Fall 2022 Conservationist

The Inside Story on Galls

by Derek Gronlund, Community Engagement Services


As summer fades into autumn, and autumn begins to crisp into winter, we find
ourselves slowing down, wrapping up in protective layers of clothing, and relying more on shelters to keep us from the cold. It may be surprising, but many insects are doing the same thing. Instead of thermal layers and heated houses, though, these tiny creatures rely on galls.

A gall is an abnormal growth on a plant caused by an external stimulus. Fungi, bacteria, nematodes (aka roundworms), and physical injuries can all cause galls, but in most cases, growth is initiated by the presence of insect larvae. In North America about 1,500 species of insects form galls on different plants, and although the exact details vary depending on the insect and the plant, the general process is the same.

In late spring when plants are actively growing, certain female insects (often types of wasps or midges) deposit eggs into plants’ leaves, buds, stems, or bark. The eggs hatch, and the resulting larvae begin to feed on the plants’ tender tissues. It’s this feeding that triggers a gall to grow. As a larva feeds, the surrounding area fills with an excess of plant growth hormones, resulting in rapid cell division. As the plant’s cells divide and grow, they form thick layers that encase the larva inside.

As fall approaches, this larval growth slows. The developing insect prepares for winter by producing concentrated levels of glycerol, which acts like antifreeze and keeps the larva’s soft body from forming damaging ice crystals. In spring when temperatures rise, the larva resumes development, eventually emerging from the gall as a winged adult, ready to initiate the cycle again.

Galls can be found in forest preserves as well as in backyards on trees, bushes, and flowers. Some can disfigure leaves or stems, but it’s uncommon for them to harm or kill a healthy plant. In fact, the presence of galls in a yard can attract insect predators that provide a natural biological control of the insects making the galls. (If galls are unsightly and affecting the aesthetics of a plant, concerned gardeners can prune back the branches.)

Aside from playing brief yet vital roles in many insects’ life cycles, galls hold an important place in local ecology. Many animals such as birds and mice peck or chew through galls to reach the developing larvae inside as a source of food. After the adults exit, the empty gall structures provide shelter for different insects and other tiny animals. Plus, galls are fun to find, interesting to look at, and a good way to draw people of all ages into the joys of observing nature.

In any case, fall in DuPage forest preserves is a good time to keep an eye out for different types of galls. Goldenrod is a common fall flower and the host plant to several gall-forming insects. The aptly named goldenrod gall fly and goldenrod gall moth both lay their eggs in the stems of goldenrod plants. The resulting galls are either smooth and spherical (ball galls) or shaped like elongated footballs (elliptical galls) and are easy to spot. The larvae overwinter in the stems and emerge the following spring.

There are also several species of midges (types of flies) that form “bunch” galls on goldenrods by laying their eggs in undeveloped leaf buds. This causes the affected stems to alter their development and grow dense clusters of leaves where the eggs were laid. The leafy rosettes are often mistaken for blooms, and thus bunch galls are sometimes known as a flower galls.

Oak trees are important plants for about 500 gall-forming insects. Oak apple galls, some of the more conspicuous, form on the ends of branches where various species of tiny wasps lay their eggs in developing leaves. The infected leaves swell and grow into spherical structures that look like miniature fruits hanging from the stem.

Other oak galls take a bit more searching to find but are worth the effort. Careful observers can find hedgehog galls on the leaves of white oaks. These galls look like miniature yellow and red sea urchins. And woolly oak galls can look like tiny yellow-pink cotton balls clinging to the underside of leaves.

The diversity of galls is staggering, and once you notice a few, you will spot them on just about every tree and plant you see. The next time you find yourself on an autumn stroll, take a closer look under a leaf or at a stem. That oddly shaped colorful bump you see may very well be hiding an insect.