Abstract
Cyhexatin was the principal acaricide used for control of the two spotted spider mite Tetranychus urticae Koch in southern Oregon pear orchards from 1972 until the early 1980’s. By 1978, a low-level resistance (4.1-fold at LC50) was found in mites from one orchard. With continued use of cyhexatin at higher rates the resistance increased by 1980 (24.9-fold at LC50). Subsequently, exclusive and intensive use of cyhexatin was discontinued in this orchard with some possible reversion of resistance levels (9.9-fold at LC50) by 1983. Resistance levels remained very high in 1983 (31.3- and 107.8-fold at LC50) in two nearby orchards where cyhexatin use was continued.
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