Gall insects do little damage to trees

While looking at my hackberry tree, the leaves were covered with this small round growth. Is it a disease, insect or just part of the tree? Will it kill my tree? How do I control it?

Those small bumps, balls and swell areas on trees are caused by insects called galls. Galls are growths on plants that may be a simple bump or a ball with complicated infrastructures. Some galls are bright colors and while others are dull, black or dark brown. Galls can be formed on the leaves, twigs and branches.

The gall insect “stings” the plant and makes it grow a home for itself. Eggs are inserted within the tissue of the growing plant where the maggot-like larvae develop. The feeding and egg laying of the adult and its continued development will result in the secretion of the developing larva within the gall. Although the gall is entirely plant tissue, the insect in some unknown manner controls and directs the form and shape to fit the larvae needs.

This is probably the most marvelous instance in nature of the influence exerted over one organism by another. Entomologists are still not sure exactly what it is that makes the plants grow these curious, often elaborate structures. The galls are so defined that the insect species may be identified by the shape of the gall.

The gall structure can be round like a grape, an oblong tubes, furry balls either singular or in clusters, resemble pine cones, spiny balls or donut-shaped knots. The gall ranges in size from one-eighth to two inches in diameter and its interior consists of a fiber or spongy mass which contains the larva.

A strange feature of the work of gall insects is that the same species of insect on different species of plants causes galls that are similar. On the other hand, several species of insects attacking the same plant can cause galls that are greatly different in appearance.

The gall wasp insects are small, dark colored insects that infests a special part of the host and produces a characteristic gall that are so definite in form that the species can be identified by them. Most galls wasp are found on oaks, hackberries and pecans; however, many other kinds of plants also serve as hosts.

A gall that is prevalent in the landscape is the oak gall. They are formed by the deformation of the leaves cause by the developing larvae. The gall appears on leaves of the red and black oak in early spring

A gall wasp is of little economic importance. A few of the galls have a slight commercial value in the manufacturing of permanent ink and as a source of dye. Only a small number of species inflict appreciable damage to plants.

One gall wasp that is a problem in the pecan industry is the phylloxera gall. It forms a small BB-like growth on pecan trees. The wasp plants her egg just as the leaf bud breaks in the spring to when new shoot growth is two inches long. Control should only be accomplished if pecans are grown commercially.

Control of gall wasp on other plants is really not necessary because of what little damage that is done and does not hurt the tree. Control after the structure is formed is nearly impossible because the pesticide cannot penetrate the gall casing to the insect. Pruning off gall-infested growth usually takes care of the problem. Better yet, leave the gall on the tree to provide addition variety to the landscape and a good learning experience for children.

Although most species of galls are associated with the gall wasp, it should be emphasized that other insects can produce galls. These insects include mites, plant lice, gall midge, flies, moths and beetles. Many of these insects can cause damage and should be controlled.

Jim Coe lives in Lawton and writes a weekly garden column for The Lawton Constitution.