Other galls can cause deformation

During the spring, many things can cause our plants to look deformed.

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Last week, I wrote about fire blight and leaf galls, which can make azaleas and camellias look deformed.

There are other galls that can attack our trees as well. The difference is that most of these galls are caused by insects. I have seen many trees and have had many samples brought into the office with these galls.

These galls can be raised spots or bumps on leaves, swollen twigs and limbs, or like the sample brought into my office that looked like a cotton boll growing on an oak tree. These galls are caused by wasps, flies, midges and mites. You will find them on oaks, elms, hickory and many ornamental trees.

The galls are formed when the insect stings or lays an egg on a leaf, stem or twig. The insect has strong plant growth-regulating chemicals or other stimuli that react with the plant hormones. The growth-regulating chemicals cause the plant cells to grow into a ball or finger-like projection. The inner walls of these galls have a covering that is made up of a rich protein that the developing larva feeds on. The gall also provides a protected environment for the larva to develop.

However, there are predators and parasites that can attack the larva inside of the gall. When the larva reaches the adult stage, it emerges from the gall.

The majority of these galls are caused by wasps. There are more than 600 species of wasps that will cause galls to form on plants. These wasps are very diverse and can cause galls to form on roots, branches, twigs, buds, flowers, acorns and leaves.

One of the more interesting galls is caused by the wool sower gall wasp. It is the size of a golf ball and looks like a cotton ball. The surface of the gall is whitish with brown spots. The larva of the wasp develops in a seedlike structure in the center of the gall.

Another gall found on oak trees is the oak apple.

When I was growing up, we stayed in the woods. We would find large structures growing at the base of the leaves. When we broke them open, we would find a mass of fibers coming from the center. The shell was paper thin and could be crushed easily. We were playing with oak apple gall. There is one wasp developing in each gall. Most of the galls that form on oak trees cause no damage to the tree.

The dogwood is one of our favorite trees. However, there is a gall insect that will attack dogwoods. I have a number of dogwoods in the woods around my home, and the other day I noticed a number of limbs that had a tubular or club-shaped swelling at the tip of a branch or the swelling was on the branch. I didn't realize these were galls caused by a small fly or midge and not a wasp. The larva of the midge overwinters on the ground in leaf litter, sod and decaying grass. During the spring, they pupate and emerge as adults about the time the new leaves are coming out. The adult midge will deposit her eggs in the terminal leaves. When the eggs hatch, the maggots work their way to the new shoots that are emerging. They will damage the shoot, and the shoot will grow around them.

There can be as many as 60 maggots in a single gall. The galls will taper at both ends, and there might be more than one gall on a twig. In late summer, the maggots exit the gall and drop to the ground to repeat the process.

Since the gall envelopes the twig, it cuts off the flow of water and nutrients to the terminal shoot. The end of the twig dies. This can cause a reduction in the number of flowers the next spring. If the infestation is heavy, the tree can be stunted.

There also are mites that can cause galls to form on trees. I see these more on elm trees than any other tree. Last week, I saw many elm trees with this pest.

The mite that causes these galls is an eriophyid mite. These are microscopic mites that feed on plants. These mites can cause many different symptoms on plants, but in elms they cause a finger like projection to grow from the surface of the leaf. They do very little damage to the tree.

When you find a bump or swollen place on a tree, it can cause much concern. However, most of these galls are harmless and the tree will outgrow the damage.

Columbia County Extension Agent Charles Phillips can be reached at (706) 868-3413 or by e-mail at charlesp@uga.edu. The Extension Web address is www.ugaextension.com/columbia.