Cooley spruce gall: what extension services say

This page describes what the named extension publications say. It does not identify a gall on any particular tree, say whether a tree is healthy, or tell anyone to treat, prune or leave it. Pesticide advice in a source is mentioned only in general terms, and the guide on who may apply pesticides covers licensing.

What it looks like

The sources agree on the shape and differ on the size. Colorado State University Extension calls the galls conspicuous cucumber-shaped growths on the new growth of spruce, 2 to 4 inches long early in the season, light green in late spring and early summer, then drying out and turning brown from mid-July. It says they are often mistaken for seed cones. Washington State University's Hortsense (last reviewed July 31, 2025) gives 1/2 inch to 3 inches, a green or purplish cone shape that turns brown and woody by midsummer or fall. The University of New Hampshire Extension fact sheet gives 1 to 2 inches long and 1/4 to 1/2 inch across, light green to dark purple while growing, on the tips of twigs, and says they are somewhat larger than the galls of the eastern spruce gall adelgid. Montana State University Extension says that at the end of summer the galls resemble miniature pinecones.

What causes it

All four sources name the Cooley spruce gall adelgid, Adelges cooleyi, an aphid-like insect. Colorado State describes the spruce side of the cycle: immature females overwinter under young branches, mature in spring and lay several hundred eggs near developing buds. The eggs hatch around bud break, and the young move to the new growth and feed at the base of the growing needles. Saliva introduced into the plant changes its development and makes the gall, and the insects develop in chambers inside it. By midsummer the gall dries out, the chambers open and winged forms emerge.

Washington State says the same insect also infests Douglas-fir but does not form galls on it. Colorado State says feeding on Douglas-fir leaves yellow spots and bent needles, and that the trees look speckled with tiny cotton-like forms of the insect.

Where the sources differ

Question	What the sources say
Does it need both hosts?	Colorado State says the adelgids normally require two hosts, spruce and Douglas-fir, to complete the full cycle, and that research indicates those leaving spruce must develop on Douglas-fir before returning, while observations in Colorado suggest spruce to spruce movement may also occur. New Hampshire says it does not need both hosts but is more serious when both are present. Montana State says it uses both.
Which spruces?	New Hampshire names Colorado blue, Sitka and Engelmann spruce. Colorado State says the galls are common and conspicuous on blue spruce. Washington State says most spruce species in the Pacific Northwest are susceptible.
Which side of the tree?	Colorado State says galls mostly form on the shadier north and east sides of trees, that trees differ widely in how readily they gall, and that greener trees tend to carry more of them.

Whether the sources call it harmful

Colorado State says the galls do little or no harm and usually cause no serious harm, that extremely heavy infestations may cause minor retardation of growth and some distortion, and that new growth covers old galls from the following year on, so within a few years they are hard to notice. It also says infestations are highly cyclical, so a heavy one in one season does not mean a heavy one in the next. Montana State says the galls rarely harm the tree, that most trees have a high resistance, and that the damage on spruce and Douglas-fir is minimal. Washington State says the branch tips usually die once the gall is brown and woody. The University of New Hampshire fact sheet, which covers spruce gall adelgids in general, says continued infestations disfigure ornamental and Christmas trees, weaken them and make them subject to attack by other pests.

What the sources say about management

Colorado State says control is rarely needed to protect tree health and has to happen before galls begin to form, because once they form the insects are protected inside. It lists several insecticide types and also says to avoid placing Douglas-fir and spruce close together. Washington State says planting the two as far apart as possible may reduce, but not prevent, infestations, that galls may be pruned out to improve short-term appearance, that long-term pruning may ruin the tree's shape, and that no chemical management is recommended. Colorado State and Montana State both say that once galls have turned brown the insects have already left, so removing them does not reduce future populations. New Hampshire is the only one of the four that lists removing green galls on a few small trees as a way to manage an infestation.

Look-alikes the sources name

New Hampshire describes the eastern spruce gall adelgid, which forms pineapple-shaped galls from swollen needle bases, mostly on Norway, white, black and red spruce. Colorado State says Cooley galls are often mistaken for seed cones. Other insect and mite galls are on the tree galls page, and growths with other causes are on the odd tree growth page.