Parasitoid-pathogen-pest interactions of Chelonus insularis, Campoletis sonorensis, and a nucleopolyhedrovirus in Spodoptera frugiperda larvae

In this study we examined interactions between two solitary endoparasitoids , the braconid Chelonus insularis and the ichneumonid Campoletis sonorensis , and a multiple-enveloped nucleopolyhedrovirus infecting Spodoptera frugip erda larvae. We examined whether ovipositing females minimize interference by discriminating amongst hosts and examined the outcome of within-host com petition between parasitoid species and between the parasitoids and the vir us. The egg-larval parasitoid Ch, insularis did not discriminate between vi rus-contaminated and uncontaminated S. frugiperda eggs; all S. frugiperda l arvae that emerged from surface-contaminated eggs died of viral infection p rior to parasitoid emergence. The larval parasitoid C, sonorensis also fail ed to discriminate between healthy and virus-infected S, frugiperda larvae or between larvae unparasitized or parasitized by Ch. insularis. Host larva e parasitized in the egg stage by Ch. insularis were suitable for the devel opment of C. sonorensis when they were multiparasitized by C, sonorensis as first, second, third, and fourth instars, whereas emergence of Ch. insular is was dramatically reduced (by 85 to 100%) in multiparasitized hosts, Nons pecific host mortality was significantly higher in multiparasitized hosts t han in singly parasitized hosts, The development time and sex ratio of C. s onorensis in multiparasitized host larvae were unaffected by the presence o f Ch. insularis larval stages, Both Ch. insularis parasitized and nonparasi tized larvae of the same instar (second, third, or fourth instars) had a si milar quantitative response to a challenge of virus inoculum, All host larv ae that ingested a lethal dose of virus were unsuitable for Ch. insularis d evelopment, In contrast, C. sonorensis did not survive in hosts that ingest ed a lethal virus dose immediately after parasitism, but parasitoid surviva l was possible with a 2-day delay between parasitism and viral infection an d the percentage of parasitoid emergence increased significantly as the int erval between parasitism and viral infection increased, The development tim e of C, sonorensis was significantly reduced in virus-infected hosts compar ed to conspecifics that developed in healthy hosts, C. sonorensis females t hat oviposited in virus-infected hosts did not transmit the virus to health y hosts that were parasitized subsequently. Field applications of virus for biocontrol of S. frugiperda may lead to substantial mortality of immature parasitoids, although field experiments have not yet demonstrated such an e ffect. (C) 2000 Academic Press.