How Some Insects Turn Plants Into Pollution Detectors
By Leslie Mertz, Ph.D.
Insect-induced galls that appear as lumps on plant stems and bumps on leaves may also be excellent pollution detectors, according to research presented at the 8th International Plant Gall Symposium 2023 held at California State University, Chico, in July.
Evolutionary ecologist Glen Ray Hood, Ph.D., of Wayne State University in Detroit, explained his lab’s work demonstrating that galls accumulated toxic soil contaminants at the sites of two highly publicized, factory-discharge incidents in southeastern Michigan: one in Ann Arbor that released 1,4-dioxane and the other in Madison Heights that released hexavalent chromium and eventually seeped as a bright green ooze onto a busy nearby freeway.
At the Plant Gall Symposium, Hood also described his lab’s current Detroit-centered project that uses plant galls to detect below-ground, anthropogenic (i.e., human-made) volatile organic compounds (VOCs), some of which have been associated with the city’s high preterm-birth rate, among other health issues.
An Idea That Grew
Hood originally got the idea to study how insect-induced plant galls interact with ground pollution while he was a postdoctoral researcher at Rice University. There, he came across a nearly 30-year-old journal article about blueberry stem galls. These often peanut-shaped galls are abnormal plant growths caused by a tiny wasp, Hemadas nubilipennis, laying its eggs in the tips of the plant’s stems. “The study authors simply asked whether heavy metal contaminants from an ore smelter accumulated in higher concentrations in some plant tissues versus others,” he says. “And they showed the plant was definitely sending higher concentrations to galls.”
The finding made sense because of the way galls are formed, according to Hood. In most cases, the insect’s egg-laying hijacks the plant’s developmental system, causing it to make a gall, and the saliva of the resulting, feeding larva incites the plant to transport soil resources to the gall, which continues to grow. “In fact,” he says, “oftentimes the gall, which is made of plant tissue and surrounds the growing larva, is thousands of times more nutrient-rich than tissue found anywhere else in the plant during its entire life cycle—so, more than nutrient-rich seeds, fruit, or even flowers.”
Hood wondered whether the plant was accumulating other subsurface contaminants in galls, and when he started his own lab at Wayne State about five years ago, he began exploring galls as so-called “phytoscreening” tools for the two southeast Michigan chemical spills.
For hexavalent chromium in Madison Heights, Hood and his group took harvested galls from various plants along half-kilometer transects running in four directions from the point source, sealed the galls in methanol-filled jars and sent them to a lab for analysis. Their goal was to determine whether gall tissue had higher concentrations of the contaminant than did other plant tissues. “About 85 percent of the time, the answer was yes,” Hood says. “And it didn’t matter which plant species or which plant tissue—stem, fruit, flower, or leaves—the gall was on or which insect was responsible for the gall. The gall always showed higher concentrations than other plant tissues.”
For 1,4-dioxane in Ann Arbor, Hood and his group, including former undergraduate Connor Socrates and current doctoral student Sarah Black, tested one gall that is nearly ubiquitous on wild grape, a very common plant, and is initiated by an aphid-like insect known as grape phylloxera (Daktulosphaira [Viteus] vitifoliae). Results of sampling and analysis showed that the galls were far more sensitive to the presence of dioxane than other plant tissues were. Since then, they have continued to sample galls in Ann Arbor, and soon-to-be-published findings of that work show gall-identified dioxane beyond the currently known extent of the dioxane plume, which is continuing to migrate in groundwater, Hood says.
“In addition, at some areas inside the plume where well water is coming back negative in traditional tests, we are finding dioxane in galls growing in plants right next to the well,” he says. “This suggests that galls might be a viable and maybe superior alternative for doing dioxane testing.”
Hood’s current Detroit VOC project is part of Wayne State’s new multidisciplinary Center for Leadership in Environmental Awareness and Research (CLEAR), funded by the National Institute of Environmental Health Sciences. For this work, he and his group are sampling galls for a wide variety of VOCs, including such notable toxins as benzene, toluene, ethylbenzene, and xylene, as well as tri- and tetrachloroethylene. They’re also collaborating with other research groups that are investigating VOC levels in the atmosphere as well as in the blood and urine of local residents.
“One of the interesting things we have already found is that VOCs are spread out all over Detroit, including both in blighted areas and affluent areas,” Hood says. “And as it turns out, some of the highest concentrations are appearing in plants growing right against homes in affluent areas.”
With that work well under way, the researchers are delving into the details. “Once we can screen numerous common and commonly galled plants, that will tell us things like which galls are the best contamination detectors, at which time of year and under which environmental conditions they work best, and which chemicals can and cannot be detected with galls,” Hood says. “Once we have those questions answered and the screening method perfected, we want to use galls to identify VOC hot spots in the city, which can help bring about cleanup efforts.”
He adds, “It’s still early on in this research, but from our studies so far, it looks like these pretty common things—galls—may be one of the best ways to quickly detect chemical contaminants in many, if not most, environments on the planet.”
Plant and Soil
“Ento-phyto-screening for organic contaminants: The use of insect-induced plant galls as a novel tool for tracking belowground chemical contaminants,” Student 10-Minute Paper; Medical, Urban, and Veterinary Entomology Section
Entomology 2023, November 5-8, National Harbor, Maryland
Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.