Dynamics of Viral Disease and Population Fluctuations in Western Tent Caterpillars (Lepidoptera: Lasiocampidae) in Southwestern British Columbia, Canada

Models of the dynamics of forest caterpillars and viral disease are sensitive to virus persistence at low host densities. Nuclear polyhedral virus could persist in the environment on tree bark or in the soil, or by infecting caterpillars even at low host population numbers. We monitored populations of western tent caterpillars at 4 locations in southwestern British Columbia, Canada, to describe the relationship between host numbers and viral infection and to determine if virus is maintained in low populations of caterpillar hosts. We evaluated the persistence of virus on tents. Viral infection was detected by DNA-dot blotting of larvae and extracts from field-collected tents. Although active virus persisted over winter on experimentally contaminated tents, few tents collected from low field populations were positive for viral contamination. Tents are therefore unlikely to be a source of environmental contamination when host numbers are low. Virus was generally, but not always present in a few larvae even in years of low host numbers. In 2 populations the level of infection increased with host numbers, and it remained high at the beginning of the decline in 1 population but decreased to a low level even at high host numbers in the other. Western tent caterpillars at 2 mainland locations did not show a typical pattern of cyclic population fluctuation and for these the levels of viral infection did not vary consistently with host numbers. Presence or absence of infection was not related to the size of the egg mass from which the family of larvae hatched although egg masses associated with contaminated tents tended to be smaller than those of noncontaminated tents.