When to Spray for Western Spruce Budworms

Significance

• The western spruce budworm (Choristoneura occidentalis) attacks both spruce and fir trees throughout western North America. The larvae of the worm feed on buds, needles, shoots and other sections of the tree, causing partial or complete defoliation and making the specimen more susceptible to attacks from other insects, such as bark beetles. Infestation primarily occurs in forest environments, but ornamental trees may also be invaded by the budworm. Control via insecticide is much easier in a landscape setting.

Spraying

• The decision to spray for western spruce budworms should be based on whether the infestation is severe or the trees in danger are high value specimens. In many cases, spraying is unnecessary and may destroy beneficial insect species. If the insecticide is to be applied, chemical control should be initiated in early spring just as the buds of the tree are about to open. At this point, the larvae of the western spruce budworm are feeding on old foliage and have not yet turned their attention to new buds.

Time Frame

• Dependent on the temperature and the elevation in which the tree resides, spraying for the budworm should occur in June and July to maximize the impact on the larvae. A biological insecticide aimed at caterpillars, Bacillus thuringiensis -- commonly called B.t. -- is also effective against the western spruce budworm and carries less risk of killing beneficial insect species. Spraying from the ground works with individual trees, while aerial applications are necessary for more extensive stands of infected trees.

Cultural Control

• Incidences of attack by the western spruce budworm increase within pure stands of its favorite host, firs and spruce. To culturally control the insect in a landscape setting, gardeners and landowners should make sure that a variety of other tree species are planted adjacent to spruce and firs. Trees that are stressed from any number of factors are also more vulnerable to the budworm. This stress may come in the form of drought, freeze damage, attacks by other insects or poor site selection.