Methidathion resistance mechanisms in Amblyseius womersleyi schicha (Acari: Phytoseiidae)

Methidathion resistance mechanisms were studied in the predaceous mite Ambl yseius womersleyi. High synergistic ratios (SR = LC50 without synergist/LC5 0 with synergist) were observed for piperonyl butoxide and 2-propynyl 2,3,6 -trichlorophenyl ether (both inhibitors of mixed-function oxidases) only in the resistant strain, suggesting that an increase in oxidative metabolism is involved in this resistance. Tests done with the synergists DEM (an inhi bitor of GSH transferases and mfo) and DEF (an inhibitor of esterases and G SH transferases) indicate that, to a lesser extent, hydrolytic reactions ar e also partially involved in this resistance. Insensitive acetylcholinester ase was thought to be a minor factor for resistance. No difference in cutic ular permeability of [C-14]methidathion was observed between the susceptibl e and resistant strains. An in vitro degradation study showed that [C-14]me thidathion was degraded 1.5 times more in the microsomal fraction of the re sistant strain than that of the susceptible one. No difference in [C-14]met hidathion degradation was observed in the soluble fraction between suscepti ble and resistant strains. From these results, oxidative degradation was th ought to have a critical role in methidathion resistance in A. womersleyi. (C) 2001 Academic Press.