As my husband and our puppy cruised ahead, I lagged behind, peering at some strange growths on the tips of a sagebrush's leaves. The more I looked, the more I found, and the more I found, the more curious I became.
The little growths were less than an inch across, many much smaller, and ranged from chestnut brown to pale greenish-pink. They were spongy to the touch, and appeared to be covered in tiny hairs.
Not wanting to dally too long, I snapped a photo for reference and moved on. Later, I shared the photo on Twitter, to see if anyone with an entomology background could point me toward relevant information.
Realizing how many insects there were to 'choose' from, I wasn't convinced I'd be able to figure out the exact one that caused the galls I'd spotted. Happily, my Twitter source, Canadian ecologist Terry Wheeler, figured the galls were caused by something in the Rhopalomyia genus, which led me to work by Ruth Hufbauer, an ecologist in Colorado, who has studied these exact galls.
This week's column
In the syndicated column, I explore the history and social uses/concerns associated with gall-producing insects, take a look at their general ecology, and dwell on the fact that we don't really know much about these diverse, numerous, and complex creatures.
The following illustration accompanies the column, which is available to subscribers of publications running Drawn to the West as a syndicated column.
- In short, a gall is a plant tumor comprised of extra cells or overly large cells. Although a range of organisms, including fungi, viruses, and nematodes, can cause galls, most are stimulated by insects. Such galls typically provide food and shelter for insect eggs and larvae. This tactic is definitely an asset – over 13,000 such species are known.
- The authors of Gall-Inducing Insects: From Anatomy to Biodiversity state galls have been of interest since Pliny the Elder wrote in the first century AD. However, scientists did not understand the link between galls and insects laying their eggs inside host plant tissues until the 1600s.
- As long as we have pondered galls, we have used them. Their functional history includes treating hemorrhoids, dying hair, and as ink, food, and jewelry. Modern agriculture utilizes some gall-producing species as biocontrols, while others are significant agricultural pests (ex: the Hessian fly, a wheat parasite).
If you'd like to read the column, let your local newspaper or magazine editor know you want to see an illustrated version of the West in their publication!
The big take-away is that 'our' scientific understanding of galls (including those on sage) is still in its infancy. It seems we have a descriptive/observation-based understanding (excluding agricultural pests which have likely been studied intensively):
- Galls are caused by many organisms.
- When insects lay eggs under the surface tissue of a plant, a gall is likely to form.
- Other insects prey on these galls/larvae.
- Galls form somewhat of a micro-ecosystem.
Beyond that, we don't really have a detailed grasp of galls' ecological significance. For example, some questions I was bouncing around with a handful of ecologists over dinner Wednesday night included:
- Do animals (like pronghorn and mule deer) that typically eat sagebrush eat the galls?
- If not, do they still eat bushes affected by galls?
- If not, how big of a benefit does this herbivore avoidance offer the individual or stand of sage affected by galls?
- Does the stress of gall 'invasion' affect the plant/soil chemistry and/or community of other species on that plant?
- Do birds focus on the galls? How does that affect their ecology?
- Why do more (and larger) galls form on larger (presumably older) sagebrush?
Below are the references I used to develop the sagebrush gall article. I found reading them fascinating. If you are not able to view a particular article (due to journal paywalls), contact the author for a copy.
- Big Sagebrush: A Sea Fragmented into Lakes, Ponds, and Puddles by Bruce L. Welch
- Cecidomyiidae, Wikipedia entry
- Galls as habitats: the inquiline communities of insect galls by Dilek Sanver, Bradford A. Hawkins
- Gall-Inducing Insects: From Anatomy to Biodiversity by Wilson et al.
- Narrow hybrid zone between two subspecies of big sagebrush (Artemisia tridentata: Asteraceae) by Graham et al.
- Observations of Sagebrush Gall Morphology and Emergence of Rhopalomyia pomum (Diptera: Cecidomyiidae) and its Parasitoids by Ruth Hufbauer
- Sagebrush in Western North America; Habitats and Species in Jeopardy from Pacific Northwest Research Station
- Seasonal patterns of arthropod diversity and abundance on big sagebrush, Artemisia trientata by M. P. Stanford and Nancy Huntley
- Summary of Food Habits of American Gall Midges by E. P. Felt