Cooley spruce gall is caused by a small insect related to aphids. It feeds at the base of new spruce shoots, and the shoot forms a gall around it: a swelling that looks like a small green pinecone at the tip of the branch. In late summer the gall dries, opens, and turns brown, and the dead shoot tips remain on the tree.
What it does to a tree
The galls rarely threaten the life of a spruce. They do kill the shoot tips they form on, and on a heavily infested tree the browned tips are visible across the canopy. Colorado blue spruce is a common host throughout the area.
How we treat
We spray affected trees twice a year, once in spring and once in fall. The insect spends most of its life protected, either sealed inside the gall during the summer or tucked under bark scales over the winter, and a spray reaches it only when it is out from under that cover. The spring and fall applications are each timed to one of those windows.
A note on Douglas-fir
The same insect spends part of its life on Douglas-fir, where it does not form galls but can cause yellow spotting on the needles. Properties with both trees can see the problem return from one to the other.
All applications are made under Brookstone Outdoors' Colorado Department of Agriculture commercial pesticide applicator license.