MODE OF ACTION OF LUFENURON ON LARVAL CAT FLEAS (SIPHONAPTERA, PULICIDAE)

Adult cat fleas, Ctenocephalides felis (Bouche), were fed suboptimal i n vitro concentrations of lufenuron in blood to allow hatching of flea larvae for cytological study. At concentrations of 0.125, 0.25, and 0 .5 ppm, larval hatch was 64, 15, and 4%, respectively. Larvae hatching from eggs laid by adults fed lufenuron at concentrations of 0.025, 0. 08, or 0.125 ppm did not differ significantly from the control. Howeve r, many larvae from the 0.08-ppm group and higher concentrations died during the 1st instar. Examination of these larvae revealed that they were dying from desiccation caused by bleeding from microscopic lesion s in the cuticle or the inability to complete the molt to the nest ins tar. Electron micrographs showed that lufenuron often disrupted format ion of the endocuticle resulting in the deposition of ari amorphous ma ss of randomly oriented chitin microfibrils. Other larvae formed norma l endocuticle but were unable to digest the old endocuticle or produce new procuticle after apolysis. Failure of larvae to digest old cuticl e or form new cuticle was caused by degeneration of the epidermal cell s needed for the synthesis of molting fluid and chitin.