Jumping Jehoshaphat! The ground is alive!
Or so it seems when hundreds of tiny round spheres the size of a pin head are hopping around on the sidewalk. This spring there are plenty of these curious little things, thanks to a bumper crop of jumping oak gall wasps.
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Oak gall wasps are unassuming little insects that go about their business without notice throughout the year. Less than 10 millimeters long at adulthood, the female wasps lay eggs in oak leaf buds in the spring. Each egg, laid singly, creates a reaction in the oak's cells.
A biochemical signal trips the plant's growth hormones, causing the oak to grow a mass of tissue around the egg. The resulting structure is a gall. Galls not only serve as protective shelter, they also provide nutritious food for the developing insect.
The egg, nestled in its new digs, develops into a larva. Jumping oak gall wasp larvae get fidgety after a while and, like kids that have been cooped up in the house too long, they start to knock against the inside wall of their galls. Thus jostled, the galls fall to the ground. A hundred jumping oak galls falling sounds like a gentle rain.
The now-grounded larvae continue to roughhouse inside their galls. Lo and behold, the force of the agitated larvae causes the galls to jump! Some people notice the startling movement on sidewalks and driveways.
The galls can also be found hopping on paths and trails in forest preserves. By some accounts, a jumping oak gall may jump up to an inch. Not too shabby for being only 1/8th of an inch in diameter.
All the hopping around helps to move the gall and its occupant into a crack in the soil or a cozy spot among the leaves where it can settle into its pupal stage. It remains as a pupa inside the gall until the following spring when it will break out of its confines and emerge as an adult.
The small adult wasps unobtrusively go about the business of procreation. The females search for newly forming oak leaf buds and lay the next generation of eggs.
Jumping oak gall wasps are only part of a large group of wasps that hone in on oak trees. According to a University of Minnesota entomologist, 80 percent of all the gall-forming insects in the United States are associated with oaks.
Collectively, wasps that cause galls on oaks are called cynipid wasps. You've probably seen many kinds of cynipids. They come in a wide assortment of sizes, colors and shapes, and they can be found on twigs, branches and leaves. They cause "oak apples," gouty oak galls, woody twig galls, oak bullet galls and more.
Many people worry about the health of gall-covered oaks.
"Most galls are harmless to trees," wrote horticulture educator Tony Bratsch of the University of Illinois Extension. "But in Illinois, both the horned and gouty oak galls can be debilitating, even killing younger trees. There is some evidence that larger trees can also be weakened and die because of excessive gall development."
In the case of jumping oak galls, the leaves become discolored and may fall prematurely. There may or may not be lasting damage to the tree.
According to Dan Balser of the Ohio Department of Natural Resources, jumping oak galls usually don't kill trees.
"However, repeated damage of any kind will weaken trees and could result in tree mortality," Balser said. "Some severely infested trees prematurely (drop) leaves. These trees might put out a second set of leaves, but regardless, defoliation this early in the season will stress the trees and reduce growth."
Our oak trees have started out the year with some wild weather and they have been stressed by the hot, dry conditions in May. This could indeed put them at risk for gall damage and insect pests of all kinds.
The recent rain came none too soon, and we can only hope that the oak trees will garner the resources to offset damage by jumping oak gall wasps and other cynipids.
Jumping galls are, at the very least, entertaining. If you find them at the right time, you're in for a treat. Keep on the alert for small wonders of the natural world like these. If galls can jump, who knows what's in store for us next?
• Valerie Blaine is a naturalist with the Forest Preserve District of Kane County. She welcomes your comments and questions at firstname.lastname@example.org.