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

When adult cat fleas, Ctenocephalides felis (Bouché), were fed concentrations of < or = 0.08 ppm lufenuron in cattle blood, egg hatch did not differ significantly from the controls. However, as the concentration 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 larvae. Microscopic examination of unhatched larvae, revealed that the cuticle epidermal cells, chorion, and vitelline membrane all were affected by lufenuron treatment. Larvae often produced 2 separate cuticles in response to treatment. The 1st cuticle consisted of an indistinct layer of epicuticle and a procuticle composed of randomly deposited chitin microfibrils. After the 1st layer of procuticle separated from the epidermal cells, a 2nd layer 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 similar to those seen in other areas of the exoskeleton. Structural defects appeared 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 reticulum and other cytoplasmic organelles was decreased). The chorion of lufenuron-treated larvae consisted of an outer layer, middle and inner layers that were thinner and less electron dense than those of controls, and lacked the innermost chorionic layer found in the control larvae. The vitelline membrane also was thinner than that of the controls. Larval hatching was prevented 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 of the larva within the egg shell.