Behavior and Survival in Corn Plants of Ostrinia nubilalis (Lepidoptera: Pyralidae) Larvae When Infected with Nosema pyrausta (Microspora: Nosematidae) and Parasitized by Macrocentrus grandii (Hymenoptera: Braconidae)

Abstract

To clarify the interaction among the European corn borer, Ostrinia nubilalis , the parasitic wasp, Macrocentrus grandii , and the entomopathogen Nosema pyrausta , we examined behavior and survival in corn stems of O. nubilalis larvae infected with N. pyrausta and parasitized by M. grandii. Fifty-four percent of heavily infected, parasitized O. nubilalis larvae emigrated from their tunnels before emergence of parasitoid larvae and did not reestablish on corn plants. One-third as many heavily infected host larvae as noninfected larvae remained in tunnels until parasitoid larval emergence. The tunnels bored by infected, parasitized O. nubilalis larvae did not differ in length from those of noninfected parasitized larvae, suggesting that emigration behavioral changes occur after feeding. Parasitoid larval and pupal mortalities were greater in infected O. nubilalis hosts than in noninfected hosts. Greater than 46% of heavily infected M. grandii broods wandered instead of remaining clustered around host carcasses and pupating. This behavior accounted for almost ≈40%of the observed mortality in infected parasitoids. Wandering was attributed to decreased production of cocoons by hosts, causing parasitoid larvae to search for suitable substrates on which to spin their own cocoons. Percentage emergence of M. grandii adults from corn stalks was significantly lower in N. pyrausta -infected treatments, but the sex ratio of the parasitoid was unaffected by the level of host infection. The mean number of M. grandii adults per brood was reduced from 39.8 in the noninfected treatment to 10.5 in heavily infected larvae.