Question
Fuzzy ball on Oak leaf
I read the question from Terry wondering about the fuzzy ball like structures on the underside of fallen oak leaves. I live in southern Minnesota. I too find it difficult to believe the growths are caused by small wasps. The sheer number of leaves with these growths would indicate a potentially worrisome infestation of wasps. I have attached a photo. Could this be a fungus?
AnswerThese are leaf galls caused by tiny wasps. A gall is an abnormal growth on a plant. The life of a gall wasp is very interesting. They do not hatch from the gall until they are a full grown adult. They quickly find another adult of the opposite sex and mate. After mating they both fall to the ground. The female burrows itself down into the ground next to the oak tree from which the gall grew. It finds the root of the tree underground and injects its eggs into the roots. The larvae hatch and feed on the roots for a year and then become pupae. The wingless females hatch underground, burrow out, and inject an egg into a leave's center vein. The larvae that hatch are round. They cause a chemical reaction that makes a gall form around the larvae. Inside the larvae eats and grows until it is an adult and then it will hatch out and the life cycle of the oak apple gall wasp will start again. These gall wasps cannot sting, so they don't bother us people and since they don't really bother trees either, galls are just something interesting to learn about.
Do not worry about these galls. The population of wasps will vary greatly from year to year and next year there may not be any depending on the weather conditions. Raking up the leaves will help some in reducing the population. Beyond that there is no real health problem for the tree.
