I’m a bit behind in posting these photos, which I took back in September. I was chatting with the blacksmith as he packed up his equipment when I glanced into the branches of the overhanging oak tree and noticed these round, light brown balls. They were about the size of golf balls, or a little smaller. I recognized them immediately as oak apple galls. I’ve looked for these before, but up till now, have found only oak bullet galls, which I wrote about here. Apple galls are larger in size, and house the larva of Amphibolips confluenta, a species of tiny gall wasp.
The best life cycle information I was able to find online tells this story: Adult wasps hatch from the galls in mid-summer. The males and females mate and then drop to the ground, where the females burrow into the soil and lay their eggs in the tree roots. Larvae hatch and live in the soil before pupating. Only wingless females hatch from the underground pupae. In spring, they crawl up the tree trunk and locate new leaves, where they inject an egg into the central vein. A tiny larva hatches inside the leaf bud, and as the leaf develops, the larva causes a chemical reaction inside the leaf that results in the formation of a gall around the larva.
The gall is thus made from a mutated leaf. Galls draw a disproportionate amount of nutrients from the tree and provide the larva with a rich supply of food, as well as a protective home. Each gall contains just one larva. When the larva is full-grown, it pupates within the gall and emerges as an adult wasp. These adult wasps have wings and can be either male or female. After drilling its way out of the gall by making a hole, each wasp finds a mate and starts the cycle again.
Wow, what a complex cycle. The galls do not injure the tree, and provide a perfect home for the larva. However, life is rarely simple. Galls are attacked by a host of predators and parasites who may occupy the gall at the expense of the original inhabitant.