Millions of tiny round hard galls are falling from oak trees and dancing on sidewalks, streets, and grasslands in California. This happens every other year, but our 2016 crop in the Sacramento Valley is especially abundant and the jumping galls are piling up beneath valley and blue oak trees.
Jumping oak galls aren’t just a visual marvel, they are tiny animals with complicated lives. This gall wasp either gets to jump or to mate, but not both. And males never jump at all. One year the galls are jumpers and all larvae are females who reproduce asexually. The next year, larvae are both male and female who don’t jump, but mate and produce eggs that become the all female jumping generation.
Each jumping gall, the size of a mustard seed, contains one larva of a tiny wasp species who will emerge as a winged parthenogenetic adult female next spring and lay eggs in an oak leaf, without having mated. Emergence of adult wasps must synchronize with oak tree spring growth so leaves are available when the wasp is ready to lay eggs.
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When the female wasp inserts an egg into a leaf this causes leaf tissue to grow around the egg and form a gall. The egg hatches into a larva inside the gall still attached to the leaf and begins wriggling around. By mid-summer this movement causes galls to fall off the leaves, pattering down like living raindrops.
On the ground, the galls continue jumping, landing in crevices and cracks, protected from the sun. The galls can jump a centimeter — 10 times greater than their size! Larvae eat the insides of the galls and when colder temperatures come in the fall, they cease movement, pupate, and overwinter still within the gall.
Next spring when the oak leaves are the right size, a parthenogenetic adult female wasp chews a hole in the gall and squirms out. She flies to an oak leaf and lays an egg that becomes encased in a gall.
This next generation of larvae don’t jump; they stay in the galls on the leaves. In autumn, they drop to the ground with the leaves and pupate. The following spring, they emerge as male and female adults who mate and lay eggs. These eggs, then, become the all female generation who jump off the tree and bounce across the ground.
You may have jumping oak galls near you in North America depending on which oak trees are around. Oaks are divided into two groups white and red oaks. White oaks have rounded leaf lobes without points or bristles, and acorns that mature in one year, among other traits. They host species of jumping oak gall wasps, but red oaks don’t. Some common white oaks include white, bur, overcup, post, chestnut, swamp chestnut, swamp white and chinkapin oaks in the eastern US and southeastern Canada. Gambel and Garry oaks grow in the west, while blue and valley oaks are endemic to California.
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