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.)