Factors Affecting Spruce Beetles During a Small Outbreak

In 1957, Dendroctonus rufipennis beetles increased to outbreak numbers in logging slash on a north-central Colorado site, entered living Picea engelmannii trees, and remained epidemic for two years. Reduced fecundity of the beetles was the first indication of the decline of the outbreak, which was caused by nematodes and unknown agents. Significant causes of summer mortality were pitch, intra- and interspecific competition for food, predation by woodpeckers and flies, and parasitization by wasps. Desiccation of both food and beetle larvae, enhanced by woodpecker feeding activity, contributed significantly to the decline. Winter mortality was attributed mainly to woodpeckers, although temperatures down to -29 deg C caused additional losses. The effects of the outbreak on the Spruce were considerable. Scattered groups of large-diameter trees were killed, and the composition of the stand was altered in favor of Abies lasiocarpa and Pinus contorta. However, mean tree diameter was not significantly reduced