After working as a chemist at a biotechnology company, I enjoy writing about science, travel, and gardening.
Living in a forested area, I have encountered many trees affected with leaf galls in our woods. This generally harmless condition might be ugly, but rarely affects the health of the tree. This condition is caused by an infestation of tiny mites or other insects and rarely persists from one growing season to the next.
What Are Those Strange Bumps on Leaves?
When a mite or other insect feeds on a leaf, their saliva causes the plant to increase growth hormone production at the site of the wound. This triggers an overgrowth of cells, forming a “gall.” Galls can be red, yellow, or black in color and may appear as a spike or a bump, depending on the insect species. The gall will continue to enlarge as the encapsulated mite or insect continues to feed and grow. Even after the insect dies, the gall may continue to enlarge for a period of time.
Saplings and immature trees are more likely to experience leaf galls than mature specimens, and leaves are more commonly affected than branches or flowers.
Common Gall Types
|Causative Insect||Tree Species Affected||Type of Gall|
Maple, ash, linden, birch, viburnum
Leaf or flower bud galls
Psyllids (jumping lice)
Cynipid gall wasps
Oak and rose
Leaf, stem, or twig galls
Adelgids (woolly aphids)
Poplar and cottonwood
Willow, Filbert, and hazelnut.
Stem and twig galls
Are Gall Mites Harmful to Trees?
Very young saplings with an overabundance of galls may experience premature leaf drop and growth stunting, as the galls require nutrients to form. Mature trees will not be harmed by the formation of galls, but repeated infestations over the course of several years may cause harm to young trees.
How Do You Get Rid of Leaf Galls?
Once galls are visible, it is not possible to eliminate the insects causing them or the growths. Fortunately, trees are not generally harmed by the condition and may not have the same infestation the next year.
For young saplings in danger of stunting or death due to repeated infestations over the course of many seasons, it is possible to kill the mites using horticultural oils or miticidal soap in early spring, before any galls form. In general, these sprays are not recommended as galls rarely harm trees and the control measures kill beneficial mite species and insects in addition to the ones causing the galls.
How to Prevent Leaf Gall
It is possible to reduce the chance of a mite infestation by doing the following tasks:
- Rake and dispose of fallen leaves affected by gall.
- Remove nearby heavily infested corn or wheat plants.
- Plant trees that are resistant to mites and gall formation.
Some trees that are not commonly affected by galls include magnolias and other trees with a thicker, heavier leaf texture. Oak trees are unlikely to form leaf galls caused by mites, but may form wasp galls on branches.
Most Common Types of Galls
There are many types of galls that form, with the most common occurring on the leaves of maple trees and in the branches of oak trees.
Maple Leaf Spindle Gall and Bladder Gall
Caused by a microscopic mite, spindle galls are often yellow and form sharp spikes on leaves. Bladder galls are smooth red bumps and are caused by the same mite that causes spindle gall. Sometimes two different gall types may be observed on the same leaf.
Cooley Spruce Gall
This gall is caused by tiny gray aphid-like insects called adelgids. Unlike other offending insects, this particular gall requires two tree species. The adelgids overwinter on the spruce trees and then migrate to spruce trees to lay eggs. The galls are 1”-1.5” in size and typically form in August or September. Cutting the gall in half will reveal the adelgids within the structure.
Oak Apple Gall
Of the approximately 800 insects that cause galls on oak trees, over 700 of the insects are represented by wasps. The gall wasp produces hormones that are similar to the plant’s growth hormones, stimulating the growth of a gall around a laid egg. For some species, the female either injects these analog hormones into the branch when she lays the egg. For other species, the egg secretes hormones that are chemically similar to the plant’s hormone. The larvae develops within the gall, which protects and nourishes the developing wasp.
Ash Flower Gall
Affecting male ash trees, the same mite that causes maple leaf galls attacks flower buds in the early spring. The flowers become deformed and turn brown. Infestations often last for more than 2 years and treatment is difficult since the galls form in very early spring, before control measures can be applied. Mites are also becoming increasingly resistant to miticidal sprays and soaps.
Willow Pine Cone Gall
These galls are caused by tiny mosquito-like insects called midges. They form large, pine cone shaped growths on willow trees and bushes. They are most common on black willows, when a female midge lays an egg on the terminal buds of the plant in early spring. If the cone is cut in half lengthwise, it is easy to see the small, orange maggot developing inside the gall.
Gall Insect Identification
Eriophyid mites are also known as blister mites or rust mites, due to the type of damage they cause to plant leaves. Unlike spider mites, these mites are very selective about the species of plant they inhabit and do not cause wide scale harm to plant life. Each female lays about 80 eggs per month, which take about 2 weeks to mature.
Gall wasp females have an ovipositor to deposit eggs onto soft branch tissue in the early spring. The gall becomes visible on the tree anywhere from 1-6 weeks from the date the egg was laid. Each wasp species typically predates a specific type of tree, such as the Beech Gall Wasp or Oak Gall Wasp.
Psyllids are also known as “jumping lice” and are small enough to get through window screens. They sometimes pose a nuisance in early autumn. When magnified, they look like tiny cicadas and have powerful hind legs that allow them to jump and fly away when bothered.
Midges are tiny flies, generally 3/16” (5 mm) long. Single eggs are laid on the tips of the plant’s branches in early spring, and the emergent larva burrows into the plant’s soft tissue. In the case of the willow pine cone midge, pupation occurs over the course of the next winter, with the adult midge emerging the following spring.
Aphids and Adelgids form galls on both pine trees and deciduous hardwoods. The Witchhazel Cone gall aphid forms cone shaped galls on the plant’s leaves, while the Cooley Spruce gall aphid affects spruce trees and Douglas firs (galls do not form on Douglas firs, but pine needles will deform due to feeding). These insects are visible to the naked eye and can be observed if the gall is cut in half.
FAQ’s About Leaf Galls
Are Leaf Galls Poisonous?
While the spiky projections might look alarming, they will not cause any harm to people, pets, or wildlife.
Which Trees Have Galls?
Leaf galls are most commonly observed on maples, poplars, hackberries, and lindens. Branch galls are very common on oak trees and can also be observed on willow trees and spruce trees.
What Insects Cause Galls?
Mites, midges, wasps, and aphids are all insects that cause gall growth on various plants.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Leah Lefler
Leah Lefler (author) from Western New York on June 23, 2021:
They are very interesting occurrences in nature! We don't have many oak trees in our yard, but our maples and linden trees definitely suffer a lot from leaf galls caused by mites. They are so strange to see, Peggy!
Peggy Woods from Houston, Texas on June 22, 2021:
I have observed those galls on our oak trees. I never knew that it was due to a wasp. Interesting!