Propagation of multicapsid nuclear polyhedrosis virus of Orgyia pseudotsugata in larvae of Trichoplusia ni

Abstract

The multicapsid nuclear polyhedrosis virus (OpMNPV) of Orgyia pseudotsugata (Lepidoptera: Lymantriidae) was adapted by serial passage to a substitute host, Trichoplusia ni (Lepidoptera: Noctuidae). During the first five virus passages, we found only localized lesions in fat body and epidermis of infected T. ni larvae. Disseminated lesions were observed in the sixth passage. Systemic fat body, tracheal, and epidermal lesions typical of nuclear polyhedrosis appeared in the seventh passage. The adapted virus (OpMNPV-Tn) maintained the high virulence of OpMNPV for larvae of the natural host, O. pseudotsugata. After one reverse passage in O. pseudotsugata, the virulence of OpMNPV-Tn for the natural host increased to tenfold that of technical-grade OpMNPV. The reverse-passaged virus had the highest activity in O. pseudotsugata thus far observed in our laboratory. The DNA profile of OpMNPV by five restriction endonucleases was maintained upon passage in T. ni. Because T. ni has many advantages over the natural host for propagation of OpMNPV, we performed several small-scale virus production trials and computed average virus yields. Presently, the average yield of OpMNPV per T. ni larva is lower than expected for larvae of this size. The substitute host, however, offers a continuous and abundant supply of nondiapausing eggs and has a very brief life cycle. For large-scale production, these advantages outweigh the reduced virus yield.