Mystery Balls

June 2nd, 2020

   One of the odder things you’ll sometimes see in nature this time of year is little green balls about the size of Ping-Pong balls.

Find one of these in the yard? It’s an oak apple wasp gall.
Credit: Linda Fortney

   Sometimes they’re very light-weight and papery with white fibers inside. Other times, they’re spongy – almost like over-ripened, mini green apples.

   People usually don’t know what to make of them. Are they some kind of seed pod? Maybe a weirdo fungus? Maybe some sort of fruit-like plant part? Something involving aliens?

   Actually, these mystery balls are none of the above. They’re plant deformities called “galls,” grown by plants in reaction to an assault by bugs, most often tiny wasps or mites.

   The little green Ping-Pong balls are a particularly curious gall known as the oak apple wasp gall.

   They grow on oak trees, look like little green apples, and are caused by wasps (hence the name).

   When they’re hanging on twigs, it looks like you have an oak tree growing apples. In landscapes, people often don’t notice them until the tree drops them into the lawn or garden beds.

   Either way, galls usually aren’t as bad as they look. Other than the occasionally large infestation, they’re fairly harmless to trees and require no treatment or sleepless nights.

   Entomologists haven’t nailed down exactly how bugs make these weird growths happen, but it’s some sort of hormonal and/or genetic reaction by the plant to secretions injected or deposited by the bug.

   Galls can take on many shapes, colors, and forms, but specific bugs always cause the same kind of gall on specifically targeted plants and plant locations.

These are leaf galls on a witch hazel.

   Oaks seem to be a favorite. They often get ball-shaped galls and other fleshy formations, including some really alien-looking ones that have finger-like projections.

   Maples and many other species commonly get leaf galls, which look more like little blisters or pimples growing on the leaves.

   In all cases, the point of gall-making for bugs is to create a shelter for the larvae to live in, along with a built-in food source – the fibers, spongy tissue, or similar plant material that the tree makes the gall out of.

   In a way, the galls are like little benign tumor-houses that increase the survival odds of the bugs that trigger them.

   When the larvae mature, they break out of the galls, and fly away to mate, lay new eggs, and/or overwinter for next year.

   Ball galls drop and dry, and galled leaves drop when the trees shed them in fall – their job complete.

   Be on the lookout for those oak apple wasp galls. If you happen to find one, break it open and see if you don’t see a single tiny wasp larvae in the middle.