Nested PCR amplification of a Spodoptera litura nucleopolyhedrovirus (SpltNPV) lef-8 gene fragment was performed on egg masses and larvae of S. litura, and resulted in detection of a latent SpltNPV in 20.0% of "healthy" laboratory stock samples, and in 22.6% of samples collected at different periods and in remote places in Kagoshima Prefecture. The PCR product sequences showed 99% similarity to the published lef-8 sequence of SpltNPV, confirming that the amplification products were derived from SpltNPV. Further, cross-inoculation of laboratory stock S. litura larvae with Mythimna separata NPV (MyseNPV) activated the latent virus, which provoked a change in the virulence of MyseNPV samples collected from cadavers of S. litura. Our data suggest that caution should be taken when using S. litura as a host to produce heterologous NPVs.

We compared the infectivity of two nucleopolyhedroviruses (NPVs), MyseNPV G isolated from Mythimna separata (Walker) (Lepidoptera: Noctuidae) and SpltNPV S isolated from Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae). MyseNPV G was more pathogenic against M. separata than against S. litura. Although SpltNPV S was more pathogenic than MyseNPV G against S. litura, it did not infect M. separata. Restriction endonuclease (REN) analysis of viral genomic DNA revealed that the two NPVs have quite different REN profiles. Based on nucleotide sequences of the coding regions of polyhedrin, lef-8 and lef-9, SpltNPV S was closely related to other SpltNPV isolates, whereas MyseNPV G appeared to belong to the Mamestra NPV group, and was distinct from a Chinese isolate of Leucania (=Mythimna) separata NPV. The potential of MyseNPV G and SpltNPV S to control pest insects is discussed.