Sagebrush Galls

Big sagebrush (Artemesia tridentata) flowers in the fall and produces small fruits with tiny seeds. So what are the ball-like growths attached to a leaf or stem?

Round and smooth, greenish or reddish, sometimes hairy, sometimes wrinkled up, often with a small hole visible on closer inspection, the balls are not fruits but galls.

Galls are tumor-like plant tissues growth in reaction to an insect. Their size shape, location, and host plant are specific to a particular species or family of insects -- be they flies, moths, beetles or other species. Grown in response to the invading insect, the gall provides shelter, food and water to its new occupant. Typically, galls do not cause harm to the host plant, though they don’t provide any benefit either.

Big sagebrush is known to host dozens of different species of gall midges – small, two-winged flies with long legs and long antennae, commonly confused with mosquitoes.

Sample of gall-producing insects. Encyclopaedia Britannica

On mating, female gall midges lay eggs on a sagebrush leaf. The egg hatches and the larva begins feeding on the leaf, using its saliva to digest the plant tissue. In response, the plant produces a tumor-like growth that forms an encircling gall. The larva may occupy its new home from a few weeks to more than a year. On a hormonal cue, the larva pupates and mines a tunnel to the gall’s surface, using newly formed antennal horns. On emergence, the pre-adult, splits the pupal exoskeleton, and flies away to find a mate and repeat the cycle.

While the gall feeds, waters, and shelters the larva, it is not immune from attack. While in the gall, the midge larva may become prey to parasitic wasps which penetrate the gall and lay an egg directly inside the body of the larva. The result should be obvious. Other insects may feed on the gall and use existing galls for their nursery.