What Is This? Solving the Mystery of Oak Apple Galls

For years I traipsed through the forests of New England and saw these spherical brown and green things on the ground. I assumed they were some sort of fungus, but never really went further than that with my inquiry. Recently, however, I decided it was time for me to solve this forest mystery. The answer surprised me. These weren’t fungi at all, but were oak apple galls, and their story is quite interesting.

The forest is full of parasites. While we tend to think of parasites as nasty things that are harmful in some ways, not all parasites hurt their hosts. In fact, most don’t actively hurt their hosts at all, because if they did, they would lose their homes, food, or energy source. Some parasites are important parts of the forest ecosystem. The oak apple gall wasp is one of those.

In the late summer, these wasps lay their eggs underground at the roots of an oak tree. When the larvae hatch from the eggs, they feed on the tree roots. This doesn’t harm the tree under normal circumstances; the larvae are tiny. Soon, the larvae pupate. They build a shell and, while in there, they develop into an adult. Then they crawl out of the ground. After mating, the females, who don’t have wings, will climb up the oak tree in the spring and inject an egg into a vein of a new leaf on the oak tree. This is when things get weird.

When the egg hatches, hormones are released into the oak leaf. This causes the leaf to change, and rather than developing into a normal oak leaf we are used to seeing, it makes a ball-like structure–the oak apple galls–that surrounds and protects the newly hatched wasp larva. Fibers form inside the ball to support the larva and gives it a place to pupate. In the fall, the adult wasp hatches from the pupa and then drills a hole through the gall where it has developed up to this point. Once it’s out, it finds a mate, goes to the roots of the tree to lay its eggs, and the cycle starts again. The entire life cycle of this wasp takes place on a single tree.

While an infestation of these wasps could harm the tree by stressing it and not allowing enough leaves to form for it to photosynthesize (make energy and food from the sun rays), this generally doesn’t happen in a healthy forest. The wasp is a fairly neutral parasite in this case. Not only that, but once the wasps vacate the galls, they are beneficial to other species. Scientists found that creatures like jumping spiders moved into the galls, using them as homes. The jumping spiders can be helpful to the tree by eating herbivorous insects, bugs that eat leaves, including some invasive species. Raccoons and other animals eat the galls themselves, and other creatures eat the wasps, controlling that population so it doesn’t overwhelm the tree. Under normal circumstances, everything stays balanced. The wasps don’t have stingers, so they don’t even bother humans like some other wasps do.

There are many different types of gall wasps, all across the United States and in other countries as well. The process of creating the oak apple galls isn’t well understood, but it seems to work the same across different species of wasp. If you have a tree in your yard that seems to have a lot of these galls on it, and the tree seems to be affected, you can knock the galls off of the tree and interrupt the life cycle of the wasp, decreasing its numbers so it doesn’t overwhelm the tree’s resources. Climate change and deforestation have caused competition for forest resources to increase, and the balances can be thrown off between parasites and hosts.

A fun project you can do with children–or even by yourself!–is to dissect an oak gall. I took a couple galls home and cut one open to reveal the fibers inside that held the pupa. I even found the shell of the pupa that the adult wasp hatched from. You just need a sharp knife like an X-Acto knife, and you can carefully cut around the circumference of the gall. Once you have a cut all the way around, gently pull the gall apart. Some of the galls will be empty, because the fibers inside are delicate and sometimes decay or get eaten, but other times you can find them intact.

Oak apple gall wasps are only one type of parasite in the forest. There are a great many more, from fungi to plants to insects. While some parasites seem like pests, in a well-balanced forest, they are all just part of the healthy ecosystem.