Mode of action of pyriproxyfen and methoprene on eggs of Ctenocephalides felis (Siphonaptera: Pulicidae).

Adult cat fleas were exposed to residues of pyriproxyfen and methoprene in glass vials, then fed on a cat 24 h later to investigate the mode of action of juvenoid growth regulators on embryonic development in flea eggs. Eggs laid by pyriproxyfen-treated fleas within 70 h after exposure to this juvenoid were often devoid of yolk and frequently collapsed after oviposition. Minimal amounts of yolk were deposited in eggs laid after 70 h, and no blastoderm was formed. These results are significant because both modes of action were different than those observed earlier by investigators studying ovicidal effects in adult insects treated with juvenile hormone. In contrast to the pyriproxyfen results, eggs laid by methoprene-treated fleas showed no gross morphological effects, and these eggs remained turgid during embryogenesis. However, the eggs either did not hatch or the larvae died within hours after hatching. Histological examination of the eggs revealed that most of the eggs contained segmented embryos which had apparently died during blastokinesis. Although eggs of some insects exposed to juvenile hormone during oogenesis fail to undergo germ band formation, there was no evidence of this effect in methoprene-treated cat fleas.