The very prickly caterpillar: Navigating Douglas-fir tussock moth outbreaks

Abstract

Defoliating insects inflict costly damage to forests, sometimes at landscape scales. Outbreaks of the native Douglasf-ir tussock moth (Orgyia pseudotsugata) occur every 5 to 10 years in the western United States and southern British Columbia. The larvae feed on the needles of Douglas-fir, grand fir, or white fir trees. If the defoliation becomes severe enough, they will kill the infested trees. Widespread outbreaks can cause extensive damage, including leaving the forest susceptible to infestation by other insect pests or wildfire. Outbreaks typically end when a naturally occurring, rapidly transmitted virus decimates the larval population. This virus has also been used as an environmentally benign insecticide to artificially end outbreaks. However, such spraying campaigns can be expensive. Land managers weigh this cost against many other considerations when deciding whether to spray. Key to this decision is being able to predict if the natural viral infection will act quickly enough to stop the outbreak ahead of substantial defoliation.

Research scientist Carlos Polivka and collaborators at the University of Chicago developed a tool that models the virus and its transmission through a Douglas-fir tussock moth population, predicting the time course of the viral infection. This helps land managers determine if they should intervene or if the natural viral infection will soon take care of the outbreak.

Citation

White, Rachel; Polivka, Carlos. 2023. The very prickly caterpillar: Navigating Douglas-fir tussock moth outbreaks. Science Findings 262. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 6p.