Acaricide Bioassays with Spider Mites (Acari: Tetranychidae) on Pome Fruits: Evaluation of Methods and Selection of Discriminating Concentrations for Resistance Monitoring

Abstract

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 and pear 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 B1 were examined. Correlations between LC50’s after 48 and 72 h were significant with both chemicals. However. large decreases in LC50’s with fenbutatin oxide from 48 to 72 h indicated that the longer time allowed a more complete assessment of mite mortality. Comparison of results from closed double-leaf and open single-leaf bioassays with fenbutatin oxide suggested that mite walk off should be included in mortality counts. Significant differences in LC50’s were found among mite species for hexythiazox and avermectin Bh but not with fenbutatin 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 acaricides within 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 and avermectin B1. Discriminating concentrations for detection of incipient levels of resistance for hexythiazox were established for each species. Discriminating concentrations also were selected for the two Tetranychus species for avermectin B1.