Oak galls in Washington and Oregon: what those balls on oak trees are, and do they hurt?

What are oak galls?

An oak gall is plant tissue, not an insect, a fungus or a seed. When a gall maker lays an egg or feeds, the egg-laying, saliva or injury sets off local plant hormones that make the surrounding tissue grow fast and wrap around the developing insect, according to the Pacific Northwest Insect Management Handbook. The tree, in effect, builds the larva a shelter.

Mites, midges, aphids, psyllids and sawflies all make galls on various plants, but on oaks the main makers are gall wasps. The PNW handbook notes that over 400 different galls have been identified on oak, and the Oregon Department of Forestry (ODF) lists where they form: leaves, branches, acorns, flowers and even roots.

The one most people notice is the oak apple gall, a round or kidney-shaped ball on a twig, 1 to 4 inches across, green at first, red on the sunny side, then brown or gray. WSU Extension’s Hortsense notes a single oak apple can house up to a dozen larvae, and the gall comes off without harming the twig.

The oak gall wasp life cycle

Most oak galls follow the loop in the diagram. A female wasp lays an egg in a bud, leaf or twig; the oak grows a gall around it; the larva feeds and grows inside the gall, which encloses and protects it; and the adult chews its way out, leaving a small exit hole. WSU notes that many galls mature in the fall.

Some species alternate between two generations with different galls, and the jumping gall wasp is the Northwest example. Its seed-sized galls drop from the leaves in fall, and the larva inside makes them hop an inch or more off the ground. The wasps overwinter in the fallen galls; in spring the females lay eggs in opening buds, which form blister-like galls, and that generation lays eggs on the leaves that become the next jumping galls (WSU; PNW handbook).

Tip: Gall wasps have their own enemies, and the PNW handbook says gall insects are heavily parasitized in most years. To see for yourself, seal a few galls in a plastic bag with a bit of tissue to absorb moisture and watch what emerges: gall wasps, or the parasitoids that eat them.

Common galls on Oregon white oak

Oregon white oak, also called Garry oak, is the only native oak in Washington and the principal one in Oregon, widespread at lower elevations in the Willamette, Umpqua and Rogue River valleys, per the USDA Forest Service silvics manual. That manual and the PNW handbook name four gall wasps as the prominent ones on it. With over 400 galls known on oaks, planted red or pin oaks can carry others; the table covers the four the sources tie to Oregon white oak.

Source: USDA Forest Service, Silvics of North America (Quercus garryana); PNW Insect Management Handbook, Oak gall wasp; WSU Hortsense oak gall fact sheets; ODF, Oak pests (2023); checked 2026-10-02.

Oregon State University’s landscape plant database names two of the four, the speckled gall and the oak apple, in its own Oregon white oak entry, and describes oak apples as 5 centimeters (about 2 inches) or more. Sizes vary by source because galls vary: WSU gives 1 to 4 inches for oak apples, the PNW handbook up to 4 to 5 inches.

Not sure your oak is an Oregon white oak? USDA’s tree records narrow it down. They list it as the only oak recorded in Washington; Oregon adds two more native tree oaks, plus a hybrid, oracle oak. Northern red, pin and English oaks have no record in either state, so if yours is one of those, it was planted, and its galls may not be the four above.

Source: USDA Forest Service FIADB reference tables, plant dictionary tree records (state distribution and U.S. nativity); checked 2026-10-02.

Do oak galls hurt the tree?

Rarely. The PNW handbook’s verdict on gall makers is that damage is generally not significant, galls rarely kill plants, and they are rarely numerous enough to warrant control. ODF describes the usual Oregon white oak picture the same way: scattered, small patches of dead leaves through the canopy that are “mostly cosmetic.”

Those dead patches are the twig galls at work. Galls in branches can rupture the bark and girdle the branch, killing the tip, and squirrels digging into twigs for the grubs add more twig dieback (ODF). WSU’s stem gall sheet says the same: twigs can be weakened or killed. On a mature oak that usually means scattered dead tips, not a dying tree.

Galls alone are no reason to hire a crew. If a big oak needs deadwood out over a roof or a driveway anyway, that is routine work covered on tree trimming, and the crew can take out gall-killed tips on the same visit.

Not galls: look-alikes and real oak problems

Several other things on an oak get mistaken for galls or blamed on them. Some are minor; a few are worth a closer look.

Leafy clumps with white berries: mistletoe

Clumps of olive-green, leathery leaves with white berries growing out of the branches are oak mistletoe, a parasitic plant. WSU lists Oregon white oak among its hosts, and the Forest Service silvics manual notes that hairy mistletoe is common on Oregon white oak in Oregon. The branch swells where the mistletoe attaches and often breaks there in storms; WSU’s advice is to prune out infected branches or pick the plants, with no chemical control.

Tiny bumps sunk into the bark: pit scale

Brown, gold or greenish bumps about 1/16 inch across, each sitting in a little pit on young twigs, are pit scale insects. WSU says they roughen twigs and in severe cases kill them, and heavy infestations combined with drought or anthracnose can severely weaken or kill a tree. Unlike galls, these are worth an arborist’s look.

Sticky leaves or a sticky car: aphids

The Forest Service lists aphids among the insects on Oregon white oak, and aphids produce honeydew, a sweet, sticky secretion that encourages black sooty mold (PNW handbook). If the patio or the car under the oak is sticky, that is honeydew, not galls; the cleanup is in how to get tree sap off a car.

Sudden branch dieback and pale sawdust: Mediterranean oak borer

ODF calls Mediterranean oak borer (Xyleborus monographus) a recently detected exotic beetle that is causing mortality in Oregon white oak. First trapped near Troutdale in 2018, it has since been found in Multnomah, Clackamas, Washington, Marion, Yamhill and Polk counties. The signs: sudden branch dieback in an otherwise full canopy, mounds of pale boring dust, round holes about 1/16 inch across, and black-stained tunnels in the sapwood. Report suspects to the Oregon Invasive Species Hotline, and don’t move firewood more than 10 miles from where it was cut (ODF).

If the dead wood is whole branches rather than scattered tips, look beyond galls: how to tell if a tree is dead covers the checks, and mushrooms on trees covers decay fungi at the base. More insects and diseases are on the pests page.

Why Oregon white oaks get special protection

Oregon white oak ranges from Vancouver Island to Los Angeles County and grows into a broad-crowned tree 40 to 90 feet tall (USDA; OSU). Large scattered trees in mixed-age stands are 270 to 330 years old, per the silvics manual, which is why a big oak is hard to replace.

Wildlife is the other reason. Washington’s Department of Fish and Wildlife lists Oregon white oak woodlands as a priority habitat; its 1998 management recommendations count about 200 species of birds, mammals, reptiles and amphibians that use the state’s oak habitat, and note that single urban oaks can qualify when they are particularly valuable to wildlife. Some cities write oaks into their tree codes (checked October 2, 2026; not legal advice):

Thresholds elsewhere are collected on tree removal permits in Oregon and tree removal permits in Washington. The tree itself, from its size and root system to the oak rules in Oak Harbor, Forest Grove and West Linn, is covered on Oregon white oak in Washington and Oregon yards. If an oak does need work, an arborist can tell gall damage from decline before anything is cut.