Juniper burr galls occur on the leaves (scales) of several species of junipers and are induced by a midge (Oligotrophus juniperi). Little is known about the life cycle (biology) of this midge. Generally, midge eggs are laid in or on the host tissue. Galls develop in response to larval secretions, with the galls developing once the larvae begin feeding on the plant tissues. When the larvae complete their development they, depending on the midge species, may pupate within the gall or drop to the ground to pupate, eventually emerging as adults.
Juniper burr galls occur singly or in clusters. The bracts surrounding the gall flare out from the sides but do not bend downward causing the gall to resemble a burr. The gall tip is open and the bracts at the top spread in all directions. Greenish in color when young, the galls eventually turn brown.
The juniper burr gall contains a single larval chamber. Midge larvae do not have chewing mouth parts, rather they have sucking mouth parts. As a result midges do not destroy the nutritive cells surrounding their chambers.
These juniper burr galls were photographed near the spillway between Crystal and Baum Lakes in Lassen County CA.