The Homeowners Column

The Homeowners Column

Trees With a Lot of Gall (growths called galls on trees)

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

As you are wandering around your yard you may notice bumps on the leaves or odd round things on the twigs of maples, oaks, hackberries and cherries to name a few plants. The bumps actually have great names such as maple bladder gall, roly-poly gall on oak, succulent oak gall and hackberry nipple gall. Some of the bumps like the ones on maples are bright red and are pretty in a weird sort of way. Galls are growths on leaves, stems and twigs of many different plants. They can be created by insects, mites, bacteria, fungi and nematodes. The ones most often brought into our office are caused by insects and mites. There are at least 1,441 species of insects and mites that cause galls in North America.

Galls are actually created by the plants themselves in response to feeding or egg-laying by certain species of insects and mites. As gall-making insects feed or lay eggs, they create mechanical damage. The damage and/or the insect saliva initiates the production of plant growth hormones. These hormones produce abnormal cell growth that results in the development of the galls. For gall formation to occur the feeding or egg laying must happen while the leaves or plant part is growing rapidly, usually in early spring.

The gall making insects and mites are very specific to the kinds of plants they attack and to the types of galls they produce. The majority of gall-making insects are wasps. Hearing the name wasp may send people running for cover. However, most wasps do not sting people. It is just a few bad tempered guys giving the rest of the wasp world a bad reputation.

The common maple bladder gall is a bladder or pouch-like growth on the leaves of maples caused by an eriophyid mite. The galls form on the tops of leaves and change in color from yellow-green to a beautiful red and eventually to black as the season progresses.

Oaks can have numerous types of galls. Out of the over 800 species of gall making wasps in North America, 731 of them attack oaks. Oak deformities are of various sizes, shapes, and colors on leaves, twigs, flowers, acorns and buds. Galls are so commonly found on oaks that many people think the galls are typical parts of the plants. An easy winter identification for shingle oak are the numerous twig galls. Some early botanical drawings actually show galls as part of the normal plant.

Oak apple gall is caused by several species of wasps. The "apple" forms on the leaf midrib or petiole. The papery gall "apple" can be the size of a large marble up to golf ball size.

Ash flower galls hang in clusters from ash trees and are especially noticeable in fall as the leaves drop. A tiny mite feeds on the male flowers of the ash. The flowers enlarge and turn from green to brown. Often the clusters stay on the tree for two years.

The good news about galls is the majority do little or no damage to trees. The trees are just well accessorized. Generally there is enough unaffected foliage for the trees to remain vigorous.

Chemical control of most galls is difficult to impossible. Timing of any sprays is difficult since sprays must be initiated before gall formation begins. Late spraying may kill the gall maker, but the gall will continue to grow. The gall maker may initiate the gall, but the gall is produced by the plant.

University of Illinois Extension Master Gardener Garden Walk

Saturday, June 24 from 9a.m. - 4 p.m.

Enjoy eight lovely gardens throughout Champaign and Urbana. Tickets are available from area garden centers, master gardeners and at the Extension office for $8 ($10 on walk day). Also, shop among garden vendors at the Idea Garden.

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