Oak galls: what extension services say

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 it. Pesticide advice in a source is mentioned only in general terms, and the guide on who may apply pesticides covers licensing.

What causes oak galls

University of Missouri Extension (G7272, reviewed September 2017) says most gall-producing wasps attack only oak, while midges form galls on a wide variety of plants. The University of Arizona's Backyard Gardener note on oak galls (January 16, 2023, Jeff Schalau) says the makers on its oaks are most commonly small cynipid wasps, but may include moths, beetles, flies or mites. It describes the method: the insect lays eggs inside plant tissue, fluid deposited with the egg makes plant cells multiply differently, the larva that hatches feeds inside and keeps the growth going, then pupates and chews its way out. It says the insect must begin at a precise moment in the plant's growth, generally around bud break when new leaves unfold in spring, or the plant may not form the gall. It also says each species makes a uniquely coloured, shaped and sized gall, and each larva develops in its own cavity.

Common oak galls, as the sources describe them

Gall	What it looks like and who makes it	What the source says about harm
Oak apple gall	Round balls on the leaves, green and turning brown. Georgia gives half an inch to 2 inches across and says a single wasp larva lives inside; Cornell gives 1 to 2 inches, with a spongy inside and a hard central core, on scarlet and black oak leaves, first noticed in May or June.	Georgia says galls rarely harm the tree.
Oak hedgehog gall	Tiny spines on spheres 1/2 inch across, often burnt red, on white oak leaves, with small wasps developing inside (Cornell).	Cornell gives no statement on this gall alone; it says most leaf galls are not harmful.
Oak flake gall	Small hemispherical galls 1/8 inch wide on new leaves, smooth and light on the upper surface and covered in white woolly hairs underneath. A wasp (Missouri).	Missouri says heavily infested leaves curl and become disfigured.
Oak blister gall	Common on oak leaves (Nebraska).	Nebraska says most oak galls are not serious.
Oak bullet gall	Circular woody growths, often in masses along stems, mainly on bur and swamp white oak (Nebraska).	Nebraska says high numbers may reduce plant growth and that the growth rate of heavily infected trees may be reduced.
Gouty oak gall	Solid, woody, irregular lumps on twigs and small limbs of scarlet, pin and black oak, up to 3 to 4 inches long and 1 1/2 inches across, often side by side along a small branch. A wasp whose cycle uses leaf blister galls in spring and twig galls after mating (Missouri). Cornell lists black, red, pin and scarlet oak.	Missouri says in heavy infestations twigs, portions of branches or entire trees may die. Cornell says small branches have been killed and broken off.
Oak vein pocket gall	Elongated swellings along the midrib and major veins of pin oak, green when fresh and brown and hard with age. A midge (Virginia Cooperative Extension).	Virginia says heavily galled leaves may look twisted and distorted.
Woolly fold gall	Found on oak leaves. A midge (Virginia Cooperative Extension).	Not published in the part of the source that names it.
Marginal fold gall	A tubular roll and swollen area on the leaf margin of pin and red oak, which may house several larvae. A midge (Missouri).	Missouri says foliage galls of this type rarely harm the host plant.

Whether the sources call them harmful

Arizona says most insect galls on its oaks are not harmful, that galled leaves tolerate them with little more than cosmetic damage, and that infested twigs are occasionally killed, which it compares to natural pruning. It adds that on eastern oaks twig galls reportedly cause significant damage to pin and willow oaks. Missouri separates the leaf galls, which it says usually allow near-normal photosynthesis, from the less obvious galls on twigs, small branches and roots, which it says can over time kill and weaken parts of a tree and lower vigor. Cornell says most leaf galls are not harmful, at most making a few leaves fall early, and that with some twig galls the stem beyond the gall may die.

What the sources say about management

Arizona says preventing gall formation is exceedingly difficult and not considered practical because the developing insect is wholly enclosed in living tissue, and that the damage does not warrant the energy, expense and environmental risk of an insecticide. Missouri says controlling galls with chemicals is not recommended in most situations, that non-systemic insecticides are virtually useless once a gall has formed, and that its own list of options for gouty oak gall is to prune out infested twigs and branches. Georgia says pesticides are not recommended in most cases because the cause is inside the plant. Cornell says a few galls seldom warrant control and may be hand picked.

Look-alikes the sources name

Arizona and Nebraska mention hackberry galls made by psyllids, covered on the hackberry nipple gall page, and Arizona notes that mite galls can cause more significant damage to non-native ornamental or fruit trees such as pears. Georgia names a fungal disease, azalea leaf gall, as a gall with a different cause. The wider group is on the tree galls page, and mistletoe, which Arizona says commonly grows on its oaks, is on the mistletoe page.