Nuclear polyhedrosis virus of the gypsy moth (Lymantria dispar L.): Environmental and ecological factors influencing transmission and host susceptibility

The gypsy moth, Lymantria dispar L., is one of the most important forest pests in the United States. Insecticides are frequently used to suppress outbreaks, however, untreated outbreaks generally collapse due to disease epizootics caused by a nuclear polyhedrosis virus (NPV). Knowledge of NPV epizootiology could aid in the use of NPV as a biological control agent. In this study, the mechanisms by which the NPV is transmitted across host generations, factors affecting host susceptibility to the pathogen, and the effects of NPV on the behavior and distribution of infected larvae in natural populations were investigated. A series of experiments were conducted in the field to determine whether maternal or environmental mechanisms are responsible for transgenerational transmission of the pathogen. Larvae hatching from egg masses oviposited in NPV-contaminated environments became infected, whereas, progeny of NPV-fed parents did not, indicating that environmental contamination of egg masses, rather than maternal transmission, is the principle means of transmission across generations. Further tests showed that egg masses acquire inoculum from the substrate during oviposition, rather than through rain-caused leaching. The pathogenesis of the virus in gypsy moth larvae, pupae, and adults was followed using nucleic acid-hybridization techniques. Virus was not detected in adult moths suggesting that vertical transmission of NPV from infected moths to progeny may not be possible in the gypsy moth. Susceptibility to disease may also be affected by disease prevalence. In laboratory assays, larvae from populations with more heavily NPV-contaminated eggs were more resistant to NPV than were populations with little inoculum present. These results suggest that exposure to NPV may favor selection for resistance to the pathogen. It was also demonstrated that NPV infection alters the behavior of larvae, resulting in the tendency of infected larvae to die in the upper crowns of trees. Thus, samples of larvae collected from the upper canopy contain a higher proportion of infected larvae compared with those collected from under burlap bands wrapped around tree trunks. Therefore, NPV prevalence is more precisely estimated by rearing larvae collecting by a stratified-random sampling design