Virulence and Multigeneration Passage of a Nuclear Polyhedrosis Virus Selected for an Increased Rate of Vertical Transmission

A strain of Spodoptera frugiperda (J. E. Smith) nuclear polyhedrosis virus (NPV) was selected in previous research for an increased rate of vertical transmission by isolating it from pupae that were infected through vertical transmission. The median lethal concentrations (LC(50)s) of the wild and selected NPVs in first instar S. frugiperda were 2.0 and 4.0 polyhedral inclusion bodies (PIB)/insect (significantly different at P < 0.05), respectively, and in fifth instars the LC(50)s were 4.18 X 10(4) and 4.45 X 10(4) PIB/mm(2) diet surface, respectively. The stability of the vertical transmission was studied by feeding fifth instars the LC(50)s of each NPV and then rearing the survivors for five to seven generations without further exposure to virus. The percentage infection by the wild NPV declined from 13.8% of the insects in the first generation after exposure to 3.9% in the fifth generation, whereas that by the selected NPV declined from 18.8% in the first generation to 12.2% in the fifth and 9.8% in the seventh generation. Relatively few of the insects infected by vertical transmission were killed by the virus. During the five generations after exposure to NPV, 4.3% of the insects were killed by the wild virus and 5.6% by the selected isolate. (C) 1992 Academic Press, Inc.