“My spruce tree has brown things all over it! Is it sick?” The caller was quite anxious. He had a beautiful Colorado Blue Spruce growing in his yard, and now it had some sort of weird alien growths at the ends of all the branches. Was it going to die?

Over the years that I volunteered at the master gardener help desk, we would often get calls like this. No, the caller’s tree wasn’t sick, not exactly. Those prickly, cucumber-shaped growths that show up on spruces from time to time are actually galls caused by an insect. They might look peculiar, but they weren’t going to cause significant harm to his spruce.

Blue spruces are one of two hosts for a tiny insect called the Cooley spruce gall adelgid. On spruce trees, these adelgids are greenish-gray aphids covered with a fine powder of wax. Like other aphids, they slurp up plant juices with their straw-like mouthparts. In the process, they cause the spruce to grow a gall (a swollen growth) at the end of the stem. We rarely notice the adelgids. as they tend to stay tucked out of sight inside the ripening gall. By the time the galls dry out and turn brown—and become more noticeable—the adelgids have flown.

Cooley spruce gall adelgids have a fascinating lifecycle. The generation that grows up on blue spruces is followed by one or two generations that live and feed on Douglas-firs. Thus, their life cycle involves two alternating hosts. This arrangement works well because these two species of conifer frequently grow together in mixed forests.

The adelgids overwinter as partially developed nymphs on the needles of the Douglas-firs. When the weather warms in spring, they molt into adults and lay their eggs in large masses on the branches. A new generation of nymphs hatch from the eggs. Hungry, they suck juices from the season’s new growth, causing the needles to become distorted and yellow. They also feed on immature fir cones.

These nymphs look quite different from their parents—they’re typical wooly aphids, resembling teeny-tiny sheep with their white, waxy fluff. Perhaps the extra covering protects them from predators, as they lack the protection of a gall at this stage.

In July, nymphs molt into either winged or wingless adults. The wingless individuals hang around and produce another generation of young right there in the same tree. Those with wings fly to a spruce and start a new generation there.

On a spruce, the eggs hatch into nymphs that feed on the juices inside the needles, causing the twig ends to morph into Cooley spruce galls. By late June and July, the galls begin to dry up and turn brown. The lack of sweet sap induces the adelgids to undergo one more molt and become fully mature winged adults. Leaving their brown homes, they then move back to the Douglas-firs and it starts all over again.

Pretty amazing, yes?

Knowing that the galls won’t significantly harm their trees, most spruce-owning gardeners just consider them a natural part of the tree, like the cones, and ignore them. However, if their appearance bothers you, you can always trim them off by hand. Sprays are available, but I question the necessity of using an insecticide against an essentially harmless pest. And while less-toxic sprays, such as insecticidal soaps, have proven effective of the insects on Douglas-firs, they don’t seem to help much on spruces.