Resistance to organophosphates and biochemical genotyping of acetylcholinesterases in Tetranychus urticae (Acari : Tetranychidae)

Resistance to organophosphorus acaricides in the two-spotted spider mite Te tranychus urticae is largely due to the reduced sensitivity of the target e nzyme acetylcholinesterase (AChE). The insensitivity of AChE to the organop hosphates (OPs) demeton-S-methyl, ethyl paraoxon, and chlorpyrifos oxon and the carbamate carbofuran was identified in a German laboratory strain of T . urticae (WI) selected with oxydemeton-methyl and a field-collected strain from Florida, T. urticae (VB). Depending on the inhibitor used, AChE in th e resistant strains was 34- to 380-fold less sensitive than the AChE of a G erman susceptible strain, T. urticae (GSS), as revealed by conventional mic rotiter plate assays. A modified, highly sensitive microfluorometric assay enabled us to measure the AChE activity in a quarter of a single mite. The assay is based on the reaction of thiocholine with a fluorogenic coumarin p henylmaleimide, yielding an intensely fluorescent product. This technique w as used to genotype individual spider mites and to examine the frequency of sensitive and insensitive AChE alleles in several strains. The kinetic par ameters (K-m and V-max) of AChE from susceptible (GSS) and resistant (WI an d VB) strains of T. urticae were compared for different substrates. In a fo liar spray application bioassay, larvae of strains WI and VB showed only mo derate resistance levels in contrast to their high degree of AChE insensiti vity relative to the GSS strain. While resistance ratios to ethyl parathion were comparable in strains WI and VB, the difference in chlorpyrifos resis tance was remarkable. The field-collected strain VB and the laboratory sele cted strain WI exhibited 78- and 9.5-fold resistance against chlorpyrifos, respectively. As there is no obvious difference in AChE inhibition kinetics between the two strains, this result suggests an additional resistance mec hanism in the VB strain against chlorpyrifos. (C) 2001 Academic Press.