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Abstract

To determine the dynamics of infection by nuclear polyhedrosis virus (NPV) in western and forest tent caterpillars, Malacosoma californicum pluviale (Dyar) and Malacosoma disstria (Hbn), I developed and tested a DNA dotblot hybridization assay for this system. The level of infection detected by the dot-blot assay was similar to that based on rearing field-collected tent caterpillars in the laboratory until viral death. Results based on testing crude caterpillar homogenates were the same as those in which DNA was extracted with phenol-chloroform. Experimental infection of second and third instar caterpillars showed that viral infection could be detected in 46% of the caterpillars two days after infection and 80 to 90% by four to five days post-infection. I monitored the levels of viral infection in six populations of forest tent caterpillars in Prince George, British Columbia. I found that high host density populations tended to have higher levels of virus than low density populations, but exceptions occurred. Virus persisted in populations through low density and the population with the highest level of virus in the peak density year collapsed following decline. A temporal series of samples of caterpillars and tent material from four populations of western tent caterpillars were analyzed. Levels of infection were related to the density of populations but not to changes in density. Infection was not related to the size of the egg mass of the caterpillar family, but in one population tents were larger for families with some individuals positive for virus than for uninfected families. Experimental contamination of tent material showed that active virus persists and tent material could provide a source of infection to the next generation. Pupae and moths from field populations of forest tent caterpillars and experimentally infected western tent caterpillars were rarely (pupae) or never (moths) positive for virus based on the sensitive polymerase chain reaction (PCR) detection assay. Approximately 2% of caterpillars reared from surface decontaminated egg masses were positive for virus based on DNA hybridization. A review of the literature revealed that low levels of viral infection are common among pupae of forest Lepidoptera and in caterpillars reared from surface decontaminated eggs. This could contribute to the persistence of virus in low density populations.