Trans-ovum transmission of a nuclear-polyhedrosis virus in the gypsy moth and the inducement of virus susceptibility

Abstract

Eggs of the gypsy moth, Porthetria dispar, were collected from six sites differing in topography from wet to dry, in density of the gypsy-moth population, and in age or duration of the outbreak. Eggs collected from one site on March 23, 1968, responded differently than eggs of any of the other collections after as little as 7 days at 0–2°C. About 80% of the larvae hatching from these eggs died of nuclear polyhedrosis in the first instar. These virus-susceptible larvae made it possible to demonstrate, by surface disinfection, that trans-ovum transmission of the nuclear-polyhedrosis virus (NPHV) takes place via the surface of the egg. The hatching larva becomes infected by ingesting part of the egg shell. (An improved method of surface disinfection with 0.1% sodium hypochlorite is described. This reduced the incidence of polyhedrosis in the susceptible larvae from 80% to 0.1%.) Larvae from eggs collected at hatching time from the same site as the susceptible group of March 23 had less than 10% polyhedrosis. This was also true for larvae from eggs collected at the other sites both at hatching time and in the previous months of December and January. Larvae from eggs collected in the winter months and refrigerated at 0–2°C for up to 7 months suffered less than 10% polyhedrosis. Thus, long periods of refrigeration did not induce susceptibility to the NPHV. The data suggest that initial mortality of first-instar larvae in the field, exclusive of larva-to-larva transmission typically would be less than 10%. By comparison with the 80% polyhedrosis observed in the virus-susceptible larvae it appears that there is normally a high incidence of transmission of the virus and that most larvae become infected as they hatch. Since less than 10% die in early instars there appears to be some natural resistance to acute infection. Apparently by March 23 eggs in the field undergo some change as the weather begins to moderate. After this the unhatched larvae cannot tolerate prolonged chilling at 0–2°C without becoming susceptible to acute infection from NPHV.