Methidathion Resistance Mechanisms in Amblyseius womersleyi Schicha (Acari: Phytoseiidae)

Methidathion resistance mechanisms were studied in the predaceous mite Amblyseius womersleyi. High synergistic ratios (SR = LC50) without synergist/LC50 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 inhibitor of GSH transferases and mfo) and DEF (an inhibitor of esterases and GSH transferases) indicate that, to a lesser extent, hydrolytic reactions are also partially involved in this resistance. Insensitive acetylcholinesterase was thought to be a minor factor for resistance. No difference in cuticular permeability of [14C]methidathion was observed between the susceptible and resistant strains. An in vitro degradation study showed that [14CJmethidathion was degraded 1.5 times more in the microsomal fraction of the resistant strain than that of the susceptible one. No difference in [14C]methidathion degradation was observed in the soluble fraction between susceptible and resistant strains. From these results, oxidative degradation was thought to have a critical role in methidathion resistance in A. womersleyi.