DOSE-DEPENDENT SEPARATION OF COTESIA CONGREGATA-ASSOCIATED POLYDNAVIRUS EFFECTS ON MANDUCA-SEXTA LARVAL DEVELOPMENT AND IMMUNITY

Cotesia congregata eggs were dissected from the reproductive tracts of adult female wasps, washed extensively to remove polydnavirus particles, and injected into fourth instar Manduca sexta larvae. Parasites developed and emerged when washed eggs were recombined with polydnavirus-containing filtered calyx fluid, indicating that the washed eggs were viable. In contrast, washed eggs injected alone invariably were encapsulated. Larvae injected with washed eggs grew abnormally large, wandered late, and rarely pupated successfully, indicating that even though the parasites failed to develop, neither did the host. We attribute this phenomenon to effects caused by a trace of polydnavirus (PDV) remaining associated with the washed eggs, as detected by results obtained using Western slot blots with polyclonal anti-virus antibodies used to detect the PDV. Injecting small amounts (0.01-0.001 wasp equivalents) of filtered calyx fluid alone caused similar disruptive effects on larval development, suggesting that the effects on development are likely to be polydnavirus-mediated. These results show that progressive dilution of filtered calyx fluid separates the effects of polydnavirus on host immunity and development. While relatively large dosages of filtered calyx fluid appeared essential for blocking encapsulation of parasitoid eggs, as little as 0.001 wasp-equivalents of PDV was sufficient to alter larval appearance and behaviour, and to arrest development prior to or during the larval-pupal transition at metamorphosis. Thus, very small doses of virus disrupt development many days following PDV injection, while relatively larger doses are needed to block encapsulation within a short time-frame following parasitization. The mechanisms by which the polydnavirus acts to disrupt development may involve the brain-neurosecretory axis or factors affecting regulation of the JH titer.