Incomplete Host Developmental Arrest Induced by Parasitism of Manduca sexta1 Larvae by Apanteles smerinthi2

Abstract

Observations of Manduca sexta (L.) larvae parasitized by Apanteles smerinthi Riley indicated that parasitism induced only partial host developmental arrest, since host molting was suppressed during parasitoid emergence but not thereafter. After parasitization of first-stage M. sexta larvae, parasitoid emergence usually occurred during the host's fourth larval stage. Two to six days later, host larvae either underwent apolysis in preparation for larval molting or exposed the dorsal vessel; in both cases, the larvae later formed nonviable larval-pupal intermediates. In many hosts, cuticle was secreted over the surface of partly emerged parasitoids. The occurrence of postemergence molting suggests that the host's ecdysteroid titer rises after emergence, in contrast to the low ecdysteroid titers measured after emergence in larvae parasitized by A. congregatus (Say), a highly co-evolved parasitoid. Together with the finding that few A. smerinthi larvae that emerged spun cocoons and eclosed as adults, the observed anomalous effects of parasitism were interpreted as indicative that this species is poorly adapted to using M. sexta as its host even though the parasitoid frequently escapes encapsulation by host hemocytes. This report is the first description of postemergence molting in larval hosts of endoparasitic Hymenoptera.