Forest Health & the Hungry Hungry Caterpillar


The life of a Douglas-fir tussock moth is not an easy one. The females can’t fly, and food is often scarce, not to mention viruses that make them explode. What’s more difficult than being a tussock moth, is having those moths in your forest.

Every ten years or so, the tussock moth population skyrockets in some areas of eastern Washington, well beyond what the forest can support. When that happens, these insects can eat so much that they literally kill the fir trees they feed on, sometimes up to 40 percent in a single stand. If a tree is lucky enough to survive the infestation, they’ll then be much more vulnerable to disease, pests and wildfire.

Often when we talk about species that destroy forests, those species are invasive. They didn’t come from the areas they’re killing. The tussock moth is actually a native species here in Washington, so what causes their once-in-ten-year eating rampage? We know that historically, the event happens approximately every ten years, but with a potentially disastrous ecological hazard, being as precise as possible is very important.


The male tussock moth has one goal in life – to mate. The Washington State Department of Natural Resources knows that this is the case, and so we use it to our advantage. Every year, we set out hundreds of traps for the male moths, baited with female pheromones.

One of the goals of the new 20-Year Forest Health Strategic Plan for eastern Washington is to produce and share better forest health data:


Tussock moth tracking is one of the tools that entomologists use to assess forest health. In 2016, the Department of Natural Resources and other agencies placed traps in 260 areas throughout eastern Washington, hoping to gather a large enough sample to measure the general population. With enough traps, we can get a good picture of how many moths are out there, and how many there are likely to be in the near future. A trap in a typical year will catch around two or three moths. But in a year proceeding the “swarm,” the trap will catch upwards of forty moths. If we can predict next year’s population, landowners can better prepare their trees. In a more big-picture sense, however, we can also prioritize which sections of our forests to modify.


The Department of Natural Resources also conducts aerial surveys of eastern Washington forests in cooperation with the U.S. Forest Service. Some of those surveys are looking for what’s called “defoliation” (trees with discolored or fewer leaves and needles). This is an excellent indicator of forest health, and it’s a good way to measure the effects of a tussock moth outbreak. Combined with trapping, we get a good idea for what’s going on across millions of acres of Washington forestlands.


Why are moth population explosions such a hazard to the health of our forests? It’s a native species, so why is there an imbalance?

Historically, wildfires were an important part of the ecosystem. These wildfires would burn through forests on a regular basis and clean the landscape of branches, smaller (less fire resistant) trees, and underbrush. Since western settlers came to this part of the state, those natural fires have been prevented. The result of this has been unnaturally dense parts of the forest with more fir trees, and it is in these dense stands that the Tussock moth thrives and spreads. In natural, spread-out stands of trees, it is much more difficult for the moth to travel and nest.

Washington State Department of Natural Resources entomologist Glenn Kohler shares forest health concerns and demonstrates tracking methods for KOMO TV.

This is why the 20-Year Forest Health Strategic Plan: Eastern Washington calls for thinning or treating 1.25 million acres of overstocked forests. The Department of Natural Resources looks to lead statewide efforts to move forests to more resilient conditions. Not only will this help manage moth outbreaks, but it will also make our land much less susceptible to catastrophic wildfire. Through mechanical thinning and prescribed burns, (more on that here) we hope to regain balance and a healthier outlook for the eastern half of the state.

Moth traps and aerial surveys are two methods the Department of Natural Resources can use to help update forest inventory data on a regular basis to reflect changes in forest conditions – including fuel conditions – from insects. One of the plan’s strategies is to, by 2020, have comprehensive maps of current forest structure and fuel conditions available for eastern. The plan also identified several other data needs: wildland-urban interface areas, areas where forest health treatments have occurred and trends in wildfire risk. This data will help us to be efficient and better understand which approaches are most effective.

Monitoring and progress reports will require collaboration and continued support of existing partnerships between the Department of Natural Resources, Department of Fish and Wildlife, USDA Forest Service, Bureau of Land Management, Natural Resource Conservation Service, tribes and private landowners.

Achieving this goal may not be much easier than the life of a Douglas-fir tussock moth. Yet, by working collaboratively, it is possible. Together, we can monitor and track our progress toward healthier, safer forests.

Learn more about creating the new 20-Year Forest Health Strategic Plan: Eastern Washington, here.