Double-leaf-disk residue assay for assessing the toxicity of repellent acaricides to spider mites (Acari: Tetranychidae)

A closed double-leaf-disk (DLD) assay is presented which prevents escape (runoff) of spider mites and exposes them to treated leaf tissue during the entire test period. The physiological (direct) toxicity of acaricide residue can be estimated with this assay without the interference of behavioral factors. In contrast, runoff is often a major component of the total mortality in open single-leaf-disk (SLD) assays, in addition to direct mortality. Fenpropathrin, a highly repellent pyrethroid acaricide, caused primarily runoff and little direct mortality in SLD assays withPanonychus ulmi (Koch). When runoff and direct mortality were combined, concentration-response lines andlc₅₀ values were identical to those obtained with the closed DLD assay. Assays with the organotin fenbutatin oxide gave similar results, suggesting that the runoff response should be included when estimating the residual toxicity of these compounds with the standard SLD assay. In fact, excluding runoff mortality from the analysis may underestimate the toxicity of foliar residue. Providing residue-free non-leaf surface inside the DLD cage caused a shift to the right of the concentration-response line, resulting in higherlc₅₀s. The DLD assay can be employed for mite species which are difficult to confine, and in toxicological studies of repellent or irritating acaricides. This assay may be particularly useful for elucidating the relationship between physiological toxicity and mite behavior in response to acaricide residue.Technical Paper 9167 of the Oregon State University Agricultural Experiment State, Corvallis, Oregon, USA.