The Nature Nerd

Focus on: Galls on goldenrod and other plants


Galls are growths on plants like goldenrod (pictured)Roachthat form whenho femaled meeflies laya eggs inside theFirstplantStagetissue GallerySubmitted photo.alls

Galls are growths on plants like goldenrod (pictured) that form when female flies lay eggs inside the plant tissue. Submitted photo.

Chickadees and small woodpeckers know all about them. Most humans give them little notice, however. As a child I was told goldenrod galls contain tiny grubs and these grubs were good bait for panfish. Two things I learned. First, it is a lot of hard work to open the galls. Second, the grubs are not good fish bait. However, I learned a good lesson about the world of nature. Lots of fascinating things are happening in our Northwoods.

The round galls we see on goldenrod stems are especially conspicuous during the winter. They measure about one inch in diameter. They are caused by the grubs of a tiny fly. The female fly inserts eggs into the stem. The gall is the response of the plant to the eggs. The eggs contain an enzyme that modifies the growth of the plant in this way. Surprisingly, there is little harm to the plant otherwise. The eggs produce grubs that feed inside the gall before hatching out as the flies. The flies are tiny fruit flies, so tiny we rarely notice them.

If you pay much attention to plants, you have seen many types of galls. Each type of gall is caused by a different insect. Goldenrod has two more types of galls. One is elongated into a streamlined shape. Another has leaves growing out of it. Galls having a similar appearance are common on oak leaves, but these are caused by a distinctly different insect.

There are other galls on other plants growing on leaves and resemble small hollow sacs or tubes and come in various colors. Some are rolled-up leaves or swellings on the stems. Each type of gall is made by a unique insect, most of which are tiny, inconspicuous, and can be identified only by specialists.

Willows shrubs have galls at the end of the stems that resemble little pine cones. These are very busy places. These galls are made by yet another tiny fly, but other insects populate these galls, too. Some feed on the gall, some shelter from winter weather in the gall, and some are carnivores that feed on the tiny insects. We cannot forget the parasites, mostly tiny wasps, that feed on some of these insects.

Each kind of parasite specializes on just one species of insect. These little galls on willow have one practical use to us (or at least to me): they are an easy way to identify willows from a distance, distinguishing them from alders, dogwoods, and other shrubs, even during winter when leaves are gone. It is common to see small cone-shaped holes carved into goldenrod galls. These galls have been attacked by birds seeking the grubs as meals of protein. I know from experience it is hard work to open these galls. These chickadees and woodpeckers must work for their dinner!

Jerry McCormick lives in Virginia. He is a retired natural resource professional and a self-described nature
nerd.

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