Cyhexatin and Fenbutatin-Oxide Resistance in Pacific Spider Mite (Acari: Tetranychidae): Stability and Mode of Inheritance

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Abstract
The Pacific spider mite, Tetranychus pacificus McGregor, collected from California almond orchards during 1984 and 1985, was tested with cyhexatin and fenbutatin-oxide to determine whether resistance (R) to these acaricides was present. A cyhexatin resistant colony was selected in the greenhouse with cyhexatin for 15 mo to obtain a pure colony for genetic analysis. Mode-of-inheritance data were consistent with a model in which cyhexatin resistance in this colony is primarily determined by a major semirecessive gene. A flowable formulation of cyhexatin was more toxic to both R and susceptible (S) strains than was the wettable powder formulation. Larvae of the R and 5 strains exhibited survival rates similar to those of adults. Concentration/mortality lines for both fenbutatin-oxide and cyhexatin were obtained for both R and S strains; results were consistent with the hypothesis that cyhexatin resistance confers fenbutatin-oxide resistance in this population. Greenhouse populations held for 9 mo without selection with cyhexatin retained their original resistance levels, as indicated by the percent survival of colonies initiated from RR, SR, RS, and SS females. Negative fitness did not appear to be associated with the resistance allele under these conditions.