# Wasp galls on oaks

> Oak apples, bullet galls, wool sower galls and woody twig knobs are all grown by the tree around a tiny wasp. They look like disease and almost never are.

Published 2026-09-15, updated 2026-10-24. Tags: Oaks, Decay & damage, How trees work. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/wasp-galls-on-oaks/

Every summer we get photographs of oaks carrying things that should not be there. They show papery brown balls hanging from the leaves, hard marbles along the twigs, a white fuzzy lump flecked with pink, or knobby swellings on the twigs. These are galls, and on an oak nearly every one is the work of a gall wasp. They look like a disease, but they are closer to an arrangement the tree never agreed to, and in almost every case the oak will be fine.

![Round green oak apple gall on the leaf of a low oak sprout among grass and ferns](https://bluewhaletrees.com/img/articles/wasp-galls-on-oaks-1-1000.jpg?v=285c1946 "An oak apple gall, still green in June, on a young oak in the Arbutus Oaks natural area in Wisconsin. By autumn it will be brown and papery.")
*Photo: wackybadger, [CC BY-SA 2.0](https://creativecommons.org/licenses/by-sa/2.0) ([source](https://commons.wikimedia.org/wiki/File%3AGall_from_Oak_Gall_Wasp_%28Cynips_quercusfolii%29.jpg))*

## What a gall is

A gall is the tree's own tissue, grown into a shape the tree would never make on its own. A female gall wasp, a cynipid a few millimeters long and easy to mistake for a gnat, lays an egg in a bud, leaf, twig or root. Chemical cues from the egg and then the larva redirect the plant's growth. The oak responds by building a chamber around the larva and lining it with nutritious tissue. It wraps the chamber in a wall that may be spongy, woolly, papery or woody. The larva eats the lining, pupates and chews its way out. The tree supplies the material and the energy; the wasp supplies the instructions.

> "Galls are irregular plant growths which are stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects or mites." (University of Kentucky Entomology, *Common Oak Galls*)

Each species of wasp makes its own gall on its own group of oaks, [red oaks or white oaks](https://bluewhaletrees.com/articles/red-oaks-and-white-oaks/index.html.md), rarely both, and the gall is usually more distinctive than the insect.

## The ones you are most likely to see

![White woolly gall flecked with pink-brown points hanging from an oak twig among lobed green leaves](https://bluewhaletrees.com/img/articles/wasp-galls-on-oaks-3-1000.jpg?v=80d225a5 "A wool sower gall at Patuxent Research Refuge, Maryland, on May 22, 2022. Under the wool are seed-like chambers, each with its own larva.")
*Photo: lwolfartist, [CC BY 2.0](https://creativecommons.org/licenses/by/2.0) ([source](https://commons.wikimedia.org/wiki/File%3AWool_sower_gall_patuxent_research_refuge_north_tract_5.22.22_DSC_6524.jpg))*

Oaks host more kinds of gall wasps than any other plant in North America, several hundred at least. A few are familiar:

| Gall | Where on the oak | What it looks like | Oaks | Harm |
| --- | --- | --- | --- | --- |
| Oak apple | Leaves | Round, up to two inches across, green in early summer and papery brown by fall | Red oaks | Cosmetic |
| Bullet gall | Twigs, in clusters | Hard, round and woody, the size of a marble | [White oaks](https://bluewhaletrees.com/plants/white-oak/index.html.md) and [bur oaks](https://bluewhaletrees.com/plants/bur-oak/index.html.md) | Cosmetic |
| Wool sower gall | Twigs, in late spring | A white, woolly ball flecked with pink, the size of a ping-pong ball | White oaks | Cosmetic |
| Jumping oak gall | Under the leaves | Tiny round galls like poppy seeds, which drop in summer and hop | White oaks | Cosmetic |
| Horned or gouty oak gall | Twigs | Woody, knobby swellings, marble-sized to two inches | [Pin oak](https://bluewhaletrees.com/plants/pin-oak/index.html.md) most of all, and black, scarlet, red and [willow oaks](https://bluewhaletrees.com/plants/willow-oak/index.html.md) | Kills the twig beyond it |

In an oak apple, a single larva sits in a chamber at the center, held by spongy fibers. Some kinds of bullet gall secrete a sweet honeydew that draws ants and yellowjackets, which end up guarding the galls from the larva's parasites. A wool sower gall is made of seed-like chambers, each with its own larva. In a jumping oak gall, the larva flexing inside makes it hop like a jumping bean. Horned oak galls sprout small horns as they mature; gouty oak galls stay smooth.

![Gray-green woody gall on an oak twig, small tan horns sticking out all over its knobby surface](https://bluewhaletrees.com/img/articles/wasp-galls-on-oaks-4-1000.jpg?v=e8d20462 "A horned oak gall in Fairfax County, Virginia, on May 8, 2021. Horned oak galls sprout small horns as they mature, and each kills the twig beyond it.")
*Photo: Jeff Clark, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0) ([source](https://commons.wikimedia.org/wiki/File%3ACallirhytis_quercuscornigera_-_iNaturalist_127504361.jpeg))*

## Two generations, two galls

Many gall wasps alternate between generations that look nothing alike. A spring generation of males and females develops in small, soft galls on leaves or catkins. Their daughters lay eggs in twigs or buds, and the next generation, all females that reproduce without mating, grows in a different gall, sometimes taking two or three years to mature. For the horned oak gall, the spring blisters along the leaf veins are easy to miss; the woody twig gall that follows is what people notice.

This is also why identifying a gall to species is hard. Many gall wasps are known from a single generation, some galls have never been matched to an adult, and the adults are minute and look alike. An extension entomologist or a gall enthusiast can usually name a common gall from a good photograph and the identity of the oak. For many of the rest, "a gall wasp on a white oak" is as far as anyone gets, and far enough.

## When galls matter

Leaf galls and most twig galls are cosmetic. The tree gives up a little leaf surface, a galled leaf may drop early, and that is the whole cost. A tree loaded with oak apples loses less than it would to a mild caterpillar year. That is the answer to almost every summer photograph that reaches Blue Whale Trees.

The exception is the woody twig galls, horned and gouty oak gall, on pin oaks and their close relatives. Each gall strangles the twig it sits on, so the branchlet beyond it dies. In a heavy year that adds up to a crown full of dead tips, and several heavy years in a row can thin a small tree badly. Even then, galls kill only a tree already struggling.

> "Most leaf galls on oak cause little or no harm to the health of a tree. However, twig or branch galls may cause injury or even death to a heavily infested tree." (Penn State Extension, *Galls on Oak*)

Pesticides are close to useless. Once a gall has formed, the larva is sealed inside living tissue where no spray reaches it. A treatment timed to the few days when adults emerge is hard to time, kills the gall maker's own parasites, and does nothing about galls already on the tree.


![Cluster of knobby woody galls swelling an oak twig beside new spring leaves](https://bluewhaletrees.com/img/articles/wasp-galls-on-oaks-2-1000.jpg?v=9d692b99 "Gouty oak galls on a twig in Plainsboro Township, New Jersey, in April 2023. Each gall strangles the twig it sits on, so the branchlet beyond it dies.")
*Photo: Ryan Hodnett, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0) ([source](https://commons.wikimedia.org/wiki/File%3AGouty_Oak_Gall_Wasp_%28Callirhytis_quercuspunctata%29_Galls_-_Plainsboro_Township%2C_New_Jersey_2023-04-20_%2803%29.jpg))*

## What to do

- On a big tree, nothing. The galls weather, fall and are replaced by the next crop, in numbers that rise and crash on the wasp's own cycle.
- On a small pin oak or red oak with woody twig galls, prune out the galled twigs in late spring, while the galls are small and the wasps are still developing inside. Cut back to a healthy side branch, and bag or burn the prunings to reduce next year's galls.
- Keep the tree vigorous, with mulch, water in a drought and no vehicles or fill on the roots; a vigorous oak outgrows its galls.
- Then enjoy them. Chickadees, titmice and woodpeckers tear galls open for the larvae in winter, and a single kind of gall may host a dozen or more other insects, from squatters to parasitic wasps hunting the gall maker. The tannin in oak galls made the iron gall ink Europe wrote with for centuries. A galled oak is a busier tree than a clean one, not a sick one.

The [oaks we grow](https://bluewhaletrees.com/shop/rare-oaks/index.html.md) will host their share sooner or later; it comes with the tree.

Wasps crowded on a trunk in late summer, feeding at rows of small weeping holes, are [a bird's doing, not a gall wasp's](https://bluewhaletrees.com/articles/sapsucker-holes-and-wasp-damage/index.html.md). Two more oak alarms end the same way: [a shelf fungus on an old trunk](https://bluewhaletrees.com/articles/fungus-is-not-a-death-sentence/index.html.md) and [a red flush on new spring leaves](https://bluewhaletrees.com/articles/why-new-leaves-come-out-red/index.html.md).

## Common questions

### What are the balls on my oak leaves?

Galls: the oak's own tissue, grown into a chamber around the larva of a wasp a few millimeters long.

### Are oak galls harmful to the tree?

Leaf galls and most twig galls cost an oak less than a mild caterpillar year, and only the woody horned and gouty twig galls, mostly on pin oak, do real damage.

### Should I spray for oak galls?

No, because the larva is sealed inside living tissue where no spray reaches it, and a spray timed to the few days of adult emergence kills the gall maker's own parasites as well.

### Do oak gall wasps sting?

No: the wasps that come out of oak galls do not sting people. Most are a few millimeters long and easy to mistake for gnats.

## Sources

- [Common Oak Galls](https://entomology.ca.uky.edu/ef408): University of Kentucky Entomology: the wasps, the galls they make, and why control is rarely warranted.
- [Galls on Oak](https://extension.psu.edu/galls-on-oak): Penn State Extension: horned and gouty oak gall on pin oak, the two-year life cycle, and why insecticides seldom help.
- [Galls Made by Wasps](https://www.pubs.ext.vt.edu/ENTO/ENTO-145/ENTO-145.html): Virginia Cooperative Extension (Day and Dellinger, 2022): oak apple, horned, gouty, wool sower and jumping oak galls, most of them harmless, and sprays that rarely work even when systemic.
- [The Arthropod Associates of 155 North American Cynipid Oak Galls](https://pmc.ncbi.nlm.nih.gov/articles/PMC9810845/): Ward and others, Zoological Studies 61 (2022), via PubMed Central: about 700 oak gall wasp species in North America, and up to 21 associated species reared from one kind of gall.
- [Conservation Corner](https://loc.gov/loc/lcib/0806/conservation.html): Library of Congress Information Bulletin (June 2008): oak galls as the main ingredient of the iron gall ink used from the 12th to the 19th century.

## Related

- Next step: [Write to us](https://bluewhaletrees.com/support/index.html.md#article=wasp-galls-on-oaks)
- [Sapsucker holes and the wasps that follow them](https://bluewhaletrees.com/articles/sapsucker-holes-and-wasp-damage/index.html.md)
- [Twigs all over the lawn: squirrels, twig girdlers, or a tree shedding its own twigs](https://bluewhaletrees.com/articles/why-trees-drop-twigs/index.html.md)
- [Fungus on a tree is not a death sentence](https://bluewhaletrees.com/articles/fungus-is-not-a-death-sentence/index.html.md)
- [Native trees for insects: the beetles, wasps, cicadas and fireflies that everything else eats](https://bluewhaletrees.com/articles/native-trees-for-insects/index.html.md)