If you have a Douglas fir, spruce or a true fir on your property, monitor it this spring for signs of defoliation, especially on the tops of the tree.
The Colorado Springs area saw the beginnings of a Douglas-fir tussock moth outbreak last year, and this year we expect it to be more widespread. The good news is that outbreaks of Douglas-fir tussock moth tend to be cyclical due to effects of several important natural controls.
At least seven species of parasitoid wasps and a tachinid fly attack and develop within developing larvae and pupae. Birds and spiders will also eat the larvae.
As an outbreak progresses, a viral disease will often knock down populations for years.
During outbreaks, the caterpillars can extensively defoliate plants, feeding first on the new growth, then later moving to older needles. While feeding is usually concentrated at the top of the tree, the whole tree can become involved during severe outbreaks.
Tops of heavily damaged trees may be killed, sometimes after only a single season of severe injury. Stresses from needle loss can also weaken plants so that they become more susceptible to effects of bark beetles, such as spruce ips, or fungi that produce cankers on branches.
Douglas-fir tussock moth caterpillars also can cause problems, because the larval hairs can be irritating and are capable of producing a painful rash. Individual reactions to the hairs are highly variable, with some people reacting strongly following exposure while others have little reaction.
If your tree is being extensively attacked, you may decide you want to use a chemical control, rather than relying on natural predation. Timing is important because younger larvae are much more effectively controlled than older larvae.
It is best to treat as soon as possible after the eggs have hatched. This means you need to monitor your trees closely, looking at new foliage as high up in the tree as is possible to reach. Even if you can’t get to the top of your tree, you might be able to note needles wilting and turning brown (use binoculars if necessary), or thinning foliage.
Depending on weather conditions, egg hatch and subsequent larval feeding occurs sometime between mid-May and mid-June, around the time of tree bud break.
For treatment, Spinosad is an organic option, and there are several nonorganic options such as permethrin or bifenthrin. It is recommended that you hire an arborist to apply the treatment rather than trying to do it yourself.
By mid-July or August, the larvae become full-grown, and many might migrate away from the infested tree to pupate. The adults emerge from late July through mid-August. There is one generation produced per year.
For more details: extension.colostate.edu/topic-areas/insects/5-542-douglas-fir- tussock-moths.