A colony of the Pacific spider mite, Tetranychus pacificus McGregor, collected from California almond orchards during 1984 was found to be resistant (R) to propargite. This propargite-resistant colony was selected 25 times with propargite for 25 mo to ensure a homogeneous colony for mode of inheritance tests. Results of these tests were generally consistent with a model in which propargite resistance in this colony is primarily determined by a major semirecessive gene. Lines for the reciprocal F1, females (RS, SR) were not different, and dominance (D) was estimated to be –0.356 and –0.315, suggesting there were no maternal effects. Greenhouse populations held for 5 mo without selection with propargite retained their original resistance levels, as indicated by the percentage of survival of colonies initiated from RR (resistant), SR, and RS females and complete concentration-mortality lines conducted at the end of the experiment. Negative fitness did not appear to be associated with the resistance allele(s) under these conditions.
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Hoy et al. (1989) studied this question.