Pine Beetles Becoming Less Selective

When a mountain pine beetle epidemic first develops in an area, the insects are selective about which patches of trees they invade most intensively, but that soon changes. At the northwestern limit of their present range, near Houston, British Columbia, the beetles have shown particular preference for lodgepole pines over 100 years old that live in forests where 40 to 70 percent of trees are pine.

The insects also target trees growing on warmer sites, particularly south and west-facing slopes at elevations of 800 to 1000 metres.

These sites are where the greatest number of recently-attacked trees were found in the Morice TSA shortly after the bark beetle epidemic began there in the mid-1990s. As the insect population built and their choicest of trees died, the beetles became less discriminating.

Four to seven years after beetles moved into a region of the TSA, the hot spots of beetle infestation showed up more randomly across the pine landscape. The age of trees and proportion of pine in heavily attacked stands came to reflect the distribution of these variables among the forests.

Mountain pine beetles at this northern edge of their range are vulnerable to cold temperatures. Nevertheless, as the epidemic grew, the beetles also moved into cooler terrain. Heavy infestations became less prevalent on south-facing slopes, and some appeared at higher elevations.