All About Galls

During the spring and summer, the Kemper Center for Home Gardening often get questions about galls, or growths found on trees and other plants. People want to know what they are, and what to do about them. The following information will tell you all you need to know about galls.

What are galls

Galls are atypical growths or swellings found on plant stems, twigs, leaves, buds, or flowers. 

What do galls look like?

Galls can be round or lumpy, smooth or hairy, large or small, and come in a wide range of colors including red, yellow, green, brown and black. 

What causes galls?

Mechanical injury and infection by bacteria, fungi, or nematodes can lead to gall formation. But the galls most commonly encountered by home gardeners are caused by insects and mites laying eggs in plant tissues.  

Galls form when meristematic plant cells are stimulated or injured and begin to divide irregularly. Meristematic plant cells are undifferentiated and under normal conditions will divide and form all the various parts of a plant. Gall-forming insects and mites use chemical signals to hijack meristem tissue to form a protective covering around their eggs. Galls protect developing larvae from predators and adverse weather, as well as provide a consistent supply of nutrition by way of regenerating plant tissue. Most species of gall-forming insects and mites are highly specialized and will produce very distinctive galls on specific parts of their preferred host plants.

Should I be concerned about galls?

Although their looks can be quite alarming, most galls do not cause long-term damage to plants and will not affect their overall health. Small amounts of galls, especially, are no need for concern. However, heavy infestations can damage young trees or particularly susceptible trees. 

What can I do about galls?

Individual galls can be pruned off, but this method of control is generally impractical for large trees. There is some anecdotal evidence that properly timed insecticidal treatments reduce populations, but these treatments also kill the numerous natural predators of wasps. The best thing you can do for a tree is reduce stress factors so the tree can naturally fight off the invaders. The best proactive steps you can take to protect your trees are to water thoroughly in periods of drought and to have your soil tested. This will inform you if any pH imbalances or nutrient deficiencies that need to be corrected to make sure your tree can withstand anything nature throws at it. 

What are common types of galls?

Gouty or horned oak gall. These large twig galls are easy to spot and are often reported on pin oaks, a common street tree found in our region’s suburban areas. They are formed by a wasp which lays its eggs in the meristematic cambium tissue just under the bark of an oak twig. As the wasp larvae develop and the gall grows larger, it can cut off the flow of water and nutrients to the end of the twig, eventually causing dieback. The two galls are caused by similar, but different wasp species.. The main difference between the two types is that the horned oak galls will produce “horns” when the wasps emerge, while the gouty oak gall does not do this. There is no treatment for gouty and horned galls, but their presence on an oak tree does not necessarily spell out doom. Major infestations of immature or already weakened trees could potentially lead to a tree’s demise, but on mature trees they are likely to only be an aesthetic concern.

Maple bladder gall. These galls are caused by eriophyd mites (Vasates quadripedes), which are frequently seen on the upper leaf surfaces of silver maples (Acer saccharinum). The small, wart-like galls appear green at first, but progress to pink, bright red and then black. The density of galls on maple leaves can vary from year to year. Heavy infestations may cause leaves to become deformed, yellow and prematurely drop, but this does not cause serious harm to the tree.

Jumping oak gall. White oaks and other oak trees in the white oak group are hosts to the jumping oak gall. Induced by a wasp (Neurotersus saltatorius), single galls are produced on the leaf undersides. The galls can cause leaves to scorch where they are attached. High numbers of galls on a leaf can cause the entire leaf to turn brown. When the gall falls to the ground in mid-summer, the activity of the insect inside causes the gall to “jump” a few centimeters off the ground, hence the name. The jumping activity enables the gall to wedge itself into the soil and leaf litter to overwinter.

Hackberry nipple gall. Hackberry nipple galls are produced by a psyllid (Pachypsylla celtidismamma) or jumping plant lice. The insect is only 1/8 of an inch long and is said to look like a miniature cicada. The green to purple-tinged, nipple-shaped galls occurs on the undersides of hackberry leaves. A heavy infestation my cause early leaf drop, but most are an aesthetic issue and don’t cause any harm to the tree.

Witch hazel cone gall. The witch hazel leaf gall is a conical-shaped gall that appears in early spring on the upper leaf surfaces of which hazel species, including cultivars. The gall is induced by an aphid (Hormaphis hamamelidis), in which young nymphs develop inside. The aphid uses both witch hazel and birch trees (Betula spp.) to complete its lifecycle. The galls will persist on the leaves throughout the season, but do not cause any harm to the tree.

Justine Kandra

Daria McKelvey
Supervisor, Home Gardening Information & Outreach

Catherine Martin
Public Information Officer

Photos by Daria McKelvey

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