Changes in the Virulence of Nuclear Polyhedrosis Viruses when Propagated in Alternate Noctuid (Lepidoptera: Noctuidae) Cell Lines and Hosts1

Abstract

Nuclear polyhedrosis viruses isolated from the alfalfa looper, Autographa californica (Speyer) (AcMNPV), the cabbage looper, Trichoplusia ni (Hubner) (TnMNPV), Rachiplusia ou (Guenee) (RoMNPV), Heliothis armigera (Hubner) (HaMNPV), and a plaque purified variant of A. californica (Ac6RMNPV), were serially passed numerous times in larvae of the cabbage looper and the beet army worm, Spodoptera exigua (Hubner). These viruses were also produced in vitro in the SF-21AE cell line from the fall army worm, Spodoptera frugiperda (J. E. Smith), and in the TN-368 cell line from T. ni . After passage in susceptible alternate in vivo and in vitro systems, all of the viruses were assayed in both neonate T. ni and S. exigua larvae to evaluate the effects on the viruses. The most significant increase in virulence for viruses produced in vivo occurred in HaMNPV, where a 12.5-fold increase occurred to neonate T. ni at the LC50after three passages in T. ni larvae after 12 initial passages in S. exigua larvae. A 14.9-fold increase in virulence of this virus at the LC50 for neonate S. exigua also occurred. Viruses obtained from the first and fifth passages in vitro exhibited the greatest variation in virulence. Virulence of HaMNPV to neonate T. ni when produced in the SF-21AE cell line was 9.4 x 106-foldgreater at the LC50 for the fifth passage virus than for the first passage virus in this cell line. The insect from which the hemolymph containing the nonoccluded virus (NOV) was taken also affected virulence of the virus produced in vitro.