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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. Description: Both the Eastern spruce gall adelgid (Adelgis abietis) and the Cooley spruce gall adelgid (Adelgis cooleyi) are widely distributed throughout the Northeast. Eastern spruce gall feeding damage typically causes galls 12 to 25 mm (1/2-1 inch) long to form in the crotches of young twigs on both Norway and white spruces. Feeding by the Cooley spruce gall adelgid will cause large, 2 1/2 - 8 cm (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 the buds. In the spring, when buds begin to swell, the adelgids become active and can lay up to 200 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 an abnormal growth, or pineapple-shaped gall, to form at the base of the branch. Each gall has cells, or locules, which house an individual growing nymph. In late summer or early fall, the galls dry and open, releasing the mature adelgids which then lay eggs on the needle tips. The overwintering adults hatch from these eggs. 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 which can be quite conspicuous. When the 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 in their individual cells 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: Eastern spruce gall adelgid: - 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 (green stage) 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: Spruce - Do
not fertilize heavily-infested trees. 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. Photos by R. Harrison.
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