Effects of parasitism by Cotesia congregata on the antibacterial responses of the tobacco hornworm, Manduca sexta

Donald Richard Ross, Purdue University

Abstract

The survival of the braconid endoparasitoid, Cotesia congregata (Say), in larvae of the tobacco hornworm, Manduca sexta (Johannson), is facilitated by a suppression of the host's cellular encapsulation response. The effect of this parasitism-induced immunosuppression on the cellular and humoral antibacterial defenses of the host was examined. Initial studies using immunologically "naive", parasitized larvae demonstrated that injection of as few as 10 viable Pseudomonas aeruginosa into host larvae, one hour to 10 days after oviposition, resulted in greater than 90% host mortality. Clearance kinetics of viable P. aeruginosa and Escherichia coli from the hemocoel of "naive", parasitized larvae suggested that the host's cellular defenses, nodule formation and phagocytosis, were impaired. Hornworm larvae injected with formalin-killed E. coli cells before and immediately after oviposition were assayed for susceptibility to P. aeruginosa. The results demonstrated that vaccination significantly increased host resistance to this bacterium. However, larvae vaccinated prior to oviposition were substantially more resistant to challenge doses than were larvae vaccinated after oviposition; suggesting that parasitism interfered with the acquisition but not the maintenance of an "immune" state. Injection of formalin-killed E. coli into parasitized larvae resulted in a significant increase in the titers of lysozyme, bactericidal, and bacteriostatic activities in serum. However, while the titer of bacteriostatic activity was not affected by parasitism the titers of lysozyme and bactericidal activities were substantially higher in serum from vaccinated nonparasitized larvae. In addition, parasitism resulted in quantitative and qualitative changes in the synthesis of the "A" and "B" families of cecropin-like bactericidal proteins. Hemolymph, homogenates, and in vitro culture of parasitoid larvae demonstrated that they neither contain nor secrete detectable levels of antibacterial factors. Studies of hemocytes from "naive" and vaccinated host larvae demonstrated that parasitism resulted in an immediate and significant reduction in both the number, and viability of circulating hemocytes. Additionally, within hours after oviposition, plasmatocytes and granulocytes became structurally and functionally transformed, and oenocytoids became more susceptible to lysis.

Degree

Ph.D.

Advisors

Dunn, Purdue University.

Subject Area

Entomology

COinS