A variety of plants play unwitting hosts to insect homes known as galls.
November 2011
By Lowell & Kaye Christie, F47246
Each season has its discoveries, and after the leaves fall in late autumn, you’re likely to notice one of the interesting interactions between insects and plants. In addition to the abandoned birds’ nests and parasitic plants such as mistletoe that adorn the bare branches of deciduous trees, you’ll often find evidence of insect homes called galls. They were developed at the expense of unsuspecting plants by any of more than 2,000 types of beetles, moths, aphids, flies, and wasps.
Insect galls occur not just on branches but year-round on leaves and plant stems. It’s just easier to discover them when the foliage doesn’t get in the way. The most obvious ones are golf-ball-size galls that often form on oak trees, but they can come in almost any size and shape. Once you become familiar with galls, you’ll notice them almost anywhere you go.
Some galls may be as large as 2 or 3 inches in diameter, while others appear as slight bulges in a plant stem or thickened areas on a leaf. They can appear in almost any form, from perfectly round balls to miniature pinecone shapes created by the gall fly on willow trees. The gall gnat produces a flower-like gall on cypresses, while one of the gall wasps creates a spiny rose gall, a round gall with sharp spines in pastel shades of pink and yellow.
Galls are made completely of plant tissue. Regardless of their shape, color, or location, they are formed by some sort of disturbance in the growing surface of a plant, usually when an insect lays eggs on it. In addition to the eggs, the insect will often inject a growth hormone that causes the gall to begin forming. In some cases, this is done by the insect larva; sometimes both the adult and the larva add the substance. When the larva produces the growth-changing chemical, the gall increases in size to keep pace with the growth of the insect.
The gall creates both a living space and a food source for the growing larva, where it will live during its formative stages. One of our favorite authors, Edwin Way Teale, described it like this: “Imagine such a room constructed of succulent, edible material, forming a house that at once provides food and shelter, plenty and protection. That is what you would find if you traded places with one of those gall insects that now live in the globular swellings on the stem of my hillside goldenrods.”
There are two basic types of galls: open and closed. For wasps, moths, and flies, where either the larval or adult stages have chewing mouthparts, the gall will tend to be a closed chamber and the adult will exit the gall either by eating its way out or through an exit hole that has been previously made by the larva. Insects with sucking mouthparts, such as aphids, create galls that already have openings or that open when it comes time for the young adults to emerge.
You often can see the exit hole when you examine an empty gall. That’s assuming a fisherman looking for bait (in the case of goldenrod galls) or some other predator that’s learned the significance of the unusual protrusions hasn’t made off with the contents. And sometimes, after the original occupant leaves, the empty chamber may be re-occupied by another insect looking for a home.
Nearly all types of trees and plants produce galls, but the most common gall hosts are oak trees; more than 750 different types of oak galls have been identified. Each gall insect tends to specialize in a particular species of plant, or even a specific part of a plant, but when they choose a different host, the actual gall takes on the same characteristics. A gall specialist often can determine the type of insect involved just by examining the gall.
Some galls seem to have special attributes beyond that of providing homes and nourishment. In central California, the valley oak has a gall that provides a bit of entertainment. It’s called the jumping oak gall, and in some ways it mimics the Mexican jumping bean. The galls themselves look like tiny yellow or brown seeds and are produced by a small cynipid wasp. As a larva matures inside, the gall falls off the tree and the movements of the immature wasp make the gall “jump” as much as several inches in the air. Naturalists believe that the critter is trying to make the gall fall into a crevice of some type to protect it from predators until the wasp reaches adulthood and chews its way out of its enclosure.
Because many galls are especially rich in nutrients, they may be eaten by other animals. In the east, a species of oak gall is gathered in the autumn and used for livestock feed. And then, some galls produce honey.
According to a Texas Parks and Wildlife publication, many of the insects that create galls produce an enzyme that changes a plant’s starch into sugar, which is used by larvae as a food source. Some of the oak galls in the Southwest contain so much sugar that it oozes to the surface and is collected as honeydew by honey ants. In one section of California, certain oak galls produce so much honeydew that honeybees gather it and may store as much as 30 or 40 pounds of it in each hive.
In most cases, galls do little or no damage to the host plants. Some gardeners dislike them, because not all galls are attractive. But they are part of the life cycles of the plant and insect worlds, and another example of the fascinating interactions that take place all around us. The next time you are outdoors, take a closer look at the surrounding trees and plants. We’re sure you’ll find examples of galls almost everywhere.