Stadium-specific degeneration of host prothoracic glands by Campoletis sonorensis calyx fluid and its association with host ecdysteroid titers

Abstract

Campoletis sonorensis is a polydnavirus-carrying wasp that parasitizes Heliothis virescens larvae. Previous studies indicated that C. sonorensis polydnavirus (CsPDV) induces degeneration of host prothoracic glands when injected into last-stadium hosts. Degeneration also occurs when last instars are naturally parasitized, but it is not observed in hosts parasitized as penultimate instars. This study investigated the effect of CsPDV on the development of penultimate-stadium hosts and the temporal sensitivity of H. virescens prothoracic glands to virally-induced degeneration. We found that calyx fluid (containing CsPDV) caused degeneration of host prothoracic glands when injected into newly-ecdysed, penultimate-stadium hosts, but degeneration was delayed until injected larvae reached the critical head capsule size for the last stadium, even when injected larvae exhibited supernumerary molts. Host prothoracic glands became sensitive to virally-induced gland degeneration just prior to apolysis to the last larval larval molt , simultaneous with an ecdysteroid peak. Prothoracic gland degeneration in response to calyx fluid was also noted in two other hosts of C. sonorensis, Helicoverpa zea and Spodoptera exiguae.