Journal of Insect Physiology 49 (2003) 483–489 www.elsevier.com/locate/jinsphys Overt viral diseases induced from apparent latency following parasitization by the ichneumonid wasp, Hyposoter exiguae D. Stoltz * , A. Makkay Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7 Received 15 July 2002; accepted 29 November 2002 Abstract Latent and/or asymptomatic viral infections are commonplace in insects, but factors inducing overt disease are poorly understood. Here we show that in Trichoplusia ni larvae parasitized by the ichneumonid wasp, Hyposoter exiguae, overt, lethal disease may on occasion be observed; however, disease has been consistently absent in control (non-parasitized) larvae. Thus far, we have detected two such diseases, one of which is caused by a granulosis virus affecting primarily fat body tissue. The other is associated with the presence of two viruses replicating together in larval midgut epithelial cells; of these, one has been identified as a non- occluded form of TnCPV. Since H. exiguae carries a polydnavirus, which is delivered to host larvae during oviposition, it is tempting to speculate that viral latency may in some cases be broken through immunosuppressive activity resulting from insect parasitism.  2003 Elsevier Science Ltd. All rights reserved. Keywords: Viral latency; Insect parasitism; Polydnaviruses 1. Introduction During the summer months of 1999 and 2000, an unexpected level of mortality was observed in Tricho- plusia ni larvae used in our laboratory as hosts for the ichneumonid parasitoid, Hyposoter exiguae. Mortality was associated with specific symptomatologies, which differed in the two types of infection observed; these diseases were not detected in 1998, nor have they appeared since the year 2000. Here we have used elec- tron microscopy to identify these diseases as viral, present a preliminary characterization of one of the agents involved, and discuss the broader significance of our observations. 2. Materials and methods 2.1. Insects H. exiguae wasps were reared largely as described (Krell and Stoltz, 1980) on T. ni larvae provided by the * Corresponding author. Tel.: +1-902-494-2590; fax: +1-902-494- 5125. E-mail address: dstoltz@is.dal.ca (D. Stoltz). 0022-1910/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0022-1910(03)00050-7 Insect Production Facility of the Great Lakes Forestry Centre, Sault Ste. Marie, Ontario. T. ni were initially provided as 1st instar larvae on a spruce budworm diet, and later switched to a high wheat germ diet originally designed for gypsy moth rearing (Bell et al., 1981). Upon reaching the 3rd or 4th stadium, larvae were trans- ferred to modified mouse cages equipped with screened ventilation holes on the sides, and lids made of glass plates. Parasitization was initiated by the addition of sev- eral mated H. exiguae females per 40–50 4th/5th instar larvae; larvae were typically exposed to wasp females for a period of 24 h. For some of the work reported here, host larvae were parasitized individually, by visual observation of two successive oviposition events, and then reared in small numbers in petri dishes, or else indi- vidually. 2.2. Electron microscopy Tissues to be processed for thin-sectioning were dis- sected out into the primary fixative (3% glutaraldehyde in 50 mM sodium cacodylate buffer containing 250 mM sucrose), and left at RT for 1 h. Postfixation was for 2 h in 2% OsO 4 in the same buffer, followed by overnight staining in 0.1% aqueous uranyl acetate. Embedment