StoryImage( ‘/Images/Story//Auto-img-11309592004160.jpg’, ‘Photo by Dr. Robert A. Hedeen’, ‘Galls on a grape leaf caused by the Grape Berry insect, which is common in the United States east of the Rocky Mountains.’);
As a biologist at the University, a frequent identification I was called upon to make was that of some type of gall that appeared on various types of plants. Usually, the individual requesting the identification was afraid their tree or shrub was infected with some type of exotic disease organism, and they were relieved when I informed them there was little danger a favorite plant would be lost.
Galls are growths on plants of various types resulting from the presence of insect eggs or larvae. Usually, the gall is more obvious than the insects producing them. No one quite understands just how these tumor-like growths form, but we do know they are formed entirely of plant tissue in response to the presence of a stage in the life cycle of a specific insect. The term gall is appropriate as when the plant is irritated, it responds by forming the protective gall.
Galls are of many shapes and forms and range in size from a tiny protuberance to the so-called oak apple, which may be several inches in diameter. Though many species of insects instigate the production of galls, the majority of galls are caused by members of two families: the gall midges (true flies) and the gall-wasps.
A remarkable fact about the gall makers is that each species infests a particular plant, and the gall produced is of a definite form and shape. Hence, when the entomologist (bug expert) or naturalist who has studied these growths examines a gall, they know what insect produced it.
Biologists have long speculated as to how galls are induced to be formed. For a time, it was supposed that when the female insect laid an egg on a plant, a drop of some chemical or poison was introduced and was responsible for the abnormal growth. By this theory, the differences between galls of different insects were explained by the supposition that each chemical produced by different species had its own unique properties. Some galls caused by plant lice and mites may be formed in this manner, but recent observations indicate this is not the case with gall midges and wasps. With these insects, the gall does not begin to form until after the egg has hatched, and the larva begins to feed on the plant. Any growth-forming substance must obviously come from the larva and not from the mother. In spite of extensive study, no gall-producing substance has been isolated from an insect.
For the most part, one should not worry that the tree or shrub will be harmed by the galls that may be present. In times of stress to a plant, such as a severe lack of rainfall, as we have seen this year in the Rock River Valley, a heavy infestation of galls may contribute to the death of a tree or shrub. The Hessian flys galls, however, may routinely cause considerable damage to wheat crops, and the clover flower midge is frequently a serious pest of red clover throughout the country. On the brighter side, galls of various types have been found to be useful as sources of certain medicines, dyes, and organic acids used in industry.
Biologists have studied gall wasps and midges for well over 100 years because many of these insects have unusual life cycles. In some, one generation will emerge in late summer and consist entirely of females that reproduce without the benefit of males (parthenogenesis). The eggs of that generation hatch in the early part of the next summer and produce both males and females that propagate in the usual manner. In a few cases, the larvae are capable of reproducing (paedogenesis).
In the first part of the 1900s, the research studies of a zoology professor at Indiana University by the name of Alfred Kinsey contributed greatly to our knowledge of the odd life cycles and sex lives of gall wasps. Kinsey was apparently stimulated by his study of the sex lives of gall wasps, and in the 1940s and 50s expanded his research to become famous for his research into the sex lives of humans, quite a transition!
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.
From the Nov. 2-8, 2005, issue