Development of insecticide resistance in Piophila casei (Diptera: Piophilidae) strains selected with low doses of deltamethrin.

Abstract

Two populations of the cheese skipper, Piophila casei (L.), were sampled. The 1st was from a sheep farm where no chemical was ever applied; the 2nd was present in a ham factory where chemical treatment with pyrethroids was applied in the past against house flies, Musca domestica L., and Dermestid beetles. A substantial difference between their resistance to deltamethrin was observed (LC50 = 11.56 versus 68.08 micrograms/cm2 for females and 1.11 versus 4.20 micrograms/cm2 for males, respectively). Laboratory strains were established from both populations and were selected at constant rates for up to 20 generations (2, 4, and 7.3 ppm for the strains derived from the 1st population and 40 ppm for that selected from the 2nd population). Males and females of both strains showed an increase in LC50 (tested at the 5th and 19th or 10th generation), except when females were selected at the lowest rate (2 ppm). At the end of the trial, slopes of logit regressions were substantially steeper in strains selected at higher rates, suggesting that the effect of the insecticide was to reduce resistance variance. Crosses between unselected and selected strains were done. Results of survival analysis in F1 hybrids were analogous for males and females and were similar in both reciprocal crosses. LC50S were intermediate between those of the parental strains.