugate on glutathione as a first metabolite: S[5-methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl]-l-glutathione. In addition, glutathione transferase with chlorodinitrobenzene as a substrate has a threefold lower Kmin R strain than in S strain. Furthermore, this reaction is competitively inhibited by methidathion with a Kiwhich is threefold lower in R than in S strain. These results indicated that in this strain of P. persimilis resistance is due to an elevated detoxication of methidathion by a glutathione transferase. Other parameters known to be able to induce resistance in arthropods have been compared in resistant and sensitive strains. Esterase and monooxygenase activity measured with chromogenic substrates are the same in the two strains as is the level of acetylcholinesterase and its inhibition by methidathion oxon. No difference between the two strains has been found in the penetration kinetics measured with [14C]methidathion. These results indicated that glutathione transferase is the only mechanism which has been selected in P. persimilis, although other mechanisms are known to be involved in resistance to other insecticides in phytoseiid mites. © 1987.
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