Hormonal basis for developmental arrest of last-instar Heliothis virescens larvae by Campoletis sonorensis polydnavirus

Abstract

Campoletis sonorensis is an ichneumonid endoparasitoid of Heliothis virescens larvae. The polydnavirus which occurs in the reproductive tract of C. sonorensis (CsV) has several effects on the host independent of the presence of parasitoid eggs or larvae. The present study found that CsV disrupted the development of last-instar H. virescens larvae by suppressing host ecdysteroid titers. When injected in large doses, CsV caused total degeneration of the glands which was associated with the permanent suppression of host hemolymph ecdysteroid titers. Total gland degeneration, therefore, prevented pupation by the host. When host larvae were injected with lower doses, however, a dose-dependent proportion exhibited only partial degeneration of the prothoracic glands. These hosts were delayed in development and remained in the stage in which they were injected due to a dose-dependent period of suppressed ecdysteroid titers. After the period of suppression, ecdysteroid titers rose to normal levels and host development continued normally. CsV had little, if any, effect on surviving prothoracic gland cells. The number of surviving gland cells per larva was inversely related to both the mean size of the cells and the length of development by the larva after injection. Intact cells apparently compensated for the lost ecdysone production of degenerated cells. However, there seemed to be a maximum rate of production by intact cells, so that fewer cells took longer to elevate the host's ecdysteroid titers to normal levels.