Deciduous Tree Galls

Authors: P.J. Liesch, UW Entomology and R. Chris Williamson, formerly UW Entomology
Revised: 1/26/2026
Item number: XHT1064

Galls are abnormal growths in response to a stimulus, such as insects, mites, nematodes, fungi, or bacteria. The Upper Midwest is home to hundreds of different types of galls—most of which are caused by insects and mites. Common examples, include ash flower gall, hackberry leaf gall, hickory pouch gall, linden nail gall, maple bladder gall, and erineum galls. In addition, oaks are particularly famous for their diversity of galls.

Galls can be surprisingly common, but often go unnoticed. Once you take a closer look, you may find that most trees around you are actually host to one, if not multiple, types of galls.

Many homeowners are alarmed when they notice gall infestations, fearing that their trees will die. While galls may look odd, they are typically just a cosmetic issue. Galls may disfigure twigs and foliage leading to aesthetic damage, but rarely affect the health or vigor of host plants—especially established trees. Occasionally, a few galls can cause more serious injury to trees if twigs and branches are heavily affected.

Each type of gall is caused by a different organism. For galls caused by insects, these are often created by various species of minute, stingless wasps. However, insects such as caterpillars, aphids, beetles, and others can also cause galls on occasion. For most of their lives, gall-making insects live inside gall tissues. They tend to emerge as adults at specific times to mate and lay eggs. For much of their life, they are protected within the galls and pesticides work poorly, if at all. In addition, attempting to treat galls on large trees is not feasible. However, since galls are conspicuous and remain in place, gall-making insects can be easy targets of natural enemies such as predators and parasitoids.

Because most galls do not pose significant health threats to plants, tolerating their presence is often the best and simplest course of action. Gall presence and abundance can vary greatly from one plant to another and also over time. The study of galls is formally known as cecidology.

Hackberry Leaf Gall

Many of the galls on hackberry leaves are induced by psyllids, aka “jumping plant lice”. These galls typically appear as small conical or cylindrical structures on hackberry leaves. The adult pysllids look like miniature cicadas. In the fall, the adults leave the galls seeking places to hibernate, often invading homes through window screens due to their small size.

Management: Insecticide treatments are not necessary, as the galls will not harm the tree. When hackberry psyllids invade structures in fall, management should focus on physical exclusion (i.e., making sure a home’s exterior seals up well through the use of weather stripping, caulk, or other means). With their small size and ability to sneak through most window screens, it can also be helpful to keep windows closed if adults are sneaking inside. A conventional home-use insecticide can be sprayed in thin bands around doors, windows, and other potential entry points in the fall before the adults attempt to sneak indoors.

Hickory Pouch Gall

Many hickory galls are caused by the feeding of aphid-like insects called phylloxera. One species produces pouch-like growths on twigs and leaves. Severely infested foliage often turns a yellow-brown color; heavily-infested leaves, petioles, and twigs may drop from the tree. The pouch galls open and phylloxera leave the galls in early summer to continue their life cycle. This insect overwinters as eggs in crevices of old galls.

Management: Diligently removing and destroying old galls may help reduce the population of the responsible insects.

Linden Nail Gall

On lindens, basswoods, and other Tilia species, a type of microscopic Eriophyid mite causes linden nail galls. These galls have a slender, carrot-like appearance and project out of the upper surface of affected leaves. Linden nail galls can vary in color from green to pink and reddish and are typically 1/4 – 1/2 inch tall. These galls can be common and are typically spotted during the summer months.

Management: Pesticide treatments are not necessary, as the galls will not harm the tree.

Maple Bladder Gall

Wart-like growths on the foliage of silver or soft maple trees are caused by tiny, Eriophyid mites. These mites overwinter in cracks and crevices of the bark, and as buds swell and break in the spring, the mites migrate out to the bud scales. As the buds open, the mites feed on newly developing leaves. The leaves respond to this feeding by developing hollow galls in which the mites live, feed, and mate. These growths are first red, then turn green and eventually black. Galls occur singly or in clusters, and can cause the leaves to become deformed and to drop early. Once formed, galls cannot be removed from the leaves. In the fall, mites move back to the bark where they overwinter.

Management: Pesticide treatments are not necessary, as the galls will not harm the tree.

Erineum Galls

Erineum galls are slightly-raised, scab-like lesions that can often be mistaken for plant diseases. They can occur on a wide range of trees and shrubs. Erineum galls are caused by microscopic Eriophyid mites, which can only be seen under magnification. These mites typically overwinter on twigs and move to new leaves in spring. In fall, the mites migrate back to twigs before the leaves drop. Erineum galls are generally round and can be over 1 inch in diameter. They are often greenish, but can also be pinkish or turn blackish over time. Confirming the presence of mites requires dissecting the galls under magnification to locate the mites.

Management: Pesticide treatments are not necessary, as the galls will not harm the plants.

Oak Galls

Oaks are well-known for their galls, with over 300 galls known on oaks in the Upper Midwest. Most of these are caused by tiny, stingless wasps from the Family Cynipidae. They have complex life cycles, which can take multiple years to complete. Given the diversity, oak galls vary greatly in size and appearance. Some are small and detachable from leaves (e.g., jumping oak galls), while others can be larger (e.g., oak apple galls) or firmly embedded within plant tissues. Depending on the gall-forming organism involved, oak galls can be present on the leaves, petioles, twigs, branches, and even the roots. While most of these are merely a cosmetic issue, a few galls affecting twigs and small branches may cause notable leaf drop or twig dieback.

Management: Since most galls on oaks pose little threat to the health of trees, pesticides are not recommended. For galls affecting twigs and causing leaf drop or twig dieback, removal of these galls soon after their formation may help. This is most feasible on small, recently-planted trees. Larger oaks generally tolerate galls with little impact on their health. Since galls can be so common on oaks, most oaks in Wisconsin will be host to many different types of galls.

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