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Acaricide bioassays with spider mites (Acari: Tetranychidae) on pome fruits: evaluation of methods and selection of discriminating concentrations for resistance monitoring

Knight, A.L., Beers, E.H., Hoyt, S.C., Riedl, H.
Journal of economic entomology 1990 v.83 no.5 pp. 1752-1760
Panonychus ulmi, avermectins, Pyrus, fenbutatin oxide, hexythiazox, monitoring, Tetranychus mcdanieli, Malus domestica, insecticide residues, insect control, Tetranychus urticae, insecticide resistance
Leaf disk bioassays with the acaricides avermectin B1, fenbutatin oxide, and hexythiazox were conducted with the mite species Panonychus ulmi(Koch), Tetranychus urticae Koch, and T. mcdanieli McGregor collected from apple andpear orchards in Washington. The effects of length of the bioassay period and inclusion of mite walk-off in mortality used to estimate LC50's with fenbutatin oxide and avermectin B1were examined. Correlations between LC50's after 48 and 72 h were significant with bothchemicals. However, large decreases in LC50's with fenbutatin oxide from 48 to 72 h indicatedthat the longer time allowed a more complete assessment of mite mortality. Comparisonof results from closed double-leaf and open single-leaf bioassays with fenbutatin oxidesuggested that mite walk-off should be included in mortality counts. Significant differences inLC50's were found among mite species for hexythiazox and avermectin B1, but not withfenbutatin oxide. P. ulmi was 20 and 2 times more tolerant to hexythiazox and avermectin B1, respectively, than the two Tetranychus species. Correlations among LC50's for acaricideswithin each species were not significant. In addition, partial correlations for species were not significant with LC50's for fenbutatin oxide and hexythiazox and fenbutatin oxide andavermectin B1. Discriminating concentrations for detection of incipient levels of resistance for hexythiazox were established for each species. Discriminating concentrations also were selectedfor the two Tetranychus species for avermectin B1.