Viral-enhancing activity of various stilbene-derived brighteners for a Spodoptera litura (Lepidoptera: Noctuidae) nucleopolyhedrovirus Shohei Okuno, a, * Jun Takatsuka, a Madoka Nakai, a Satoshi Ototake, b Akio Masui, b and Yasuhisa Kunimi a a Department of Applied Biological Science, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Saiwai, Fuchu, Tokyo 183-8509, Japan b Agro and Fine Chemicals Research Laboratory, Nippon Kayaku Co., Ltd., Koshikiya, Ageo, Saitama 362-0064, Japan Received 26 December 2001; accepted 11 September 2002 Abstract Previous reports have shown that stilbene-derived optical brighteners enhance viral activity in some baculovirus–lepidopteran host systems. To investigate the relationship between chemical structure and enhancement activity, we synthesized 11 stilbene- derived brighteners and compared their enhancing activity with a nucleopolyhedrovirus (NPV) with that of Tinopal UNPA-GX in common cutworm larvae, Spodoptera litura. Tinopal UNPA-GX significantly increased larval susceptibility to NPV infection, and the LD 50 of NPV was reduced about 2200-fold by the addition of 1% Tinopal to the virus suspension. Five of the 11 brighteners enhanced NPV infection to a similar extent as Tinopal, and one brightener enhanced virus infection more effectively than Tinopal. Replacing the diethyl amino group of Tinopal UNPA-GX with other chemical groups changed the enhancing activity of the brighteners. In addition, disruption of the peritrophic membrane (PM) was observed only when the cutworm larvae were given brighteners that had viral-enhancing activity. This suggests that brighteners disrupt the PM that lines the insect midgut and facilitate NPV infection of midgut epithelial cells. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Spodoptera litura; Nucleopolyhedrovirus; Peritrophic membrane; Viral enhancement; Stilbene-derived brighteners 1. Introduction The common cutworm, Spodoptera litura (Fabricius), is a significant insect pest of many forage crops and vegetables in Japan, China, and Southeast Asia. Bacul- oviruses are safe and selective biological control agents because they have little negative impact on non-target organisms and the environment. However, their slow action and high production costs are factors that limit the commercialization of these insect viruses as biopes- ticides. Previous reports have shown that stilbene-derived optical brighteners increase the susceptibility of eco- nomically important lepidopteran pests to viral infec- tions and reduce the larval survival time (Hamm and Shapiro, 1992; Li and Otvos, 1999a,b; Shapiro and Robertson, 1992; Shapiro and Vaughn, 1995; Vail et al., 1996; Zou and Young, 1994, 1996). In addition, optical brighteners protect the virus from solar radiation. Since optical brighteners absorb UV light and emit visible blue light, a coating of optical brighteners on viral occlusion bodies (OBs) absorbs UV light, thereby protecting the virus from inactivation (Doughtery et al., 1996; Morales et al., 2001; Shapiro, 1992). The enhancing effects of optical brighteners are well documented for several host-virus systems. The in- creased viral efficacy caused by optical brighteners has the potential to make baculoviruses commercially fea- sible. However, the mechanism of viral enhancement has not been elucidated. To date, two different modes of action have been proposed: the optical brightener may increase virus infection by preventing the sloughing of virus-infected midgut epithelial cells (Washburn et al., 1998) and/or preventing formation of, or severely Biological Control 26 (2003) 146–152 www.elsevier.com/locate/ybcon * Corresponding author. Fax: +81-423-67-5696. E-mail address: sokuno@cc.tuat.ac.jp (S. Okuno). 1049-9644/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII:S1049-9644(02)00122-6