Bumblebees and butterflies are more than just colorful springtime visitors. These pollinators are keystone species—and with several having experienced significant rates of decline over the last few decades, it’s critical that they be protected.
To do so, transportation agencies and ecologists have looked at roadsides as potential habitats to restore for pollinators. In the U.S., roadsides make up some of the only land that is not used for livestock, agriculture, and other activities, which opens thousands of acres of potential habitat. Plus, in Minnesota, roadsides harbor milkweed and other plants that pollinator species feed on.
While restoring roadside habitats seems like an ideal solution, a key question remains: How do pollutants factor into the equation? A study by University of Minnesota (UMN) researchers revealed that while microplastic particles from tires are commonly found on roadside plants, early results show minimal impacts on pollinator species.
A closer look at roadside pollutants
Emilie Snell-Rood, professor of ecology, evolution, and behavior at the UMN, presented her lab’s findings at the annual Center for Transportation Studies Transportation Research Conference in November.
“Roadsides have a lot of potential as pollinator habitat. At the same time, though, we worry that they could be an ecological trap,” Snell-Rood said. Drawing pollinators to roadsides could expose them to a slew of past and current pollutants—including lead and gasoline, heavy metals from the wear and tear of brake pads and tires, and pesticide spillover from adjacent agriculture—as well as harmful levels of sodium and nitrogen.
As part of work funded by the Minnesota Department of Transportation (MnDOT) and the Minnesota Environment and Natural Resources Trust Fund (MN-ENRTF), Snell-Rood’s lab has studied the risks of roadside pollutants for the last decade, with a current focus on microplastics. Synthetic rubber from tires is one of the largest terrestrial sources of microplastics, which can end up on roadside plants that feed pollinators. On average, researchers found that tire particles made up 3.6 percent of the dry mass of milkweed leaves.
To determine the level of risk associated with microplastic ingestion, the research team incorporated varying levels of ground-up tires into the diets of cabbage white caterpillars, with compositions ranging from zero to 16 percent. In a study of 400 caterpillars, researchers concluded that the species is tolerant of ingesting particle levels of up to 8 percent of their diet. There was no difference in the proportion of caterpillars who died versus those who survived, and no significant effect on development time from egg to adult. When repeating the experiment on monarchs, the team found similar results: Tire compositions of up to 8 percent of the diet had no significant effect on survival rates.
Proceeding with cautious optimism
While researchers are still in the process of investigating the risks microplastics pose to caterpillars’ body size, fecundity, and life span, these initial results are optimistic. “The levels of microplastics that we’re seeing on the leaves do seem to be below thresholds of concern for caterpillars, at least in respect to survival and development time,” Snell-Rood said. Researchers also found similar patterns among other pollutants in previous studies.
These results support a positive outlook for using roadsides as habitats for pollinators, but Snell-Rood advised agencies to continue with interventions to reduce or mitigate pollution. “This is good news, but not a license to pollute,” she said.