Study Examines Insects’ Role in Plastic Pollution
By Paige Embry
Microplastics permeate the world. They can float through the air and have been found in Antarctic ice, the deep ocean, drinking water, and inside an array of animals. Microplastic pollution, mostly in the oceans, has been getting a lot of attention in the last few years but microplastics’ ubiquity means that scientists researching them have to find ways to limit contamination—and assess its extent when it inevitably happens. Max Helmberger, a Ph.D. student in entomology at Michigan State University, has researched several soil-dwelling organisms’ ability to create microplastics from larger plastic debris. He says labs have had to come up with “all sorts of creative solutions” to the contamination problem, with at least one dying all their lab coats bright pink so it would be obvious when bits invade a sample. Helmberger says, “Being persnickity is kind of a must in microplastic research because microplastics are everywhere.”
Microplastics come in two basic forms: primary and secondary. Primary microplastics are ones that are manufactured in sizes smaller than 5 millimeter (think sesame seed). Nurdles, the pre-production pellets used to make plastic products, are an example of a primary microplastic. Secondary microplastics are tiny bits that have broken off larger pieces. It is this second type of microplastic that Helmberger and colleagues recently studied in relation to insects and other invertebrates. Findings from their research were published in February in the open-access Journal of Insect Science.
Helmberger and his colleagues wanted to look at an array of different types of soil-dwelling organisms and assess their ability to fragment plastic in a fairly short period of time. His chosen animals were Acheta domesticus (a house cricket), Oniscus asellus (an isopod, sometimes known as a sowbug or woodlouse), Zophobas morio larvae (a beetle), and Cornu aspersum (a snail). Helmberger put each animal in an “arena”—a small glass jar. The bottom was filled with plaster of Paris and topped with sand that had been heated to 500 degrees Celsius to burn off organics and plastics. The animals went into the jar with pieces of both pristine and weathered polystyrene, along with one oat flake of real food to sustain them. He left them there for 24 hours.
Afterward, Helmberger counted the number of microplastic particles in the animal poop, the sand, and within the dead animal itself. To discriminate between the plastics and other tiny bits of stuff in the sample, Helmberger used two fluorescent stains: “Nile red” and a mix of “calcofluor white” and “Evans blue.” Nile red is commonly used to detect microplastics, but Helmberger says, “Nile red also binds to chitin—which, if you study insects, is kind of a problem.” Chitin is a key component in arthropod skeletons, so he needed a way to differentiate between the plastic and chitin. The white/blue mixture was the answer because it binds to chitin but not plastic. Helmberger weeded out anything that fluoresced for both dyes (or only the white/blue mixture). Each particle that fluoresced only under Nile red and also looked like polystyrene was poked with a soldering iron. If it melted or deformed, he knew it was plastic and counted it. Helmberger also set up various anti-contamination protocols, and they largely seemed to work based on the contamination assessment tests he ran, he says.
Helmberger and team found that the beetle larvae were the fragmentation kings, fragmenting both pristine and weathered polystyrene. The crickets and woodlice fragmented only the weathered polystyrene, and the snails “did not appreciably fragment anything.” A second experiment offered the isopod the option of pristine polystyrene, polystyrene exposed to UV rays (as if it had been sitting out in the sun), and polystyrene that had been soaked in a soil suspension. The isopod far preferred the last, showing that the condition of the plastic may play an important role in how tasty a given organism views a piece of plastic.
These experiments produced some expected results—that the beetle larvae fragmented the polystyrene—and some unexpected ones—the snails didn’t. In at least one other experiment, snails did fragment plastic. The researchers speculate that perhaps the difference lay in the snails, which were different species, or in the time the snails were around the plastic, 24 hours in the new study versus four weeks in the past experiment. Helmberger says they picked 24 hours because it seemed like a more realistic amount of time for an organism out in the world to be in contact with a bit of plastic.
This research adds to the evidence that organisms in the environment may not be just passive recipients of microplastic pollution; some may be active creators of it. It also shows how new, and complicated, microplastic research is with protocols still being developed—whether it’s wearing hot pink lab coats or using a soldering iron to figure out what is truly plastic.
Journal of Insect Science
Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.