Effect of lufenuron on chorionic and cuticular structure of unhatched larval Ctenocephalides felis (Siphonaptera : Pulicidae)

When adult cat fleas, Ctenocephalides felis (Bouche), were fed concentratio ns of less than or equal to 0.08 ppm lufenuron in cattle blood, egg hatch d id not differ significantly from the controls. However, as the concentratio n of lufenuron in blood increased from 0.125 to 1.0 ppm egg hatch decreased to 64 and 2%, respectively. Most of these eggs contained fully developed l arvae. Microscopic examination of unhatched larvae, revealed that the cutic le, epidermal cells, chorion, and vitelline membrane all were affected by l ufenuron treatment. Larvae often produced 2 separate cuticles in response t o treatment. The Ist cuticle consisted of an indistinct layer of epicuticle and a procuticle composed of randomly deposited chitin microfibrils. After the Ist layer of procuticle separated from the epidermal cells, a 2nd laye r of procuticle was deposited. It was not possible to determine whether the egg tooth was functional during larval hatch. The surface of the egg tooth appeared normal, but the cuticle may have had structural abnormalities sim ilar to those seen in other areas of the exoskeleton. Structural defects ap peared to be due to the cytotoxic effects of lufenuron. The epidermal cells of treated larvae showed evidence of disintegration (i.e., the nuclei and mitochondria appeared to be degenerating and the amount of endoplasmic reti culum and other cytoplasmic organelles was decreased). The chorion of lufen uron-treated larvae consisted of an outer layer, middle and inner layers th at were thinner and less electron dense than those of controls, and lacked the innermost chorionic layer found in the control larvae. The vitelline me mbrane also was thinner than that of the controls. Larval hatching was prev ented by ruptures in the cuticle, which opened during eclosion resulting in the loss of hemolymph and desiccation of the larva. Evidently, tearing of the cuticle was caused by abnormal formation of the procuticle that was not strong enough to withstand the cuticular expansion and muscular movement o f the larva within the egg shell.