J. HYM. RES. 1(1), 1992 pp. 125-139 Viruses and Virus-like Entities in the Parasitic Hymenoptera Don Stoltz and James B. Whitfield (DS) Department of Microbiology and Immunology, Dalhousie University, Halifax, NS, Canada B3H 4H7; (JBW) Depart-ment of Biology, University of Missouri, St. Louis, MO, 63121-4499, U.S.A. 1 Abstract. — We provide here an overview of viruses and virus-like agents affecting the parasitic Hymenoptera. An amazingly complex variety of such agents is now known. By far the majority are non-pathogenic, replicating primarily in the female reproductive tract, from which they are often if not invariably delivered into host insects during oviposition; most, in fact, would appear to be beneficial to the parasitoids in which they are found. Emphasis is necessarily placed on the better known polydnaviruses, which are carried by perhaps tens of thousands of braconid and ichneumonid species. Polydna virus DNA appears to be permanently integrated into parasitoid genomes, thereby ensuring transmission to all progeny; in keeping with this observation, these, where present, are known to be required for successful parasitism. Relationships among the polydnaviruses are discussed in terms of their possible relevance to parasitoid phylogeny and classification. Like other insects, the parasitic Hymenoptera must presumably from time to time be subject to infectious viral disease. There are, to be sure, nu-merous opportunities. For example, viruses could readily spread horizontally between larval cohorts of those species which are either gregarious or polyembryonic. The host itself could represent a source of infectious virus for developing parasi-toid larvae, and possibly as well for adult wasps feeding upon hemolymph exuded from oviposi-tion sites. Finally, infectious disease agents could be transmitted mechanically via contaminated ovi-positors of hyperparasites. Yet, oddly enough, vi-ral diseases in the parasitic Hymenoptera are seemingly rare. Reasons for this are not immedi-ately evident. Conceivably, many parasitoids suc-cumb to disease prior to emergence from their hosts; such events might easily go undetected, even in a laboratory situation. A SURVEY In fact, we are aware of only one example of a recognized viral pathogen of parasitoids: a re-cently described baculovirus from the braconid, Microplitis croceipes (Cresson) (Hamm et al. 1988); thus far, replication has been observed only in fat body tissue. A baculovirus has also been described from the ichneumonid parasitoid, Mesoleius tenthredinis (Morley) (Stoltz et al. 1981). Like most of the viruses and virus-like agents that are de-1 Current address (JBW): Department of Entomology, University of Arkansas, Fayetteville, AK 72701, U.S.A. scribed below, this virus appears to replicate only in the female reproductive tract and /or its acces-sory glands; like all of the agents described below, the Mesoleius baculovirus is apparently non-pathogenic (at least for the parasitoid host). The best-known of the parasitoid-associated viruses are the polydnaviruses, which now con-stitute a recognized virus family (Polydnaviridae) within which 2 groups are recognized, namely the bracoviruses and ichnoviruses, which differ in terms of both morphology and host range; the polydnaviruses will be subsequently examined in considerable depth, and so are not discussed fur-ther here. Emerging as potentially a second virus family is a growing assemblage of entities having as a distinct common morphology a long, filamen-tous, enveloped nucleocapsid. Examples of these viruses are known from both braconid (Stoltz and Vinson 1977 and 1979a; Styer et al. 1987; Hamm et al. 1990;) and ichneumonid (Krell 1987) parasitoids; one has been shown to replicate in host tissues (Styer et al. 1987). In terms of nucleocapsid mor-phology, the filamentous virus observed in the braconid Cotesia congregata (Say) (Stoltz and Vinson 1977, 1979a) very closely resembles a filamentous virus from the tsetse fly Glossina pallidipes Austen (Odindo et al. 1986). It should be noted that a somewhat similar virus has been described from honey bees (Clark 1978; Bailey et al. 1981). A variety of other viruses (again all non-pathogenic) have been reported from parasitoid species (Table 1). Stoltz et al. (1988a) describe an unusual virus (designated CmV2) which appar-ently replicates in all individuals of certain Cotesia
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