The impact of host developmental status on baculovirus replication

The yield of progeny occlusion bodies from a baculovirus infection is a critical parameter governing viral population dynamics in the field. Previous evidence has suggested that the ability of the virus to block host development may be an important factor in determining yield of progeny virus. Here, we explore the relationship between yield, dose, and host developmental status at the time of infection during Autographa californica nucleopolyhedrovirus infection of final instar Heliothis virescens. The data indicate that occlusion body yield is strongly inversely dependent on the time elapsed after ecdysis before the insect is infected. The later in the instar the infective dose is received, the lower the efficiency with which the virus can block host development. The occlusion body yield from insects whose development is completely arrested is more than fourfold greater than the yield from insects that have initiated prepupal development. Dose is also an important factor, with high-dose infections more likely to lead to developmental arrest. Thus, the infection parameters that give rise to optimal progeny virus yield are infection at high dose early in the instar. Analysis of ecdysteroid titers demonstrated that only low levels of ecdysteroids are detectable in insects whose development is completely arrested. In contrast, in insects whose development was only partially arrested, extremely high ecdysteroid titers were frequently observed. These data support the hypothesis that the function of the baculovirus ecdysteroid UDP-glucosyltransferase gene is to delay or block host development, with the benefit of increasing the yield of progeny virus. Copyright 1998 Academic Press.