Tree galls: what extension services describe
Galls are odd lumps, bumps and swellings on leaves, twigs and roots, most often made by wasps, midges, mites, aphids or psyllids. This page lists the common ones that Missouri, Nebraska, Cornell, Georgia and Arizona extension publications describe, with a page each for maple bladder gall, oak galls, hackberry nipple gall, ash flower gall and Cooley spruce gall.
Checked against the sources at the bottom of this page on October 8, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named extension publications say. It does not identify a gall on any particular tree, say whether a tree is healthy, or tell anyone to treat, prune or leave one. Pesticide advice in a source is mentioned only in general terms; the guide on who may apply pesticides covers licensing.
What causes galls
University of Missouri Extension (G7272, reviewed September 2017) says galls are abnormal growths found on nearly every plant part, and typically result from a chemical stimulus from the pest interacting with the plant's hormones. It counts more than 1,500 gall producers, mostly insects and mites, with wasps, midges, eriophyid mites, aphids and psyllids the main ones, and says most gall-producing wasps attack only oak. Cornell University (July 1972) adds bacteria and fungi as causes. It names crown gall, a bacterial tumor usually at or near the soil level, as a gall that is harmful and often kills plants, and cedar apple leaf galls as fungal.
Whether the sources call them harmful
Missouri says galls generally cause little real damage and infested leaves usually photosynthesize at near-normal levels, but that the less obvious galls on twigs, small branches and roots can over time kill and weaken parts of a tree and lower vigor. Nebraska Extension in Lancaster County (May 14, 2024) says leaf galls seldom, if ever, cause permanent injury. The University of Arizona Backyard Gardener note on oak galls says most insect-caused galls on Arizona oaks are not harmful, that infested twigs are occasionally killed, and that on eastern oaks twig galls reportedly cause significant damage to pin and willow oaks. Cornell says that with some twig galls the stem beyond the gall may die.
Common galls, as the sources describe them
| Gall | Host and cause | What the source says about harm |
|---|---|---|
| Maple bladder gall | Red and silver maple leaves. A mite (Missouri, Nebraska, Cornell). See the page on maple bladder gall. | Rarely detrimental to the overall health of large trees (Missouri). |
| Maple gouty vein gall | Sugar and red maple. A midge (Missouri). | Does not seriously affect the health of the tree (Missouri). |
| Maple spindle gall | Soft maples and sugar maple. Mites (Cornell, Nebraska). | Rarely distorts the leaf, but many make leaves unsightly (Nebraska). |
| Oak apple gall (see the page on oak galls) | Round green balls, half an inch to 2 inches across, that turn brown, on oak leaves. A small wasp (Georgia, Cornell). | Georgia says galls rarely harm the tree. |
| Gouty oak gall | Hard woody lumps on twigs and small limbs of scarlet, pin and black oak, up to 3 to 4 inches long. A wasp (Missouri, Cornell). | Missouri says in heavy infestations twigs, branches or whole trees may die. Cornell says small branches have been killed and broken off. |
| Oak bullet gall | Woody round growths in masses on stems, mainly bur and swamp white oak (Nebraska). | Nebraska says the growth rate of heavily infected trees may be reduced. |
| Hackberry nipple gall and petiole gall | Hackberry leaf undersides and leaf stems. A psyllid (Nebraska, Arizona). See the page on hackberry nipple gall. | Little lasting damage (Arizona). |
| Ash flower gall | Black clusters at branch tips of ash. Eriophyid mites in the male flower clusters (Nebraska). See the page on ash flower gall. | Unsightly but does not affect tree health (Nebraska). |
| Cooley spruce gall | Cone-shaped overgrowths on spruce branch tips. Adelgids (Cornell). See the page on Cooley spruce gall. | Cornell gives no statement on harm for this gall. |
What the sources say about management
The sources agree that control is rarely discussed as worthwhile. Nebraska says nothing removes galls after they have formed, and that control is neither recommended nor necessary. Georgia says pesticides are usually ineffective because the insect or disease causing the gall is inside the plant, and Missouri says non-systemic insecticides are virtually useless once a gall has formed. Cornell says a few galls seldom warrant control. Where Missouri and Arizona list options for twig galls or heavy infestations, those are in the publications themselves.
Growths with other causes are on the pages for odd tree growth and things growing on trees. A pest that regulators track is on the pests and diseases guide.
Galls, one page each
Sources
- University of Missouri Extension, Insect and Mite Galls on Missouri Trees (G7272), reviewed September 2017, read October 8, 2026
- Nebraska Extension in Lancaster County, Common Tree Galls, May 14, 2024, read October 8, 2026
- Cornell University, Galls on Plants, July 1972, as posted by the Maine Department of Agriculture, Conservation and Forestry, read October 8, 2026
- University of Georgia Extension, Towns/Union County, Galls, December 6, 2022, read October 8, 2026
- University of Arizona Cooperative Extension, Backyard Gardener 61: Oak Galls, read October 8, 2026