Acaricide resistance of the two-spotted spider mite in California 'Bartlett' pears

Acaricide resistance of two-spotted spider mite, Tetranychus urticae Koch, was studied on 'Bartlett' pears in California. Residual bioassays detected 37-fold resistance to cyhexatin and 478-fold resistance to fenbutatin-oxide. Field trials indicated loss of efficacy of these compounds in an orchard with a high frequency of resistant spider mites. Resistance to both chemicals was widespread in Lake and Mendocino Counties, but not in Sacramento County. When field treatment ceased, frequencies of resistant types to cyhexatin stayed approximately the same while resistance to fenbutatin-oxide varied.; A good correlation among the resistance frequencies of spider mites from pear orchards and the neighboring crops in nine pear-growing counties suggested gene-flow among neighboring populations. Studies indicated that two-spotted spider mites are capable of moving between pears and alternative host crops. Spider mites in counties with a lower percentage of fenbutatin-oxide-treated hosts had lower frequencies of resistance than those with a higher percentage. These results suggest that non-treated alternative hosts near pear orchards may provide susceptible immigrants which slow or prevent the development of resistance.; Fenbutatin-oxide resistant and susceptible spider mite lineages were founded with females sampled from two field sites in June and September 1989. The F2 spider mites were bioassayed at the fenbutatin-oxide discriminating concentration. The frequency of resistant lineages was higher in the September than in the June collections although fenbutatin-oxide was not applied between the two sample dates in the field. This suggests that the frequency of resistant genotypes fluctuates with a seasonal pattern.; Over the last 10 years, resistances of Tetranychus spider mites to several selective acaricides have been documented on almond, cotton, pear and strawberry in California. The collective results suggest that factors promoting the development of resistance are chemical use and lack of dispersal of susceptible spider mites from alternative non-treated host crops. Resistance management, therefore, can be achieved by enhancing non-chemical control tactics and juxtaposing treated and non-treated host fields. Using chemical rotations to overcome resistance is not appropriate in the pear system because the frequencies of resistant types do not decline sufficiently.