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 resistance to methiocarb was collected from a Danish greenhouse for investigation of possible resistance mechanisms. Bioassays with methiocarb showed moderate (10-fold) resistance at LC 50 in the greenhouse-collected strain compared to a susceptible laboratory strain. Piperonyl butoxide, a cytochrome P450 monooxygenase inhibitor, showed no synergistic effect on the tolerance level to methiocarb. In vitro assays of enzyme activities showed significantly increased activity of acetylcholinesterase (2.6-fold), general esterases (1.3-fold), and glutathione S -transferases (1.2-fold) in the resistant strain. Assays of acetylcholinesterase sensitivity to inhibition by methiocarb, dichlorvos, and eserine showed no indications of insensitive acetylcholinesterase in the resistant strain. These results indicated that increased activity of acetylcholinesterase contributed to methiocarb resistance in the resistant strain. The slightly increased activity of general esterases and glutathione S -transferases may also have contributed to methiocarb resistance.