EFFICACY AND TOXICODYNAMICS OF PYRIPROXYFEN AFTER TREATMENT OF INSECTICIDE-SUSCEPTIBLE AND INSECTICIDE-RESISTANT STRAINS OF THE HOUSE-FLY (DIPTERA, MUSCIDAE)

When applied as a treatment of larval rearing medium in laboratory tests, the insect growth regulator (ICR) pyriproxyfen was approximate to 32-fold more effective in reducing the emergence of F-1 adult progeny of a susceptible strain (S) of the house fly, Musca domestica L., than a strain (Rmp) that is highly resistant to the organophosphorus insecticide methyl parathion. Pyriproxyfen did not evoke a response when applied directly to eggs, but it did reduce the numbers of F-1 pupae produced by S or Rmp adults treated by exposure to residues of the IGR on a glass surface; this effect on pupal development was significantly greater in the S strain. Results of studies comparing the in vivo absorption and metabolism of topically applied [C-14]pyriproxyfen by S and Rmp adult females indicated that cuticular penetration of the IGR was reduced and that its metabolic degradation enhanced in the resistant strain. Tests involving the coadministration of metabolic synergists with pyriproxyfen suggested that microsomal oxidases had a major role in this metabolic transformation. Studies with third-stage larvae indicated no differences between the S and Rmp strains in the extent of cuticular penetration, internal accumulation, or excretion of radioactive material, but metabolic degradation of [C-14]pyriproxyfen was enhanced in Rmp larvae. Selection of a substrain of the Rmp colony by treating larval rearing medium with increasing concentrations of pyriproxyfen (0.5-2.0 ppm) in alternating generations led to a substantial increase in tolerance (15-fold) to the IGR after only six treatments, but we observed no appreciable change in cuticular penetration or metabolic degradation of the compound.