Spruce Gall Adelgids

Two species of adelgids, often incorrectly called aphids, can damage at least three species of spruce and will also attack Douglas fir. These insects are not usually  serious pests, especially in forest conditions, but damage can be quite disfiguring when trees are planted in monoculture situations, such as Christmas tree plantations. Both the Eastern spruce gall adelgid (Adelgis abietis) and the Cooley spruce gall adelgid (A. cooleyi) are widely distributed throughout the Northeast.

Eastern Spruce Gall Adelgid

Eastern spruce gall adelgid feeding damage typically causes ½ –1-inch long pineapple- shaped galls to form in the crotches of young twigs on both Norway and white spruces. Feeding by the Cooley spruce gall adelgid will cause large, 1–3-inch galls to form on the tips of Colorado blue spruce. Cooley spruce gall feeding injury on Douglas fir causes yellow spots on the needles. If uncontrolled, the needles will bend and the tree will be covered with adults that look like tiny cotton balls.

The Eastern spruce gall adelgid overwinters as an immature female under a thread-like cover of wax near the base of buds. In the spring, when buds begin to swell, the adelgids become active and lay eggs at the base of the bud. Young nymphs hatch in less than two weeks and immediately begin feeding on the needles of emerging shoots. Continued feeding induces the galls to form at the base of the branch. In late summer or early fall, the galls dry and open, releasing mature adelgids which then lay eggs on the needle tips. The overwintering adults hatch from these eggs.

Management practices for Eastern spruce gall adelgid include:

• Do not fertilize heavily infested trees. There is some evidence that fertilizing without chemical control will actually enhance insect population growth.

• If practical, hand pick and burn or destroy galls before insects emerge. It is too late after the galls turn brown and the holes appear. Also, if practical, rogue out most heavily infested plants early on to encourage growth of the more resistant trees.

Horticultural oil applied when plants are dormant (mid-April to early May) is reasonably  effective. Another application of horticultural oil or insecticidal soap in mid- to late September, as insects emerges from galls, will provide excellent control.

Cooley Spruce Gall Adelgid

The Cooley spruce gall adelgid overwinters as an immature female beneath bark scales on spruce or Douglas fir. In the spring they lay their eggs under a cottony, waxy covering that can be quite conspicuous. When eggs hatch in the spring, the nymphs crawl to the developing buds and begin to feed. This feeding results in swelling of tissue, and eventually a gall will develop on the tips of Colorado blue spruce branches. The young adelgids will live within the gall until mid-summer when winged adults emerge and migrate to other spruce or Douglas Fir. Many references incorrectly state that the Cooley spruce gall adelgid requires both Colorado blue spruce and Douglas Fir to complete its lifecycle. In fact, in the absence of the alternate host Douglas Fir, the spruce gall adelgid can complete its lifecycle on spruce.

Management practices for Cooley spruce gall adelgid include:

For spruce:

Do not fertilize heavily-infested trees.

If practical, hand pick and destroy galls at the green stage.

Rogue most susceptible plants when young.

Do not plant Colorado blue spruce close to Douglas fir.

Apply horticultural oil at bud-break. Make second application in late-July to mid-August.

For Douglas Fir:

No effective parasites or predators have been found; chemical control remains the best option.

Apply horticultural oil when trees are dormant, but use caution on trees that will be sold in the same year, as oils can reduce tree color. Apply in early May and, if necessary, again in July.

By David Wallace, Plant Protection Specialist, 1999