Pesticide Biochemistry and Physiology 69, 1–12 (2001) doi:10.1006/pest.2000.2523, available online at http://www.idealibrary.com on Methidathion Resistance Mechanisms in Amblyseius womersleyi Schicha (Acari: Phytoseiidae) Ma ´rio Eidi Sato, 1 Tadashi Miyata, Akira Kawai,* and Octavio Nakano² Laboratory of Applied Entomology, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan; *National Institute of Vegetables, Ornamental Plants and Tea, Aki, Mie 514-2392, Japan; and ² ESALQ, Universidade de Sa ˜o Paulo, Piracicaba, SP, C.P. 9, CEP 13418-900, Brazil Received August 23, 1999, accepted March 22, 2000 Methidathion resistance mechanisms were studied in the predaceous mite Amblyseius womersleyi. High synergistic ratios (SR = LC 50 without synergist/LC 50 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 [ 14 C]methidathion was observed between the susceptible and resistant strains. An in vitro degradation study showed that [ 14 C]methidathion was degraded 1.5 times more in the microsomal fraction of the resistant strain than that of the susceptible one. No difference in [ 14 C]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.  2001 Aca- demic Press INTRODUCTION other chemicals (organophosphosphate, carba- mate, and pyrethroid insecticides) in tea fields in Japan, especially in Shizuoka Prefecture (4–7). Insecticide treatments used for the control of The main mechanisms by which an arthropod insect crop pests are one of the main causes of can develop resistance to insecticides have been pest outbreaks (1) and are responsible for the documented (8). In Phytoseiidae, insecticide reduction or suppression of beneficial species resistance can be due to increased activity of such as phytoseiid mites (2). Amblyseius womer- biotransformation enzymes, such as glutathione sleyi Schicha (Acari: Phytoseiidae) is a promis- transferases (9–11), mixed-function oxidases ing predator of tetranychid mites and presents (12), and esterases (13, 14). Decreased affinity wide geographic distribution, occurring in many of acetylcholinesterase for insecticides (organo- countries in Asia and Oceania (3). This predator phosphorus and carbamate compounds) has also is an important natural control agent against the been found to be involved in resistance (15, 16). Kanzawa spider mite, Tetranychus kanzawai The methidathion resistance in A. womersleyi Kishida, on tea in Japan (4). is known to be mainly due to a single gene (4) Methidathion is an organophosphorus com- but so far, there is no information about the pound currently used on tea in Japan. It has mechanisms of resistance involved in the resis- recently been observed that mites of the species tance of this predaceous mite to this insecticide. A. womersleyi are resistant to methidathion and The objectives of this study were to determine the methidathion resistance mechanisms in A. 1 Present address: Biological Institute, Caixa Postal 70, CEP 13001-970, Campinas, SP, Brazil. womersleyi and to provide basic information for 1 0048-3575/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.