DEFINITION: Insect galls are growths that develop on various plant parts in reaction to the feeding stimulus of insects and mites. Galls may be simple enlargements or swellings of stems or leaves, or highly complex novelties of plant anatomy, but they are always specific to the gall former. Galls are formed mainly by gall midges and some other flies (Diptera), gall wasps (Hymenoptera), and mites (Acarina), but are also caused by aphids (Homoptera), sawflies (Hymenoptera), and a few moths (Lepidoptera) and beetles (Coleoptera).
Sometimes, complexity is added by nature to the pattern of insects emerging from galls. The original egg-laying insect may be eaten by another parasite or starved when the invader eats its food. Thus, the emerging insect may not be the causes of the original gall.
A collection of galls may be made to show the diversity included in this interesting subject. Nearly 1,500 insect species in the U. S. produce galls, creating a real challenge. Eight hundred species of gall insects are known just from the oaks in North America. The identity of the gall maker is usually possible by examining the structure and form of the gall. Some from a collection should be cut in half to show the internal structures, and the insects that have emerged can be glued on paper points next to the galls. Save not only the adult stage of the insect but any immature forms also. One of the plants most commonly harboring galls is the goldenrod. In the winter, one may find two kinds of goldenrod galls. One is caused by caterpillars on stems of the plant, and has a spindle shape of an inch or so. Exit holes will be seen on this kind of gall, where the caterpillar prepared its escape before pupating into a moth in the fall of the year. The other goldenrod gall is caused by fly maggots (Eurosta solidaginis) of the Family Trypetidae, and is a round swelling about an inch in diameter. Inside is a maggot that is fully grown, waiting in a suspended state of animation [diapause] until the spring when it will pupate into a fly and leave the gall. People who fish during the winter sometimes break open these goldenrod galls and use the fly maggot as bait. Close to the base of the stem can be found at least two kinds of galls formed by gall midges, one woody, the other soft and spongy. These also diapause through the winter and emerge as adults in spring when goldenrod begin to grow.
Rearing insects from galls is fun and relatively easy. The galls should be kept in a jar or similar enclosure, with some moisture. An option, especially for galls on twigs, is to tie a sleeve of cheese-cloth or muslin around the area to capture the insects that emerge from the galls. Some galls must be kept through the winter because the larvae require winter diapause. Some gall makers leave the galls as immature forms and drop to the soil where they may live out the winter. One can trap these in plastic bags and transfer them to small containers of peat moss that can be left outdoors for the winter and brought indoors in spring to catch the emerging adults. Part of the fun of studying galls is learning about the diverse life histories of the gall makers.
Damage caused. Some galls that cause leaf distortion can be damaging to trees and shrubs if they occur in large numbers, but most do not do so every year or appear to cause great plant damage or be economically important. Certain trees in a planting may be more susceptible than their neighbors. Most plants and gall makers have evolved together for many years and appear to have developed an equilibrium. Some slight unsightliness after gall formers have left some of the more succulent galls is counterbalanced by their interesting presence.
Benefits of galls. Many products are gained from galls. Tannic acid is a primary product, and is used in a certain kind of insecticide. Inks of best quality have been made from galls, the most prominent coming from the Aleppo gall from oaks in Europe and Asia. Galls are interesting to study, have a fascinating diversity, and make good teaching subjects. Sometimes, galls have been used as food. The "pomme de sauge," in the Near East, is valued as food due to its aromatic and acid flavor. In the Ozarks of the United States, a tiny black gall fed to livestock contains 64% carbohydrates and more than 9% protein.
Felt, E. P. 1940. Plant Galls and Gall Makers. Comstock Publishing Co., Ithaca, New York.
Gagne, R. J. 1989. The Plant-Feeding Gall Midges of North America. Cornell University Press, Ithaca, New York.
Russo, R. A. 1979. Plant Galls of the California Region. Boxwood Press, Pacific Grove, California.
Prepared by the Department of Systematic Biology, Entomology Section,
National Museum of Natural History, in cooperation with Public Inquiry Services,
Information Sheet Number 171