House fly, Musca domestica L., populations sampled from eight dairies in the Chino Basin area of southern California were evaluated by topical application for permethrin resistance. Resistance ratios at LD95, ranged from 15.2- to 185.2-fold, roughly in agreement with frequency of permethrin usage. Combined use of permethrin with piperonyl butoxide (1:5 ratio) increased toxicity of permethrin in laboratory bioassays; synergism ratios ranged from 12.4- to 34.3-fold and were significantly correlated with resistance ratios. Permethrin resistance greatly decreased in a population that remained unselected in the laboratory from November 1984 to July 1985. Selection of that colony for two consecutive generations caused a 12.5-fold increase in the LD95. Withdrawal of permethrin from one dairy during 1985 caused the LD95 to decline from 9.26 to 1.84 μg per fly. Mortality counts of house flies taken at 24 and 48 h after treatment showed that some flies recovered from initial knock-down recorded at 24 h, thus causing a 50% increase in the LD95. The possibility of using piperonyl butoxide to manage permethrin resistance is discussed.
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Meyer et al. (1987) studied this question.