Looking for early signs of western spruce budworm activity

In 2011, western spruce budworm activity was highly apparent in Douglas-fir and grand fir forests of the eastern slope of the Cascade Mountains north of I-90 and across parts of northeastern Washington. Many people noticed defoliated branch tips and tree tops for the first time as an ongoing outbreak spread to include their land or as the damage intensified to a more conspicuous level. This article describes the early annual signs of budworm activity so you can continue looking for the effects of western spruce budworm.

The DNR 2011 Forest Health Highlights report  (pgs 12-13) describes recent and predicted western spruce budworm defoliation in Washington State. Stevens, Ferry, Okanogan, Chelan, Kittitas and Yakima counties all are likely to have sites with significant budworm populations in 2012. If you own or visit forest land with the preferred host trees (Douglas-fir, grand fir, subalpine fir, Engelmann spruce), keep your eyes open for the early symptoms of budworm activity.

Newly hatched budworm caterpillar
PHOTO 1: Newly hatched budworm caterpillars can be observed in late summer. Photo: Mike Johnson/DNR.

Through winter until approximately May, tiny caterpillars (Photo 1) that hatched last summer have been overwintering in silken shelters called hibernacula. These shelters (Photo 2) are usually built in protected locations on tree surfaces, such as in bark crevices or under lichens.

As the days lengthen, budworm larvae emerge from their shelters. They crawl toward the tops and branch tips of trees. They may release a silken thread and disperse, carried by the wind, through the forest.

Silken shelters called 'hibernacula'
PHOTO 2: Silken shelters called 'hibernacula' protect western spruce budworm over the winter. Photo: US Forest Service.

Each caterpillar’s goal is to successfully locate a host tree with swelling buds. Cone and pollen buds are more desirable and more nutritious than vegetative buds, but the key is that the buds must be starting to swell for the larvae to be able to chew in. If the soil conditions are still too cold for the tree to be emerging from its dormancy and initiating bud-swell, the caterpillars may be too early and unable to penetrate the hard, dense buds. When that occurs, the caterpillars have to seek food and shelter by mining inside the older needles, a low-nutrition waiting area. (Photo 3) Although the upper parts of tree crowns likely have more caterpillars than those branches in arm’s reach, it’s still worth looking their signs. Look for tiny bits of frass (feces and chewed debris) that accumulate on the surface of an occupied needle near a tiny hole.

Budworm larva mining in needle
PHOTO 3: Budworm larva mining in needle. Source: Ladd Livingston/ForestryImages.org
Western spruce budworm larva consuming the foliage inside an expanding bud.
PHOTO 4: Western spruce budworm larva consuming the foliage inside an expanding bud. Photo: David McComb/Forestry Images.

As time passes, the larvae move to the swelling buds and chew their way inside (Photo 4). Some frass may accumulate, and, as the buds expand, some webbing may be constructed to maintain a shelter among the unfurling needles (Photo 5). Look for this activity in June.


Budworm protected by webbing
PHOTO 5: Budworm are protected by webbing as they feed on and in expanding buds. Photo: David McComb/Forestry Images

Needles damaged and consumed by western spruce budworm

As the larvae grow and more of the needles have been consumed, the damage becomes more conspicuous (Photo 6). By late June, more silk webbing accumulates and some partially eaten needles dry and frizzle. The caterpillars prefer to stay protected within the webbing and sometimes incorporate several adjacent buds into one shelter. If the branch is jarred, alarming the caterpillar, or if all the new needles in its immediate site have been consumed, the budworm may move on to a different site, but western spruce budworm eats only the newly expanding needles.

Branch with many new buds damaged by one year of feeding by western spruce budworm.
PHOTO 7: Branch with many new buds damaged by one year of feeding by western spruce budworm. Photo: Kenneth Gibson

The damage becomes more conspicuous (Photo 7) as the caterpillars grow (they eat more of the foliage), and as the weather warms (partially eaten needles dry out). After many years of having all the new foliage eaten, forests with bare tree tops and branches appear grey (Photo 8).


Many years of cumulative defoliation has denuded tree tops.
PHOTO 8: Many years of cumulative defoliation has denuded tree tops. Photo: Jeff Jones/ American Forest Land Company

Sometimes, after many successive years of defoliation or a damaging weather event has killed most of the available buds, the larvae may finish the needles and consume even the succulent newly expanding twig shoots. This may be a sign that the caterpillars are starving. Although there has likely been significant damage inflicted on the forest by then, it can signal that the caterpillars are desperate for nutrients and the population will “crash” the coming year.

budworm in pupal stage
PHOTO 9: When budworm enter the pupal stage they no longer feed on foliage and are not susceptible to most forest insecticides. Photo: Mike Johnson/DNR

When landowners witness and experience budworm activity, they may feel desperate to understand and make a plan of action. Consider whether stewardship activities could decrease your vulnerability to this caterpillar. In the long term, forest management that enhances the proportion of healthy pine and larch, reduces canopy layers, and increases the vigor of fir host trees will shift the balance away from budworm habitat. Some landowners may seek to spray forest insecticides to provide temporary foliage protection for their trees. This is an option, but must be planned well in advance with appropriate timing and permits for the products and application method you choose. The caterpillars are only vulnerable to the least toxic insecticides for a few weeks in mid-June (when the new foliage has expanded enough that the insecticide can land on it, yet the caterpillars are still eating and have not yet entered a protected pupa stage (Photo 9). Moreover, aerial insecticide applications require complex advance permitting and contracting.

In the next few months and years there will be many opportunities to learn more about the western spruce budworm. Start now with the Forest Service’s Leaflet number 53

Karen Ripley, Forest Health Program Manager,
Washington State Department of Natural Resources