Effect of parasitism by Cotesia congregata on the susceptibility of Manduca sexta larvae to bacterial infection.

Eggs and larvae of the braconid Cotesia congregata are not encapsulated within the hemocoel of their habitual host, Manduca sexta. Experiments were performed to evaluate the status of antibacterial defensive responses in M. sexta larvae parasitized by this gregarious endoparasitoid. Previous investigations have shown that immunologically naive, nonparasitized M. sexta larvae are resistant to infection by Pseudomonas aeruginosa. Studies using naive, parasitized larvae demonstrated that inoculation with 10 colony forming units of P. aeruginosa 9027, one hour after oviposition, resulted in greater than 90% host mortality. Examination of the fate of P. aeruginosa and Escherichia coli injected into parasitized larvae suggested that the cellular antibacterial defenses of the host, nodule formation, and phagocytosis, were impaired.