If you’ve stopped trailside to look at a big sagebrush (Artemesia tridentata), it’s likely you’ve noticed grape-like growths attached to a leaf or stem. Some are round, smooth and reddish, other are covered with a mass of white hairs. Some are dried and shriveled up, with a tiny hole visible. Each is gall: a tumor-like growth of plant tissue triggered by an insect. The plant gall grows quickly in response to an invading insect, providing it with all needed water, food, and shelter, usually without causing harm to the host plant. Galls are often specific in shape, color, and size, so an observer can identify the kind of gall-inducing insect just by looking.
Worldwide, there are about 13,000 species of gall-inducing arthropods, including mites, aphids, beetles, moths, fruit flies, wasps, and sawflies, each interdependent upon a specific plant species. Our Wenatchee Valley big sagebrush is known to host at least 32 different species of gall midges, a group of tiny (2-3 mm.) delicate two-winged flies with long legs and long antennae ( Family Cecidomyiidae).
As flying adults, gall midges live only a few days, long enough to mate. A female lays eggs, either singly or in groups, on top of a sagebrush leaf. The egg hatches quickly, and the larva immediately begins feeding, using its saliva to dissolve plant tissue. The saliva triggers the plant’s own hormones to induce rapid atypical growth that encircles the larva, forming a gall. The plant tissue inside the gall provides all needed food and water for the larva. The length of time in the larval stages varies by species from a few weeks to up to two years. Some over-wintering gall midges go into a dormant stage to slow development and allow the best timing of adult emergence to match the host plant’s life cycle. Next, a very brief pupa stage occurs with the formation of sharp antennal horns, used by the the pre-adult to mine a passageway to the gall’s surface, and dig an exit hole. Then, the exoskeleton of the pupa splits open, and the adult gall midge flies out of the hole, seeking a mate during its final days of life.
Life inside a gall is filled with life-threatening danger for growing gall midges. A variety of types of parasitic wasps seek out a gall, penetrate it, and lay an egg directly inside the body of the resident gall midge larva. Still other hyperparasitic insects repeat this process by laying eggs inside the body of the first invading parasite’s larva. Galls attract vegetarian insects that lay eggs inside the gall selected for its abundance of plant tissue. Oddly, sometimes these plant-eaters will attack and kill their roommates inside the gall. Some species trigger gall formation within a gall and as the new gall grows, it can crush and kill adjacent gall midge larva. Finally, some generalist insects will happen upon a gall, tear it apart, and feed on whatever is inside!
When leaves with galls fall off the shrub, ground-feeding birds like the spotted towhee eat these packets of protein. Mice, shrews, and wood rats feed on fallen galls. Fly-catching birds like the Say’s Phoebe capture winged adult gall midges in mid-air. Once an adult female gall midge lands to lay an egg, it becomes fair game for an insect-gleaning ruby-crowned kinglet or black-capped chickadee. A variety of birds are known to peck on galls to excavate larva as food.
This dramatic story of life and death is happening within most big sagebrush shrubs growing in our foothills. Next time you are out, take a minute to search for a sagebrush gall, and marvel at the complex interdependence between a gall midge and a leaf.
To write this article, Susan utilized a terrific 2006 field guide, Field Guide to Plant Galls of California and Other Western States, California Natural History Guide Series No. 91, by Ron Russo. Wenatchee Valley College Entomologist, Dr. Bob Gillespie, provided technical review and assistance with taxonomy.