Those Round Balls on Your Oak: Galls, Explained Plainly

Here is the short version. A gall is a growth the tree builds around an insect, almost always a tiny gall wasp. The wasp lays an egg in a bud, leaf, or twig. The tree reacts by growing extra tissue around it. The larva lives inside, safe and fed, until it chews its way out as an adult. The tree did the building. The insect just gave the order.

The galls you will actually see in North Texas

Start with the oak apple gall. It is the one people call about most. A round, apple-sized ball, green in spring and turning brown by fall, hanging from a twig or leaf stem. Cut one open in June and you will find a single larva in the middle, surrounded by spongy tissue. It looks alarming. It is not. A healthy oak shrugs off a dozen of them.

Bullet galls are smaller and harder, about the size of a marble, stuck tight to twigs. They show up on post oaks and blackjack oaks, the tough native oaks that cover so much of Collin County. They stay on the tree through winter, which is why people notice them after the leaves drop. Woolly galls look like cotton balls glued to a twig, and they are just as harmless. Horned oak galls have little points sticking out of them. Gouty oak galls cluster along twigs in rough, knobby bunches.

Then there is the jumping oak gall, which is the odd one. It forms as a tiny seed-sized gall on the underside of leaves in spring, drops to the ground in summer, and actually jumps when disturbed, thanks to the larva moving inside. Homeowners in Melissa and Anna sometimes find hundreds of them on a patio and assume something is wrong. What happened is a big gall year, and it usually follows a mild spring.

Do galls hurt the tree?

Almost never in a way that matters. Leaf galls cost the tree a little leaf area and nothing else. Twig galls like the oak apple type take a small amount of energy. A mature live oak, bur oak, or post oak can carry hundreds of galls and keep growing like nothing happened. Trees have lived with gall wasps for millions of years. If galls killed oaks, there would be no oaks left in Texas.

There are two exceptions worth knowing. The first is a heavy load of gouty oak galls on a young tree. When knobby galls ring a twig all the way around, they can choke off growth past that point, and a small tree with galls on most of its twigs can end up stunted or misshapen. The second is a severe jumping oak gall year that strips enough leaves to stress a tree that is already struggling, say from drought or recent construction damage. Stress plus defoliation is a bad combination. Either case is a reason to get eyes on the tree, not a reason to panic.

What not to do about galls

Do not spray. By the time you see a gall, the wasp is already inside, protected by layers of plant tissue. Insecticide on the outside of a gall does nothing to the insect inside, and it kills the beneficial insects that actually keep gall wasp numbers down. Spraying a gall is like locking the barn after the horse moved in and furnished the place.

Do not pick them all off, either. Pulling leaf galls off in spring does not help, because the wasps that matter already laid their eggs or already left, depending on the species and the season. And do not fertilize a galled tree hoping to fix it. Extra nitrogen pushes soft new growth, which is exactly what gall wasps like to lay eggs in. If the tree is otherwise healthy, the best treatment for galls is nothing.

One more thing people get wrong: galls are not contagious in the way a disease is. A gall on your oak does not mean your neighbor's oak is doomed, and it does not mean your pecan or crape myrtle will get them. Each gall wasp species uses one host, usually one oak species. The wasps are already everywhere. Gall years just vary with the weather. Acorn crops run on a similar boom and bust cycle, covered in our acorn mast year guide.

Telling galls apart from real trouble

This is the part that matters, because some real problems get mistaken for galls and some galls get mistaken for real problems. Oak wilt does not make round balls. It makes leaves brown from the veins outward, fast, usually starting in one section of the canopy. If your oak has galls and sudden browning together, the browning is the problem, not the galls. Call someone about the browning.

Burls are different too. A burl is a hard, woody swelling on the trunk or a big limb that keeps growing year after year. Galls stay small and stop growing once the wasp leaves. Cankers are sunken, dead patches of bark, not raised balls. If you are not sure what you are looking at, a photo of the growth and a photo of the whole canopy will usually settle it.

When it is worth a call

Most gall calls end with reassurance, and that is fine. But get a crew out if you see any of these: twigs dying back past heavy gall clusters, a young tree with galls on most of its branches, a tree that lost most of its leaves to jumping oak galls and already looks thin, or galls plus sudden leaf browning that looks like oak wilt. Winter is the time to prune out the worst galled twigs on a young tree, well outside the February through June oak pruning window when fresh cuts invite trouble.

The bottom line is simple. Galls look weird. Weird is not the same as dangerous. Leave them alone, keep the tree watered through dry spells, and spend your worry on the things that actually kill North Texas oaks: oak wilt, drought, and buried root flares. The round balls on the twigs are just the tree doing what it has always done. For anything else on the tree, trimming and a straight assessment beat a spray rig every time, and you can reach us through the Melissa tree service page.