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  • R. Gary Raham

Inspired by Galling Mysteries

By R. Gary Raham:

Galls are unusual plant growths stimulated by insects and mites to help insure that their young will live long and prosper. ILLUSTRATION:  R. GARY RAHAM
Galls are unusual plant growths stimulated by insects and mites to help insure that their young will live long and prosper. ILLUSTRATION: R. GARY RAHAM

Colleagues love to share epochal moments. These are times when expectations are dashed and they remember why their professional obsession of choice ignited their passion in the first place. I follow the blog of Chris Helzer, a Nebraskan who identifies himself as the Prairie Ecologist. In June of this year, while photographing a gall on a goldenrod leaf, he observed something that “blew him away.” It started a chain reaction in my mind that resulted in this article. But before I try to short circuit your mental tranquility, it behooves me to remind you of a few details about those bizarre, cancer-like growths called galls that appear on some of your favorite plants.

Arthropods and flowering plants share a history that extends back at least a hundred million years to the heyday of the dinosaurs. It’s no wonder that their relationships have become complex enough to amaze and confound our humble species with a history less than a hundredth of that time span. Galls are unusual plant growths stimulated by insects and mites to help insure that their young will live long and prosper. While scientists have not nailed down all the details about how this happens, it appears that the arthropods modify chemicals in their saliva to act like natural plant growth hormones that induce the host plant to build custom-made birth chambers for their young larvae. Remember that factoid. Normally, the gall-makers force the plant to make their birthing chambers.

Over the course of a hundred million summers, various insects and mites (a spider relative) have found numerous ways to make galls. Four major arthropod groups have specialized in this form of plant assault: Aphids and their relatives, Eriophyid mites, certain midges and flies, and a consortium of wasps who should be awarded prizes for creative plant deformation.

In my own yard I have a hackberry tree that has long suffered the attentions of psyllids (pronounced sillids). These critters are commonly called jumping plant lice because of their habit of skydiving from leaves in the fall when they hatch out of their characteristic “nipple galls.” The precise forms vary for different infestations. My hackberry hosts a species that produces smooth nipples with a tiny smear of brown on the tip. Other species produce nipples that look distinctly hairy. Still others resemble indented mushroom caps, pudgy doughnuts, rounded disks or cones. When I first saw them years ago I sounded the alarm to Colorado State University’s extension service, but found out, to my relief, that these critters don’t seriously harm the tree. The jumping lice become tiny imitations of cicadas in the fall and may latch on to window screens on the back door and windows.

Eriophyid mites are microscopic, but build finger-like houses on chokecherry or wild plums. They may also be found on boxelder, aspen, mountain maple, and other trees.

Various wasps create quite elaborate galls on oaks, roses, and other plants. Not a big fan of these stinging creatures, I haven’t studied their galls. I’ll leave that to another naturalist.

Gall midges and flies create their nursery structures on various trees and shrubs. One species called the Hessian fly (Mayetiola destructor) acquired its ominous name by attacking Colorado wheat on the eastern plains. Not a benign gall maker, it causes severe stunting of the crop.

That brings me back to Chris Helzer’s amazing tale that involves a gall midge (Asteromyia carbonifera), goldenrod (Solidago canadensis), and a fungus (Botryosphaeria dothidae). The fungus added a wrinkle to the traditional gall stories that shook Helzer’s expectations.

While photographing the goldenrod gall’s surface, Helzer noticed what looked like tiny flowers. He knew real flowers were not an option, but suspected he could be looking at structures created by a fungus. A little research on his part proved that guess to be correct. It revealed a tangled web of relationships between a midge, their favorite host plant, a helpful fungus, and some frustrated wasps. Suddenly, nature proved his (and my) preconceptions about galls and how they form to be woefully inadequate—or at least incomplete. (But that is, truly, part of the joy of science.)

Certain parasitic wasps most likely inspired the 1979 movie, Alien. These wasps like to plant their eggs in juicy insect larvae that serve as incubation chambers. The wasps eat their hosts from the inside out, eventually popping out of the larval shell like a Jack-in-the-Box. Sometimes wasps accomplish this feat by poking their egg-laying ovipositor through the wall of a gall, and then fishing around a bit until they locate the midge larva. Such was the fate of the midge A. carbonifera before it teamed up with a suitable symbiotic fungus buddy.

The female midge somehow acquires fungal spores that accompany her to suitable goldenrod real estate. The fungus then actually builds the gall rather than the plant host. These fungus-made gall walls are robust. Wasps can still pierce them, but no longer have the luxury of fishing around for the larvae. This restricts their success rate and improves prospects for the midges. The midges expand the range of the fungus in the process.

In his blog Helzer said, “I’ve been working in prairies for more than 30 years and hope to be doing it at least 30 more. Some interactions I come upon are merely interesting. Others, like this one, blow me away. What a fantastic world we live in!”


Gary Raham writes and illustrates both science fact and science fiction. Keep in touch with his work at



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