The Cooley spruce gall is a brown, cone-like swelling found at the tip of spruce branches. The Cooley spruce gall is a harmless growth that looks like a pine cone.
Small, sap-feeding insects called aphids, cause galls to form early in the spring when young aphids hatch from eggs. The aphid young crawl to the base of new needles on the growing tips of the spruce branches, where they feed on tree sap. As the aphids feed, they inject a growth hormone into the tree. This causes the tree to form a woody growth, or gall, around the insect. This woody home protects the aphid from harmful predators and harsh weather.
As the insects grow and mature, the gall dries out. Openings in the gall allow the adult aphid to emerge. Once an aphid leaves the gall, it flies from the spruce to another tree where it spends the summer feeding and reproducing. In late fall, the aphids return to the spruce tree, laying eggs that hatch. The new young spend the winter protected by the spruce branch.
The insect damage to the spruce is unsightly but isn’t harmful. Control usually is unnecessary and is often ineffective. Once an aphid is inside a gall, insecticides can’t reach it. Sprays must be applied in early spring when the young are exposed on branches and before a gall forms. This step requires close attention because the insects are tiny, making recognition difficult.
For more information, see the following Colorado State University Extension fact sheet(s).