Synergism of the IGRs Methoprene and Pyriproxyfen Against Larval Cat Fleas (Siphonaptera: Pulicidae)

Insect growth regulators (IGRs) methoprene and pyriproxyfen are widely used as topical treatments to pets or applied to the indoor environment to control cat fleas, Ctenocephalides felis (Bouché). The toxicity of methoprene, pyriproxyfen, and combinations of both IGRs to cat flea larvae was determined. The LC₅₀ of methoprene and pyriproxyfen applied to larval rearing medium was 0.39 and 0.19 ppm, respectively. Combinations of methoprene:pyriproxyfen in ratios of 1:1, 5:1, 10:1, and 20:1 produced LC₅₀s of 0.06, 0.09, 0.19, and 0.13 ppm, respectively. The pyriproxyfen synergized the activity of methoprene as indicated by the combination indices (CI). The ratio of methoprene:pyriproxyfen (40:1) provided an LC₅₀ of 0.42 ppm and the pyriproxyfen was not synergistic. Combinations of pyriproxyfen:methoprene in ratios of 5:1, 10:1, and 20:1 provided LC₅₀s of 0.14, 0.20, 0.20 ppm, respectively, and the methoprene did not synergize the activity of pyriproxyfen. The dose-reduction indices (DRIs) indicated that the concentrations of IGRs in the combinations of methoprene:pyriproxyfen (ratios of 20:1 or less) could be reduced by at least one-third of the amount required by methoprene alone to provide similar larval mortality. Combinations of methoprene and pyriproxyfen may be effective in increasing the residual activity on pets and assist in reducing the likelihood of insecticide resistance developing to IGRs.