Pathology and Properties of the Tetravirus Helicoverpa armigera Stunt Virus Peter D. Christian, 1 Susan J. Dorrian, Karl H. J. Gordon, and Terry N. Hanzlik CSIRO Entomology, P.O. Box 1700, Canberra, ACT 2601, Australia Received May 11, 2000; accepted September 29, 2000; published online December 12, 2000 A quantitative study of the pathogenicity of Heli- coverpa armigera stunt virus (HaSV) (Tetraviridae) isolates toward larvae of several heliothine species was conducted along with studies on the stability of the virus to a variety of chemical, enzymic, and tem- perature treatments. Surface contamination bioas- says of several HaSV isolates against H. armigera produced 50% effective concentration (EC 50 ) esti- mates ranging between 568 and 9244 virus particles (vp)/mm 2 . Against mid 1st instar larvae of H. ar- migera, H. punctigera, and Heliothis punctifera, EC 50 estimates for one isolate were 1288, 16,137, and 2667 vp/mm 2 , respectively. The virulence of HaSV infection varied markedly with the age at which larvae were exposed to the virus. Presentation of the virus to the first three instars of H. armigera was accompanied by cessation of feeding, growth retar- dation, and eventual lethality, whereas no adverse effects were observed when later instars were ex- posed to the virus, even at very high concentrations. Active HaSV was recovered from frass of larvae ex- posed to the virus as 1st instars. Household bleach (1% v/v; 0.04% w/v available chlorine, 0.004% w/v NaOH), formaldehyde (1% w/v), and temperatures >65°C completely inactivated HaSV in suspension. Treatments with ether, proteinase K (1 mg/ml), H. armigera gut contents, and temperatures between 22 and 55°C partially inactivated virus activity. No observable inactivation was observed after treat- ment with chloroform, chymotrypsin (1 mg/ml), tryp- sin (1 mg/ml), or RNase A (1 mg/ml). The virus was stable between pH 2.8 and pH 10.0 with around 60% loss of activity observed at pH 11.4. The pattern of pathogenic effects seen in several other insect spe- cies challenged by high concentrations of HaSV in- dicated that the host range of the virus is limited to species within the lepidopteran family Noctuidae. The apparently restricted host range of HaSV along with a number of other features indicate that this virus has considerable potential for the develop- ment of novel control agents for use against helioth- ine pests. © 2000 Academic Press Key Words: Helicoverpa armigera stunt virus; tetra- virus; Helicoverpa armigera; Helicoverpa punctigera; Heliothis punctifera; virus pathogenicity; virus sta- bility. INTRODUCTION Helicoverpa armigera stunt virus (HaSV) is one of only three small (40 nm in diameter) nonenveloped RNA-containing viruses isolated from a species of the Heliothinae (Lepidoptera: Noctuidae) (Rubinstein, 1979; Scotti et al., 1981; Hanzlik et al., 1993). It was first detected in a laboratory culture of Helicoverpa armigera (Hu ¨ bner) in Australia but has been detected in field populations and laboratory colonies elsewhere (our unpublished observations). The virus’s nominal host makes it of particular interest to agricultural en- tomologists and biotechnologists, as this species and other heliothines are pests of paramount importance in many of the world’s major agroecosystems. These pest species include H. armigera (distributed throughout Africa, Asia, and Australasia), Helicoverpa zea (Bod- die) and Heliothis virescens (Fabricius) (the New World), and Helicoverpa punctigera (Wallengren) (Aus- tralasia). Over the recent past much interest has fo- cused upon exploiting the unique properties of these simple viruses for controlling heliothine pests and in the design of novel pest control strategies (Christian et al., 1992; Hanzlik and Gordon, 1997). HaSV is one of the best characterized members of Tetraviridae, a family of viruses isolated exclusively from lepidopterous insects. The family comprises around 10 virus species distributed in two genera (Murphy et al., 1995; Hanzlik and Gordon, 1997). HaSV has a bipartite, positive-sense, RNA genome and belongs to the Nudaurelia capensis omega ()-like ge- nus of the family. The 5.3-kb RNA1 encodes the 187- kDa replicase (Gordon et al., 1995) and the 2.5-kb RNA2 encodes the coat protein precursor and a 17-kDa protein of unknown function (Hanzlik et al., 1995). The 1 To whom correspondence should be addressed. Fax: 61-2- 62464173. E-mail: Peter.Christian@ento.csiro.au. Biological Control 20, 65–75 (2001) doi:10.1006/bcon.2000.0887, available online at http://www.idealibrary.com on 65 1049-9644/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved.