Oak-Apple Gall

Oak Apples, Woolly Catkins, and Leaf Blisters are appearing on oaks belonging to the red oak group in Ohio. While the galls may be very apparent, they cause no appreciable harm to their oak hosts.

There are somewhere around 800 different types of arthropod galls found on oaks in the U.S. About 700 are produced by tiny wasps (order Hymenoptera) belonging to the family Cynipidae. Of those, only a handful represent a serious threat to the overall health of their oak hosts.

An overly generous conference moderator once introduced a talk I was about to give titled, “Tree Galls: A Management Conundrum,” as being presented by a “gall expert.” I looked around to see if they’d made a last-minute speaker substitution.

I came across a number of interesting oak leaf insect galls during hikes this week in several local parks. Oaks are prime gall-fodder providing support for over 800 different types of galls. It's why I find oaks so interesting. Three-quarters of the gall-makers belong to two families: the wasp family Cynipidae; and the "gall midge" family Cecidomyiidae (cecido means "gall").

Several types of "oak-apple" galls are now obvious on the leaves of their namesake oak hosts in Ohio. These unusual plant growths range in size at maturity from 1/2 - 2" in diameter and are named for their resemblance to apples.