The galls formed by insects and mites are particularly conspicuous and abundant. There are well over 1,000 species of gall-forming insects in North America. Most of these parasites are capable of forming galls on only one plant species or on a few closely related species of plants, and even then only on a particular part of the plant. The growth of plant cells in galls has been likened to the growth of cancerous tissues in animals. The gall forms where the adult female midge lays one or more eggs on or in the terminal bud of a dogwood twig.
Dogwood is a common flowering tree of the eastern United States.
The galls are abnormal swellings of 1 to 2 cm on the twigs at the ends of the branches. The galls may not seem obvious at first, but you can soon learn to recognize them. The midges that form the galls are very small, so we will not be looking at them directly, but only at the galls they form. A random distribution is what one would expect if all trees were equally likely to receive galls, and trees with galls were equally likely to receive more galls as trees without galls. This does not mean that all trees would have an equal number of galls, because some could have more than others simply by chance.