Defoliators in eastern Oregon and Washington

Defoliating insects are major disturbance agents affecting forest health and productivity in eastern Oregon and Washington. Information on the four main defoliators of conifers in eastern Oregon and Washington is abundant. Because of concerns about growth suppression and mortality of trees during widespread defoliator outbreaks, much research effort has been focused on these species. They are western spruce budworm (Choristoneura occidentalis), Douglas-fir tussock moth (Orgyia pseudotsugata), pandora moth (Coloradia pandora), and larch casebearer (Coleophora laricella). Various interactions of defoliators with other system components and natural regulatory processes have been described, as have monitoring and suppression techniques using pheromone traps. Large-scale suppression projects using both chemical and biological materials have been used in attempts to control some defoliator outbreaks. While such suppression projects have prevented some tree mortality and growth loss, they have been largely ineffective in changing the outbreak behavior of these insects. Some suppression materials have undesirable side effects on non-target insects such as sensitive or endangered moths, butterflies, and skippers, or on other ecological processes. Successful control of larch casebearer has been achieved by introduction of parasitic wasps. For most defoliators, the recommended strategy is preventative: silvicultural treatment to promote a diversity of tree species, stand structures, and moderate stocking levels. Decision-support tools UPEST and UTOOLS analyze insect and disease risks, and the Forest Vegetation Simulator models effects of insects and disease on stand growth.