A tree gall is an abnormal growth of plant tissue triggered by an outside organism, usually an insect, mite, fungus, or bacterium. These growths come in a remarkable range of shapes: smooth spheres, spiky knobs, wart-like bumps, felt-like patches, and woody bulges. They can appear on leaves, twigs, branches, bark, or roots. Most galls look alarming but cause little to no lasting harm to the tree.
How Galls Form
Gall formation starts when an organism manipulates a tree’s own growth hormones. In insect-caused galls, an adult female typically lays eggs in or on plant tissue. After the eggs hatch, the larvae secrete chemicals that hijack two key hormones the tree uses for normal growth: auxins (which control cell elongation) and cytokinins (which drive cell division). Research on oak gall wasps has shown that wasp larvae can actually produce these hormones themselves, flooding the surrounding tissue with growth signals the tree didn’t generate on its own.
At the same time, the larvae suppress the tree’s defense chemistry. The plant’s production of jasmonic acid, a hormone that normally helps fight off attackers, drops sharply in gall tissue. The result is a structure that grows rapidly, doesn’t fight back, and feeds the organism inside it. It’s a hijacking of the tree’s biology at the molecular level.
What’s Inside a Gall
Cut open a typical insect gall and you’ll find a layered structure built around a central chamber. The innermost layer is called nutritive tissue, a protein- and fat-rich lining that the larva feeds on directly. In young galls, this tissue is soft and wall-less, almost like a paste, which makes it easy for newly hatched larvae with tiny mouthparts to consume. As the gall matures, the nutritive cells develop walls and become more structured. Surrounding this inner layer are protective layers of dense plant tissue, sometimes hardened into a woody shell, that shield the larva from predators and weather.
A single gall typically harbors one legless larva, though some gall types house entire colonies. The larva feeds, pupates, and eventually emerges as an adult, leaving behind the now-empty structure on the tree.
What Causes Galls
The list of gall-causing organisms is surprisingly long. Insects are the most common culprits, with gall midges and gall wasps responsible for the majority of cases. Aphids, sawflies, certain moths, and some beetles also produce galls. Tiny eriophyid mites cause many leaf galls, including the familiar bladder galls on maples.
Fungi cause galls too. Cedar-apple rust, caused by the fungal pathogen Gymnosporangium juniperi-virginianae, produces round, reddish-brown galls on cedar and juniper trees. This fungus has a two-year life cycle that requires two different host trees. It overwinters as a gall on cedar, then in spring rains the gall swells and produces gelatinous orange tentacles that release spores carried by wind to apple trees. Spores produced on apple later travel back to infect cedars, starting the cycle again.
Bacteria can also trigger galls. Crown gall disease, caused by the soil bacterium Agrobacterium tumefaciens, works by literally inserting a piece of its own DNA into the plant’s genome. The transferred DNA reprograms plant cells to divide uncontrollably, producing tumor-like growths at the base of the trunk or along roots. Crown gall is one of the few gall types that can cause serious, lasting damage.
Common Galls by Tree Species
Oaks are the undisputed champions of gall diversity. Over 300 different gall types have been documented on oaks in the Upper Midwest alone. Oak apple galls are among the most recognizable: round, papery spheres up to two inches across, each containing a single wasp larva. Jumping oak galls are tiny and detach from leaves, visibly bouncing on the ground as the larva inside shifts its weight. Horned oak galls and gouty oak galls form on twigs and branches, creating hard, knobby masses studded with small horns.
Maple bladder galls are extremely common on silver maples. Caused by eriophyid mites, they appear as small wart-like bumps on leaves that start out red, turn green, and eventually blacken. They can cover a leaf densely enough to cause curling and early leaf drop, but they don’t threaten the tree’s health.
Other frequently encountered galls include hackberry leaf galls (bumpy, nipple-shaped growths caused by psyllid insects), hickory pouch galls, linden nail galls (slender, nail-shaped projections on leaf surfaces), and ash flower galls caused by mites. On goldenrod stems, which aren’t trees but are commonly noticed, a round swelling about an inch across is caused by fly larvae, while a spindle-shaped gall on the same plant comes from moth caterpillars.
Leaf Galls vs. Stem Galls
Leaf galls generally form as hollow pockets, bumps, or extensions of the leaf surface. Some are raised blisters, others look like tiny pouches hanging from the underside of a leaf. A distinct category called erineum galls consists not of swollen tissue but of thousands of tightly packed leaf hairs that shelter mites on the leaf surface. On beech trees, erineum forms at the junction of veins on the leaf underside. On sugar maples, it appears on the upper leaf surface and is often bright pink or red.
Stem and twig galls tend to be woodier, harder, and longer-lasting. Because they form on permanent structures rather than leaves that drop each fall, they can persist for years and sometimes interfere with the flow of water and nutrients through a branch. This is why twig galls are the type most likely to cause real problems for a tree.
Do Galls Hurt the Tree?
Most leaf galls cause little or no harm. They’re cosmetically unappealing but don’t affect the tree’s overall health or vigor. The tree drops its leaves in autumn, and the galls go with them.
The exceptions worth knowing about are twig and branch galls, particularly horned oak galls and gouty oak galls. These two species of gall wasp can cause serious injury, and heavy infestations over multiple years may kill branches or, in severe cases, an entire tree. Crown gall disease from bacteria is another exception, as it can weaken trees and create entry points for other infections. Cedar-apple rust galls typically do more damage to the apple host (causing leaf spots and fruit blemishes) than to the cedar.
Managing Galls
For most galls, the best approach is patience. Gall populations tend to cycle naturally between abundant years and scarce ones, partly because parasitic wasps and other natural enemies that live inside galls help keep populations in check. Removing every gall from a tree can actually backfire by eliminating those natural enemies along with the pests.
When action is warranted, timing matters more than the method. For twig galls like horned oak galls, prune young expanding galls as soon as they appear in spring. Removing old, dried-out galls serves no purpose since the insects have already emerged. For mite-caused galls like ash flower gall or hackberry leaf gall, a dormant application of horticultural oil just before bud break in spring targets mites when they’re most vulnerable and beginning to move. Insecticidal soap, timed correctly, can be equally effective against mite and adelgid galls.
For hackberry galls, it’s worth leaving some galled leaves at the base of the tree rather than cleaning them all up. Those fallen galls harbor natural enemies of the gall makers that will emerge the following spring and help control new infestations. A few galls left in place can actually improve long-term stability more than total removal would.