Acaricide toxicity and resistance in larvae of different strains of Tetranychus urticae and Panonychus ulmi (Acari : Tetranychidae)

The toxicities of eight structurally different acaricidal compounds to six- legged larvae (first motile stage) of three laboratory strains of the two-s potted spider mite, Tetranychus urticae, and the European red mite, Panonyc hus ulmi, were evaluated following spray application. The larvae of five fi eld-derived strains of T urticae originating from France, Italy, Brazil, Ca lifornia and Florida were also tested for their susceptibilities to discrim inating concentrations of several acaricides resulting in 95% mortality whe n applied to the organophosphate-resistant laboratory reference strain WI. The spray bioassay used was robust and gave repeatable results with a wide range of acaricidal compounds, irrespective of their mode of action (ovo-la rvicides or primarily acting on motile life stages). Compounds tested were abamectin, azocyclotin, chlorpyrifos, clofentezine, deltamethrin, fenpyroxi mate, hexythiazox and pyridaben. Larvae of one of the laboratory strains of T urticae, AK, originally collected in Japan in 1996 and maintained withou t further selection pressure, exhibited 2000- and >4000-fold resistance to the mitochondrial electron transport inhibitors pyridaben and fenpyroximate , respectively. Another strain of T urticae, AU, obtained from Australia an d maintained in the laboratory under selection with hexythiazox and clofent ezine since 1987 showed >770- and >1000-fold resistance to clofentezine and hexythiazox, respectively. The same resistance pattern was observed agains t larvae of a laboratory strain of P ulmi, CE, also selected with hexythiaz ox. Larvae of one of the field-derived strains of T urticae, BR, showed a l ower susceptibility to a number of compounds, whilst the others were suscep tible to all compounds except the organophosphates. (C) 2001 Society of Che mical Industry.