By Todd Lanigan, DNR Forest Health Specialist, Eau Claire, todd.lanigan@wisconsin.gov or 715-210-0150
This time of year, you may be noticing some large growths on oak leaves. You are either seeing oak apple gall and/or wool sower gall. These galls are formed by a small, stingless wasp, known as a Cynipid Wasp.
For the oak apple gall, when the female lays her egg, she injects a growth regulator that causes the leaf tissue to form around the egg. When the larvae begin to feed for the wool sower gall, this causes the gall to form. The galls in both cases protect the developing wasps from the elements and predators.
Oak apple galls are round and initially are green in color. Eventually, the gall turns brown as the wasp larva matures inside.

An oak apple gall