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Publication #SP123

Insect Galls1

D.E. Short and J.L. Castner2

Galls occur on a wide variety of 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) (Plate 4). Of the more than 2,000 gall-producing insects in the United States, l,500 are either gall gnats or gall wasps. 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. 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.

Plant galls are abnormal growths of plant cells formed as a response to the insect's stimulus caused by egg laying, or larvae or nymphs feeding. In the spring, before the leaves are fully developed, eggs are laid in the leaf or stem. Gall production is believed to result when the cambium and other meristematic tissues react to stimuli produced by the larvae and cause the abnormal growths. The immature insects often can be found in a cell or cells within the developing gall. After a brief period of cell growth, all 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.

Galls are found most commonly on the stems and leaves, but also occur on trunks, flowers, fruit, leaf-shoot terminals, petioles and roots. Each gall-forming insect produces a gall that is characteristic of that particular insect. Some galls may be two 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 seem to cause a lot of concern to the general public. Generally they do not seriously harm the plant. Most ornamental plants and trees are not apparently injured even by relatively large numbers of galls. Some people actually look for plants which are susceptible to gall formation because of their unusual and aesthetic appearance.

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. 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 them healthy.

Insecticidal control is usually not practical because:

  1. Damage is not significant most of the time. Parasites normally build up and suppress populations of gall-makers before serious injury occurs.

  2. Correct timing of application is essential to control the adult pests before the gall is formed.

  3. Environmental contamination and expense are involved, especially on larger trees. Some galls that occur on the stems and limbs may be handpicked and destroyed.

List of plates

Plate 1. 

Horned oak gall.

Plate 2. 

Goudy oak gall.

Plate 3. 

Oak apple gall.

Plate 4. 

Gall wasp inside oak apple gall.

Plate 5. 

Phytophaga wellsi.

Plate 6. 

Gall formed by hickory midge fly.

Plate 7. 

Roly-poly gall on oak.

Plate 8. 

Oak fig gall.

Plate 9. 

Phylloxera gall on wild grape.

Plate 10. 

Psyllid gall on red bay.

Footnotes

1.

This document is SP123, one of a series of the Department of Entomology and Nematology, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. This document is available for sale as a high-quality, color publication. For ordering information or to order using VISA or MasterCard, call 1-800-226-1764. Date first printed: November 1992. Reprinted: February 1997. Reviewed: June 2005. Please visit the EDIS Website at http://edis.ifas.ufl.edu.

2.

D.E. Short, retired professor; J.L. Castner, scientific photographer; Department of Entomology and Nematology, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.The term plates, where used in this document, refers to color photographs that can be displayed on screen from EDIS. These photographs are not included in the printed document.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.