What is a Gall, after All?

An oak gall hangs from the branch of a tree.

An oak gall hangs from the branch of a tree.

Note: “Science and Nature for a Pie” is now on Facebook!  Stop by and “like” the page for ongoing stories, photos, and discussions about the great outdoors! https://www.facebook.com/scienceandnatureforapie

“You have some gall saying that to me!” “Man, that guy galls me!” It’s difficult to determine precisely how common these expressions still are, but most will recognize the idea that having “gall” or to “gall” someone is not a good thing. It relates to substances like bile (an acid that aids in digestion; too much of it can produce uncomfortable results), or to afflict someone or something with a sore or growth. In the plant world, a gall develops when a wasp or mite deposits an egg on the petiole of a plant. Whether it’s a good thing or a bad thing may depend on taking the perspective of the plant or the insect.

DSC08212
It’s not unusual to see a golf ball-sized green object hanging among the leaves of oak trees in the spring. Commonly known as oak apple galls because of their resemblance to crab apples, these are part of the life cycle of a wasp from the family cynipidae. In some cases, the remains of the oak leaf may still be visible. As the leaf develops from a bud, the gall expands, producing a flexible exterior. Inside, a web-like spongy material surrounds the pupa of the wasp. At this stage, the pupa receives nutrients from this material as it continues to develop over as long as a year or more.

An oak gall in late spring. Note the spongy material inside, along with the tiny wasp pupa on the tip of the knife.

An oak gall in late spring. Note the spongy material inside, along with the tiny wasp pupa on the tip of the knife.

When the time comes, the adult wasp emerges. At 1/16 – 3/8” (2 – 8 mm.) long, these look a bit like a chunky flying ant. They mate, lay eggs in the buds of the oaks, and die. The cycle begins again as the next generation develops inside another gall. For the most part, this process does not hurt the tree, as the population is too limited to affect leaf production.
Mites, which cause a different sort of gall, are a type of arachnid. According to the National Audubon Society’s Field Guide to Insects and Spiders, there are “more than 30,000 known species of mites with probably at least a million more waiting to be identified.”

These galls are produced by mites.

These galls are produced by mites.

Despite its etymological connection to pain and deformity in humans, plant galls, though they affect one or more leaves of a plant, are not usually harmful to the host plant overall. So while people who have a lot of gall may not be particularly popular, plants with galls are a crucial part of the lives of woodland critters.

 

 

 

DSC08360

A closeup of the galls pictured above.

Sources: National Audubon Society Field Guide to Insects and Spiders; Henderson State University (www.henderson.edu)

Leave a comment