Polydnavirus of Microplitis croceipes prolongs the larval period and changes hemolymph protein content of the host, Heliothis virescens

Polyhednavirus from certain parasitoid Hymenoptera have been reported to interfere with both host immunity and host development. Heliothis virescens larvae in injected with either calyx fluid or sucrose gradient-puritied polydnavirus from Microplitis croceipes (McPDV) gained less weight than saline-injected larvae. The active feeding portion of the fifth stadium larva (time to reach the burrowing-digging stage) was doubled (7.0 vs. 3.4 days) when a 0.25 wasp equivalent (WE) of sucrose gradient-purified McPDV was injected into a newly ecdysed fifth stadium host. Many of the treated larvae were unable to pupate successfully and died at a point of incomplete larval-pupal ecdysis. Pupae that did result from the treated larvae weighed significantly less than controls, even at 0.025 WE. The rate of weight gain and extent of delay of development were dose-dependent: as little as 0.1 WE extended the time of active feeding by 1.5 days and yielded only 25% adults. A 0.05 WE dose yielded 78% adults compared to 95% for controls. The total protein content of hemolymph from individuals injected with McPDV was significantly less than that of controls at any McPDV dose equal to or greater than 0.1 WE. SDS-PAGE profiles of hemolymph proteins from control and McPDV-injected larvae revealed a marked inhibition of the normal accumulation of storage proteins during the fifth stadium and a lesser reduction of serine protease inhibitor protein. Thus. McPDV-injected larvae exhibited some symptoms (less total hemolymph protein and reduced amounts of storage protein) similar to those shown by both parasitized larvae and by larvae injected with M. croceipes tertocytes. However, McPDV affected development during the active feeding stage of the larva, while tertocytes primarily impacted larvae at the time when larval-pupal transformation processes are initiated.