Can apples grow on oak trees? Will pine cones form on willows? Not true ones, but there are oddities of nature that might make us think they can - "galls". These unusually-shaped growths and swellings are a plant's reaction to an irritation. The irritation can be caused physically or chemically. What causes it? Primarily insects.

Over 1500 species of insects in North America cause galls, and several hundred more are the result of fungi and mites (spider relatives). That many gall-makers results in a vast variety of shapes, sizes and colors of galls. These deformities are common occurrences. Over one-half of all plant families are attacked by gall-making insects.

But why do insects do it? The answer lies in the food and protection that the plant tissues provide for the developing insect.

Galls are more easily identified than the gall makers. Each species of insects makes only one type of gall, but you may find several species using the same plant. All plant parts may exhibit galls - leaves, stems, buds, flowers, twigs, and even roots.

Oaks are the most commonly attacked plants. Over one-half of all galls produced can be found on oaks. The insect you find inside the gall however may not be the one that made it! One investigator found 75 different insects utilizing oak apple galls, some as parasites to the make, others as a convenient place to call home.

The life history of each species of gall maker can be quite different, ranging from the simple to the complex. As a result, galls have received a great deal of scientific interest.

A field of goldenrod will yield the easiest to find of all galls, in quantity and variety. The spotted-winged-fly (Eurosta solidaginis) causes the ball-shaped swellings found on stems. Another fly (Rhopalmivia solidaginis) produces a flowery-appearing structure of stunted leaf growth near the top of the plant.
 

Elliptical goldenrod gall is caused by the larva of a moth, an unusual event, since few moths cause galls. In the fall of the year, the adult moth lays eggs singly on the lower leaves and plant stem. The larva hatches in spring and crawls to the new plant growth, burrows into a bud and down the stem. The plant then forms the gall around the larva at this point. The immature insect feeds off the tissues and tunnels to the edge of the gall. It then plugs the tunnel with silk and plant tissue and returns to the center of the swelling. It pupates and emerges via the tunnel as an adult moth in late summer.

Willow pine cone gall forms at the branch tips of willows. The swelling appear as gray-colored cones, complete with overlapping scales. A small gnat (Rhavdophaga stobiloides) is the maker, which over-winters as a larva in the gall, emerging as an adult in spring. Numerous other insects use this gall for their needs. In one study, two dozen galls produced 564 insects! Only 15 were the original gnat hosts. Others included wasps, other species of gnats and grasshopper eggs.

The majority of galls have little effect upon their host plants other than changing their outward appearance. Besides stimulating human curiosity with their odd appearance, galls have generated other interest and use. The oddities are studied in cancer research, as they are believed to be examples of tumorous growth. Tissues of many galls are high in protein and have been utilized for livestock feed. Tannic acid in oak galls has found uses in tanning leather, dyes, inks and medicines.

Written by Nick Kerlin