A common question we get at the extension office has to do with little round balls falling out of trees

Sometimes they are fuzzy, and sometimes they are smooth. The fuzzy balls falling to the ground are called woolly oak leaf galls. They are usually attached to the lower surface of an oak leaf and fall off the leaf. The smooth BB-like gall is called the live oak pea gall. When I first encountered these galls, I wondered what made them. After a little research, I found out there are many of types of galls on trees and they can be caused by insects, fungi and even bacteria. Most are harmless and are more interesting than they are detrimental. The galls we will be learning about in this article are all formed by insects.

The most prolific insect that causes galls is called a gall wasp

Most gall wasps are in the Cynipidae family and are called cynipid wasps. These wasps are very small and all, but a few species are less than 1/4 inch in length. Their color varies greatly. Some species are black, others are red, yellow, or amber. The larvae are legless and both larvae and pupae are white in color. The female gall wasp will lay her eggs in actively growing plant tissue. The plant tissue then starts growing around the egg, protecting and providing food for the gall-maker until it matures.

Most oak wasp galls are harmless to the tree

In some cases, the galls can cause superficial damage and only on occasion, can they cause dieback in heavy infestations. Chemical control is usually not very effective. However, if a tree is continuously being infested, target the adult gall-makers before they lay eggs in the spring with a contact insecticide. Correct timing is very important.

One of the most noticeable galls on oaks looks like a sandspur attached to the twig

It is called the leafy oak gall. If you carefully cut the gall in half, you often see the developing larvae inside. Another common gall is called the oak apple gall. They are round, green galls that are filled with a spongy mass. There is one larva encased in the center of the gall. I once convinced a co-worker that it was the fruit of an oak tree.

Lately, we have been seeing a very severe infestation of roughbullet galls on live oaks in commercial plantings

It appears that in the most severe cases, the tips of heavily infested branches die back. Some researchers are looking into whether or not there are any live oak cultivars that are more or less resistant to roughbullet galls.

Pruning and destroying infested plant parts is an effective, but labor-intensive, way to minimize gall problems

Without pruning, leaf galls will either drop off with the leaves or “jump” off by themselves. However, those on the branches, roots, and stems of trees may persist harmlessly for several years.

Host plant selection is important, but more work is needed to identify resistant plant varieties and species to certain gall-makers. Gall susceptibility is likely linked to a plant’s genetics and age. In general, galls are more likely to form on actively growing plant parts. Maintain normal fertilization, irrigation, and other approved practices to keep plants healthy. For more information on insect galls go to: https://hgic.clemson.edu/factsheet/galls-outgrowths/

Article Written by Larry Figart, Urban Forestry Agent, Program Coordinator, October 2023 

0