Some trees have a lot of gall

Along a trail in Sullivan County a couple of weeks back, I spotted what appeared to be a large grape lying on the ground. It was green in color with some brown spots and had a section of oak leaf attached to it. Handling this curious object revealed that it had a smooth skin and was somewhat rigid, but not hard. The three-quarter-inch green ball was almost perfectly round.

The creator of this object was the oak apple gall wasp (Amphibolips confluenta). In the spring, adult female wasps lay a single egg underneath the dermal layer of an oak leaf, and after hatching, the larva secretes a chemical that causes the leaf to form a ball or gall around the larva. This protects the larva somewhat from predators, and also provides a food source in the form of gall tissue.

When the larva reaches adulthood, it frees itself from the gall by burrowing a tiny escape hole and flying off to find a mate to repeat this curious cycle of life. These wasps are mostly harmless to the host tree. However, there are several other species of gall wasp that can cause problems with trees, especially the twig and branch burrower species.

In the fall, these “oak apples” that haven’t been gnawed on can be found on the ground among the leaf litter in oak forest as brown one- to two-inch balls that appear largely hollow if you break them.