Galls are the result of the abnormal growth of plant cells. They are caused by insects, mites, nematodes, bac-teria or fungi. Insects that cause galls include certain small wasps (cynipids), jumping plant lice (psyllids), aphids, thrips, moth caterpillars and beetles (Table 1). The most common galls on ornamental plants are those caused by insects and mites. Galls develop characteristic sizes, shapes and colors. Some galls have been used to produce inks and dyes, and some are acceptable food for animals and man.

Biology

The biology of gall-making wasps can be complex. Some wasps form different types of galls in alternating generations. In some species, the adults and galls are similar in the first and third generation, while very dif-ferent in the second generation. The two generations of the mealy oak gall wasp are described below as an ex-ample of a common gall-making insect.

Asexual generation: Spherical galls, 1/8 to 1 inch in diameter, appear on branches and twigs of live oak in late summer and early fall. When first formed, they are pink to pinkish brown and the yellow-green tissue inside is moist and soft. Adults emerge by chewing holes in the bases of galls during December. All adults are female; they do not mate before laying eggs on swollen leaf buds.

Sexual generation: Eggs laid by adults of the asexual generation hatch in early spring as leaf buds begin to open. Larvae develop quickly in leaf tissue and stimu-late the development of small, beige-colored galls re-sembling kernels of wheat. Adults of both sexes emerge from these galls after a few weeks. After mating, females lay eggs in post oak twigs and branches. These eggs remain dormant for 3 to 5 months. They then hatch and stimulate the formation of galls of the asexual generation.

Gall formation

Gall tissue is formed when a plant reacts to some stimulus from insects and mites. This stimulus might be : 1) a fluid injected by adults laying eggs; 2) the pres-ence of the insect or mite in or on plant tissue; 3) insect or mite saliva; or 4) insect excretions. Genetic changes in the plant tissue also may occur during gall formation. After a brief period of growth gall development stops completely. The insects or mites inside galls become surrounded by their food source and are protected by the gall.

Damage

The biggest problem with gall-infested plants is that they may seem unsightly. However, some people con-sider certain galls attractive and use them in flower arrangements and other crafts. Galls don’t usually damage plants, although leaves with insect-induced galls may fall from the plant earlier than noninfested leaves. Twig and stem galls usually persist for more than a year. They can weaken stems and twigs and cause them to drop during storms. In the case of pecan stem phyllox-era, this can reduce pecan yield.

Common plant galls and their inhabitants

Galls occur on a wide variety of plants. Some plants are hosts for only one or two species of gall-forming insect or mite species, while others such as oak and hack-berry trees can be hosts to many species (Table 1). Certain plants and plant varieties are particularly attractive to gall-forming insects and mites.

Galls commonly occur on leaves and stems, but also may occur on flowers, fruits, twigs, branches, trunks and roots. Some of the common galls are easily recognizable. Common terms used to describe galls are blis-ter galls, bud galls, bullet galls, erineum galls, flower galls, fruit galls, leaf galls, leaf spots, oak apples, pouch galls, roly-poly galls, root galls, rosette galls and stem or twig galls.

Management of gall-forming insects and mites

Prevention.. To avoid gall-forming insects, choose plants that are not known to be hosts to these pests. If planting a susceptible plant, select a good site and plant it properly. Should a susceptible plant already be in place, good horticultural practices will help keep it healthy. Gall-forming insects must attack the host plant at a very precise stage of plant development in order to form galls. Occasionally, certain trees will bear more galls than adjacent trees of the same species. Studies have shown that oak trees whose buds opened earlier than those of nearby trees had many more galls, because the wasp causing the galls needed open buds in which to lay its eggs.

Tolerance, hand picking and pruning. Since most galls and gall-forming insects are not a threat to plant health, attempts to suppress them are not usually warranted. Learning to recognize different galls and the insects or mites responsible may even be one way to enjoy nature! If galls are unsightly, they can be hand picked, or infested plant parts pruned and discarded. However, this may not prevent infestations the following season. Removing the host plant and replacing it with a non-susceptible plant is the only sure method of control.

Biological control. Several kinds of wasps parasitize gall-forming insects and limit the number of galls formed. These wasps are natural biological control agents and should be encouraged. To protect these beneficial wasps, don’t use insecticide when they are searching for their insect hosts (particularly from late spring through early summer).

Newly planted susceptible plants may have galls for a year or so before parasites find the galls and begin their attack on gall-forming insects.

Adult gall-forming insects leave galls through exit holes. The vacated space is almost immediately occupied by small spiders and beneficial insects such as lacewing larvae, ants or parasitic wasps. Thus, old galls house beneficial organisms that feed on the harmful insects. The honeydew-like substances associated with some galls attract ants, wasps and bees.

Chemical control. Although some pesticides (insecti-cides and miticides) are registered for controlling gall-making insects, their use is generally unwarranted and ineffective. Unless these products can be applied when adults are actively laying eggs, they give no control. Once galls begin to form, the insects and mites inside are protected and can not be killed with either a surface- applied pesticide or a systemic pesticide. Once gall growth begins it cannot be stopped with insecticides.

Table 1. Common gall-making insects and mites in Texas.

Educational programs conducted by the Texas Agricultural Extension Service are open to all people without regard to race, color, sex, disability, religion, age or national origin.

Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Zerle L. Carpenter, Director, Texas Agricultural Extension Service, The Texas A&M University System.

15M-10-94, Revised

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Last modified: June 18, 1999 by Paul O.