Population density and transmission of virus in experimental populations of the western tent caterpillar (Lepidoptera : Lasiocampidae)

Epizootics of nuclearpolyhedrovirus (NPV) are associated with population fl uctuations of western tent caterpillars, Malacosoma californicum pluviale ( Dyar), in western Canada. To experimentally quantify the spread of NPV in t he western tent caterpillar, we measured the transmission of NPV within and among colonies of larvae introduced to alder trees. We altered both host p opulation size (number of larvae per colony on a tree with 4 colonies) and pathogen density (number of infected individuals introduced to 1 of 4 colon ies per tree). At the scale of the tree, the transmission coefficient (prop ortion of new infections per initially infected caterpillar) was independen t of host density, but varied with initial pathogen dose. Low doses of viru s had higher transmission coefficients than high doses. Transmission of vir us between colonies within trees was related to the size of the colonies an d virus was more widely spread among the 4 colonies when colonies were larg e (256 larvae per colony). This could be attributed to variation in the dev elopment rate and mobility of caterpillars from small and large colonies. T he dynamics of NPV in populations of tent caterpillars will be influenced b y movement of caterpillars among colonies, but increases in the amount of v irus above a threshold level will not lead directly to increased numbers of infected individuals. Individual tent caterpillar moths lay all of their e ggs in a single egg mass, and in declining populations, egg masses are smal ler. The slower development of small colonies arising from these smaller eg g masses may delay the recovery of declining populations, restrict the spre ad of virus, and influence the population dynamics of tent caterpillars.