Imagine that when you were very little, your mom drilled a small hole in a plant and placed you inside. The plant’s reaction to the hole was to quickly enlarge that section of the plant where the tiny you was nestled. This new plant growth gives you a round home that protects you from the weather and provides all the food you need to grow into an adult. This round home is called a gall.
Most galls are formed by insects, others by fungus or bacteria. Each gall is unique to the plant that it grows on. Galls come in a variety of colors from red to yellow, green and black. There are three types of galls:
Leaf galls are the most common. They can be found on the lower or upper part of the leaf, and they may deform the leaf.
Twig and stem galls look like an odd growth on the stems and twigs of plants. They can be small or large.
Flower or bud galls disfigure a plant’s flowers or buds.
The Smithsonian notes that in North America alone, there are almost 1,500 different insect species that cause plant galls and most of them, over 800 species, make galls on oak trees.
Here is a sampling of some of the many galls you may see during your winter walks:
Oak apple gall
Walking under an oak tree, you may notice small, brown balls about the size of a ping pong ball hanging from the branches. These galls are the home of the oak apple gall wasp, Amphibolips confluenta. The galls are hard on the outside and soft on the inside. If you see a small hole in the gall, that hole was made by the adult wasp as it was emerging from the gall last summer. Oak apple galls are commonly seen on scarlet and black oaks.
Goldenrod ball gall
Round galls on a goldenrod stem are an indication that a goldenrod gall fly (Eurosta solidaginis) is living inside the gall. Fly larvae live inside the gall all winter and emerge in the spring. You can find these swollen stems in meadows and along paths.
Eastern spruce gall
Eastern spruce gall adelgids (Adelges abietis) cause galls on both Norway and white spruce. Look for the small (½”-1” long) pineapple shaped galls near the tips of the branches. One sign of eastern spruce gall adelgid is a scattering of brown spruce branch tips on the ground under a spruce tree. The tips break off during a heavy snow storm or wind storm.
Cedar-apple gall and rust
The fungus Gymnosporangium juniperi-virginiana infects both cedar trees and apple trees during its two-year life cycle. In the winter, look for the fungus on eastern red cedar (Juniperus virginiana; also known as eastern juniper) trees. Spores develop in the gall during the early spring and they are released in May and June after a warm rain. If the spores land on apple or pear trees, they will infect the leaves of the tree where they land. Spores from cedar-apple gall on apple or pear leaves are released in June and July and may infect nearby cedars and other kinds of junipers and the cycle continues again.
One bacteria that forms galls is Rhizobium Rhizobium radiobacter. Grapes, walnuts, black cherry and other stone fruits are susceptible to Rhizobium radiobacter. The bacteria enters a plant when an insect or weather-related event injures the plant. For example, if the winter is severe, plants are more likely to get damaged from strong winds and crown gall bacteria will enter the plant. These galls can become enormous, over three feet in diameter when in the crown or body of trees, thus the name crown gall.
References and Additional Resources:
Brandeis University Biology Department, About Insect Galls.
Cornell University, Insect Diagnostic Laboratory, Galls on Plants.
Felt, Ephraim Porter and Millet Taylor Thompson, Key to American Insect Galls, New York State Museum Bulletin, The University of the State of New York, No. 200, August 1, 1917.
Jorgensen, Neil, A Sierra Club Naturalist Guide: Southern New England, Sierra Club Books, 1978.
Morton Arboretum, Plant Galls.
NYS Dept. of Conservation, Gall Making Insects Unsightly, Yet Ingenious,
Smithsonian BugInfo Insect and Mite Galls.
Stokes, Donald, Stokes Nature Guides: A Guide to Nature in Winter, Little, Brown and Company, 1976.
Thompson, Millet Taylor, Illustrated Catalogue of American Insect Galls Rhode Island Hospital Trust Company, 1907.
University of Minnesota Extension, Insect and Mite Galls.