RESISTANCE OF TETRANYCHUS-URTICAE KOCH (ACARI, TETRANYCHIDAE) TO CYHEXATIN AND FENBUTATIN-OXIDE IN CALIFORNIA PEARS

Twospotted spider mite, Tetranychus urticae Koch, sampled from 'Bartlett' pears in California, was established in the laboratory and tested for susceptibility to cyhexatin and fenbutatin-oxide. At LC50, the maximum difference in susceptibility between colonies to cyhexatin was 38-fold; to fenbutatin-oxide, the maximum difference was 478-fold. Field trials indicated loss of efficacy of these compounds in an orchard containing twospotted spider mites for which bioassays showed low susceptibility. Results of both the laboratory bioassays and field trials suggest that these mites were resistant A survey showed that resistance to both chemicals was widespread in Lake and Mendocino counties. We detected no resistance in Sacramento County except for one collection with a very low frequency of resistance to fenbutatin-oxide only. When field treatment ceased, resistance frequencies to cyhexatin stayed approximately the same, but resistance to fenbutatin-oxide varied. Although correlation between resistance to cyhexatin and fenbutatin-oxide among 39 colonies was significant, lack of a change in cyhexatin resistance after fenbutatin-oxide application in the field suggests multiple resistance to these compounds.