Response of two-spotted spider mite to pyrethroid residues on leaf surfaces.
Understanding the causes of spider mite (Acari : Tetranychidae) outbreaks following the application of synthetic organic insecticides has been the subject of much research during the past 20 years. Pyrethroids are the most recent group of insecticides to have been implicated in promoting spider mite outbreaks, and a wide variety of explanations for the cause of this phenomenon have now been suggested (Penman & Chapman, 1988; Gerson & Cohen, 1989). They include effects on the natural enemies of spider mites, as well as effects on mating, reproduction, development, longevity, diapause and behaviour of spider mites, each of which might be mediated through direct exposure, and/or indirectly through the effects that these insecticides have on their host plants.
Because pyrethroids possess significant irritant/repellent properties (Elliot et al., 1978) much attention has been focused on the effects that these insecticides have on spider mite behaviour, especially at sublethal concentrations. The main behaviourial response to pyrethroids is dispersion, and this occurs when spider mites are sprayed directly, or come in contact with residual deposits (Penman et al., 1984; Iftner et al., 1986). Insecticides which cause dispersal can release mites from density-dependent suppression and the reproductivity and longevity of individuals may increase as a consequence (Hal1, 1979; Penman & Chapman, 1983; Iftner et al., 1986). Therefore, it can be hypothesised that the success of spider mite populations (and hence outbreak or resurgence) following a pyrethroid application may depend, in part, on the presence of suitable unsprayed areas within the mites' dispersal range, and their ability to detect and colonise these areas.
The main aim of the experiments reported in this paper was to determine the ability of two-spotted spider mite (TSM), Tetranychus urticae Koch, to detect and colonise untreated areas of varying size when these were in different positions on pyrethroid-treated leaf surfaces. The study also involved a comparison between a pyrethroid insecticide which commonly induces spider mite outbreaks (esfenvalerate), and one that seldom does (fluvalinate).