Population Density and Transmission of Virus in Experimental Populations of the Western Tent Caterpillar (Lepidoptera: Lasiocampidae)

Abstract

Epizootics of nuclearpolyhedrovirus (NPV) are associated with population fluctuations of western tent caterpillars, Malacosoma californicum pluviale (Dyar), in western Canada. To experimentally quantify the spread of NPV in the western tent caterpillar, we measured the transmission of NPV within and among colonies of larvae introduced to alder trees. We altered both host population size (number of larvae per colony on a tree with 4 colonies) and pathogen density (number of infected individuals introduced to 1 of 4 colonies per tree). At the scale of the tree, the transmission coefficient (proportion of new infections per initially infected caterpillar) was independent of host density, but varied with initial pathogen dose. Low doses of virus had higher transmission coefficients than high doses. Transmission of virus between colonies within trees was related to the size of the colonies and virus was more widely spread among the 4 colonies when colonies were large (256 larvae per colony). This could be attributed to variation in the development rate and mobility of caterpillars from small and large colonies. The dynamics of NPV in populations of tent caterpillars will be influenced by movement of caterpillars among colonies, but increases in the amount of virus above a threshold level will not lead directly to increased numbers of infected individuals. Individual tent caterpillar moths lay all of their eggs in a single egg mass, and in declining populations, egg masses are smaller. The slower development of small colonies arising from these smaller egg masses may delay the recovery of declining populations, restrict the spread of virus, and influence the population dynamics of tent caterpillars.