Galls

The larva of a goldenrod gall fly was found inside this stem gall. Over 2,000 North American insect species form galls of many types.

The swollen goldenrod stem looked like it had a tumor in it. I carefully slit it open with the tip of my knife revealing a larva of the goldenrod gall fly. It had been placed inside the stem as an egg by its mother. As the larva hatched and began feeding, it released chemicals that coerced the plant into creating a gall that formed comfortable and safe living quarters for the larva.

Over 2,000 types of beetles, wasps, moths, flies and aphids, as well as nematodes, mites, lice, fungi and bacteria all cause abnormal plant growths usually called galls. They may occur on any part of the plant: roots, stems, leaves, flowers and even under the bark.

Galls form when chemicals infused by the critter take control of the growth and the development of the gall. The plant is helpless to do anything about the runaway cell division. Some scientists believe these same chemicals might be at the root of many human cancers.

Galls come in a fascinating array of shapes and sizes. They can be less than one sixteenth of an inch to over one meter long, depending on the infecting species and the plant itself. Galls produced by insects and other critters tend to be small, up to two inches in diameter. Those produced by fungi and bacteria can be much larger.

Each organism has its own signature gall. They vary from a round hard ball called an oak apple to the fuzzy wool-sower gall that looks like a small flattened ball of wool with red “seeds” in it. Some look like ugly cancers. Other galls are so unique or attractive that they are used for decoration.

Some species attack only a specific plant, but others may have several alternate hosts. However, the chemicals create similar galls even with different hosts. Truly, the insect has full control of the gall forming process.

Galls come in two main classes. For insects that have chewing mouthparts such as wasps and flies, galls are completely enclosed like that on the goldenrod. When it comes time for the adult to leave the gall, it simply chews its way out. Insects and other critters with sucking mouthparts must have a gall that is partially open or one that splits open when it is time to emerge.

Once vacated by their original occupants, some galls can become homes for others. For instance, a small wasp uses the goldenrod gall as a nursery. She deposits an egg and food into the gall and then seals the hole.

Galls can occur on many plants, but there are some species that are more susceptible than others. Look for galls on oaks, members of the rose family which include chokecherries and serviceberries, willows, cottonwoods and members of the aster family, including goldenrod.

Plant systems are hijacked by the gall producing animals but in the long run, the galls are of little consequence to the plant. There is generally no need to attempt to control insects that cause galls.

On the other hand, I find it curious to contemplate what we would think about an insect capable of injecting humans with chemicals that basically hijack our minds or bodies. I suspect we would wage war and eradicate them if we could—while at the same time defending our right to do the same thing with chemicals of our own choosing.