ACETYLCHOLINESTERASE ACTIVITY ASSOCIATED WITH METHIOCARB RESISTANCE IN A STRAIN OF WESTERN FLOWER THRIPS, FRANKLINIELLA-OCCIDENTALIS (PERGANDE)

A strain of western flower thrips (Frankliniella occidentalis) with re sistance to methiocarb was collected from a Danish greenhouse for inve stigation of possible resistance mechanisms. Bioassays with methiocarb showed moderate (10-fold) resistance at LC50 in the greenhouse-collec ted strain compared to a susceptible laboratory strain. Piperonyl buto xide, a cytochrome P450 monooxygenase inhibitor, showed no synergistic effect on the tolerance level to methiocarb. In vitro assays of enzym e activities showed significantly increased activity of acetylcholines terase (2.6-fold), general esterases (1.3-fold), and glutathione S-tra nsferases (1.2-fold) in the resistant strain. Assays of acetylcholines terase sensitivity to inhibition by methiocarb, dichlorvos, and eserin e showed no indications of insensitive acetylcholinesterase in the res istant strain. These results indicated that increased activity of acet ylcholinesterase contributed to methiocarb resistance in the resistant strain. The slightly increased activity of general esterases and glut athione S-transferases may also have contributed to methiocarb resista nce. (C) 1998 Academic Press.