Integrated pest management of the Douglas-fir tussock moth

Abstract

The Douglas-fir tussock moth is one of the most destructive forest defoliators in western North America. Densities of most tussock-moth populations fluctuate over time with considerable regularity. Fluctuations in density on warm, dry sites where populations have a high intrinsic rate of increase are more likely to periodically reach outbreak numbers than where rates of increase are relatively small. Such cycles of abundance are inherent properties of the tussock-moth system that are not likely to change in susceptible host types. Modern pest-management programs emphasize the annual monitoring of insects in forests with outbreak histories to determine early changes in population numbers and to predict trends. When outbreaks develop, several environmentally safe chemical and microbial insecticides are effective in reducing larval numbers and preventing serious defoliation. Computer models predicting growth loss, tree mortality, and top-kill during outbreaks are available as aids to making management decisions. Silvicultural practices favoring seral nonhost species on high-risk sites may be the best prescription for reducing the impact of tussock moth outbreaks.