Inside the gall, the larvae of the wasp is curled into a "C" shape, eventually reaching 1.5 mm long. The larvae lives inside the gall feeding on the interior of the gall wall.
The jumping gall wasp completes two generations each year. The first generation is "gamic", consisting of both males and females, while the second generation is made up of only females. The first generation is started by the females emerging from underground and laying up to 150 eggs in the swelling buds of Garry Oak in the second half of March. Adults emerge from these galls in the first half of May. Some clumps of galls produce males and other females, but not both.
Females are darker, with more rounded abdomens than males.
After mating, a gamic female lays up to 70 eggs one at a time, on the underside of the leaf, preferring the most recently formed unhardened leaves at the end of the branches. In early June the "agamic" generation have hatched and tiny galls begin to form on the lower surface of the leaf and by mid-June many of these galls have matured into mustard seed-like galls, 1.0–1.5 mm in diameter.
The galls start to fall off the leaf in late June to mid July with a small number dropping off in August and September.
Once on the ground the curled larvae flexed and the flexible walls of the round gall move. The purpose of this movement is to work the gall into the soil to overwinter there. Often however, the gall jumps from this flexing motion, hence the name. On dry July days you can hear the noise of jumping galls as they move and land on the dry leaf litter. The "jumping period" can last for 8 weeks. The wasp pupates in the gall, underground and is fully adult inside the gall by October, but stays inside until spring.
Heavily infested trees can have a scorched appearance by mid summer, and it can cause leaf drop. An excellent extenion pamphlet on the life history of this species can be found by clicking here.