INSECT GALLS

General Description

Galls occur on a wide variety of woody plants. These growths may be the result of fungi, bacteria, nematodes or mites, but insects are the prime cause. Gall-forming insects include aphids, phylloxerans, psyllids, midges (gall gnats) and cynipid wasps (gall wasps). The gall wasps (order Hymenoptera, family Cynipidae) are the most important insects that induce plant galls. These wasps are very small and, all but a few species, are less than 1/4 inch in length. Color varies greatly. Some species are black, others are red, yellow or amber. The larvae are legless and both larvae and pupae are white in color.

Gall wasp inside oak apple gall

These growths are called galls because they contain large amounts of tannin which has a very bitter taste. Long ago, they were known as gallnuts because they tasted as bitter as gall. Galls vary widely in size, shape, texture and color. It is not uncommon to find several different species on the same woody plant or tree. If galls are cut open larvae, pupae and adults may be observed providing they have not previously emerged.

For detailed information on galls in Florida, see the UF/IFAS publication Insect Galls of Florida (SP 343).

Important Species

Of the more than 2,000 gall-producing insects in the United States, 1,500 are either gall gnats or gall wasps. Some species encountered in Florida on woody plants are:

cypress twig gall midge

Taxodiomyia cupressiananassa

Biology

Plant galls are abnormal growths of plant cells formed as a response to the insect's stimulus caused by egg-laying, larvae or nymph feeding. In the spring, before leaves are fully developed, the eggs are laid in the leaf or stem.

Insect gall on twig Small holes indicate adult insects have emerged

The immature insects can often be found in a cell or cells within the developing gall. After a brief period of cell growth, gall development stops. The insect becomes enclosed by the gall and feeds only on gall tissue during its development. Small holes on the outside of the gall indicate that the adult insects have emerged.

Host Plants

Galls are primarily a pest of trees. A few occur on woody ornamental shrubs and flowers. They are virtually non-existent on bedding and foliage plants. About 80 percent of the gall wasps produce galls specifically on oak trees. In fact, 60 percent of all known insect galls occur in the oak family and 30 percent occur in the daisy, rose and willow families.

Seasonal Abundance

Gall development usually begins in the spring and progresses throughout the summer and fall months.

Damage

Gall production is believed to result from the reaction of cambium and other meristematic tissues to stimuli produced by the larvae to cause the abnormal growths.

Each gall-forming insect produces a gall that is characteristic of that particular insect. Some galls may be 2 inches in diameter while others are so small they are rarely noticed. They occur in almost every conceivable form and color and their shapes range from spheres to tubes. The surface may be smooth, hairy or covered with spines. Gall susceptibility varies greatly between species in the same location. This is probably due to the general condition of the particular plant and its natural resistance.

Galls are found most commonly on the stems and leaves but also occur on trunks, flowers, leafshoot terminals, petioles and roots.

Sampling

The insects that produce the galls are seldom noticed because the major gall-forming insects, such as the wasps and gnats, are extremely small. However, the actual galls can readily be observed.

Cultural Control

Generally galls do not seriously harm the plant. If the tree or ornamental plant is unhealthy or under stress it will be damaged to a greater extent than healthy ones. Fertilize, irrigate, prune and carry out other approved practices to keep plants healthy. If practical, some galls that occur on the stems and limbs may be pruned and destroyed.

After formation, it is impossible to eliminate the galls or the pests with insecticides because they are enclosed and well protected inside the gall. Those that occur on the leaves will drop off with the leaves, but those occurring on the trunk, roots, and stems may persist for several years.

Insecticidal control is usually not practical because: (1) most of the time, injury is not serious; (2) correct timing of application is essential to control the adult pests before the gall is formed; and (3) there are considerations of environmental contamination and expense involved, especially on larger trees.

For the most current insecticide recommendations to control these pests, please contact the local county office of the Cooperative Extension Service. If you have access to the World Wide Web you can consult the University of Florida's Insect Management Guide at:

http://edis.ifas.ufl.edu/TOPIC_GUIDE_Insect_Management_Guide

Back to Insect and Mite Menu or Back to Main Menu