The western tent caterpillar: Qualitative differences and the action of natural enemies

Abstract

A population of the western tent caterpillar, Malacosoma californicum pluviale (DYAR), is composed of several types of individuals that differ in their behavior and vigor (WELLINGTON 1957). In the larval stage, Type I individuals are the most active, responding most effectively to environmental stimuli as soon as they hatch. Type IIa larvae are nearly as active as Type I, but less directed in their movements; Type IIb are much less active, and incapable of independent orientation; and Type IIc individuals are so sluggish that they usually die early in the larval stage. As adults, Type I individuals have the greatest flight capacity, and their progeny include higher percentages of Types I and IIa larvae than offspring of any Type II female. As each female deposits all its eggs in a single mass, and the emerging progeny remain together in a communal nest for most of their larval stage, colonies that contain the highest proportion of I and IIa individuals thus are the most active of the possible mixtures. Because of their higher group activity, they construct elongate tents at nesting sites, instead of the compact structures spun by the less active colonies produced by IIb mothers. Members of elongate colonies feed more regularly than larvae in compact colonies, so they usually survive better in the field and in the laboratory. The proportions of elongate and compact colonies in a locality vary with the infestation history (WELLINGTON 1957, 1960, 1962, 1964). During 1965-66 we had an opportunity to study the action of several biotic agents on the different types of colonies and the various kinds of individuals within them. During that period, the population on the Saanich Peninsula of southern Vancouver Island was well below its 1963 peak, so we were observing a declining population. This paper only summarized the results, as the original data and many of the analyses are too lengthy to include here. We have deposited essential data at the Kyoto laboratory, where they are available for inspection.