Adelgids (Adelges cooleyii, A. lariciatus, A strobilobius, Pineus pinifoliae, P. similis) are soft-bodied insects closely related to aphids.
The first generation feeds on the succulent new needles at the branch terminals. The insect injects a hormone during feeding which mimics plant hormones responsible for cell division.
The tree responds by forming a gall at these feeding sites. The young adelgids enter this gall and are sheltered inside it through their growth cycle.
Hand pick the newly formed galls off the twigs in June and destroy them. Adelgids are an aesthetic issue on an otherwise healthy tree, so chemical control is rarely, if ever, needed.