Dynamics of Azinphosmethyl Resistance in Epiphyas postvittana (Lepidoptera: Tortricidae)

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Abstract
Three insecticide application methods (topical application, direct spray, and diet incorporation) were used to estimate concentration-mortality regressions for azinphosmethyl resistant (R) and susceptible (S), and F1 Epiphyas postvittana (Walker) larvae. The three techniques yielded different estimates of dominance. Topical application of mid-instar F1 larvae gave a dominant response with a degree of dominance of +0.61. Direct spray of insecticide onto first instars gave intermediate responses for the F1 generation, with a degree of dominance of -0.09 and +0.14 in tests on the same strains two generations apart. Backcross and F2 larvae showed greater susceptibility than F1 larvae, although the resistance was too low to allow Mendelian analysis. Concentration-mortality lines were estimated for 8-d-old larvae exposed to azinphosmethyl in artificial diet for 3 wk: the degree of dominance was +0.31. Tethered virgin female moths from the susceptible laboratory strain were placed within apple orchards in which E. postvittana resistant to azinphosmethyl were present, and they were allowed to mate at will with local males. Larval offspring were exposed to diagnostic concentrations that cause 100% mortality of susceptible larvae to determine the phenotypic frequency of resistance. The resistance phenotype frequency was >85%, and gene flow between resistant and susceptible individuals was demonstrated to have occurred in the field. Males from five replicate strains yielding survivors were backcrossed to a susceptible strain, and the offspring were again selected on artificial diet containing azinphosmethyl. Selection and backcrossing for five or six generations did not result in a decrease in the resistance with either direct spray or diet incorporation techniques. Monogenic inheritance could not be proven from the data, although no conflict with this hypothesis was detected.

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