The ubiquitous use of rock salt to melt snow and ice on winter roads in the U.S. may be changing the physiology of the monarch butterfly. When snow melts, road salt runs into nearby soil and into the roots of common plants, which are in turn ingested by caterpillars. Then, as they become adult butterflies, their brains and muscles develop abnormally: The females develop larger eyes and brains, and males develop more muscle.
These are potentially positive changes, as they aid the butterflies in elements of mating and reproduction. Larger muscles help males fly longer and farther, enabling them to find more mates. Larger eyes allow the females to seek out better plants on which to lay their eggs.
Just like in people, sodium is good for butterflies in low quantities: It’s one of most important elements of muscle and brain development.
But there’s a catch: “As [the salt] increases a little bit, that’s probably a good thing for them,” said study co-author Emilie Snell-Rood, a biologist at the University of Minnesota. “As it increases even more, it becomes toxic and stressful.”
And there’s evidence that road salt is on the rise: Following the harsh winters of 2011 and 2013, the sodium content of roadside plants increased by as much as 30 times compared with previous years, according to the study, published June 9 in the Proceedings of the National Academy of Sciences.
Snell-Rood’s research came about in 2011, when she moved to the Twin Cities metropolitan area and noticed how much road salt is applied to the roads, more than 350,000 tons annually. She suspected that this overabundance of sodium could be affecting animals that feed on roadside plants.
Her team decided to study monarch butterflies because the species feeds on milkweed, a common roadside plant in Minnesota. From 2011 to 2013, they raised monarch butterflies on two types of diets: sodium-rich roadside plants and low-sodium prairie plants.
After six weeks the team recorded both groups’ survival rates. The results showed that although the caterpillars grew bigger, it wasn’t ultimately beneficial: The caterpillars that ate roadside plants had a 40 percent survival rate, compared with a 58 percent survival rate for those fed prairie plants.
The team also created artificial food with even higher concentrations of salt, and fed these to cabbage white butterflies. This led to “much higher mortality,” Snell-Rood added.
Numbers drop from “40 to 50 percent survival to 10 percent survival as you ramp up sodium levels even more,” she said.
As the study caterpillars that ate sodium-rich plants grew into butterflies, the developmental differences were immediately evident.
Male butterflies used the extra sodium to boost muscle, while the females grew bigger eyes and brains, which could help them locate plants on which to lay their eggs, said Snell-Rood.
That the female butterflies are not also using the extra sodium to develop larger muscles is “a little weird,” said Snell-Rood. It could mean that they are deciding not to forage for other plants because they think the plants in their vicinity will be better for their eggs.
The added sodium has “huge implications” for monarch butterflies’ development, said study co-author Anne Espeset. It “can change whether or not they have the ability to migrate,” since the females would not have the muscle capacity to handle migration.
That could be particularly worrisome for some monarchs, whose annual migration south to Mexico each winter has sharply declined in recent decades.
Whether these differences would be seen across other species and in other locations around the country is also not yet known.
“We were just looking at one road. We have no idea to what extent this is happening along interstates or highways,” said Snell-Rood. “Is it elevated to the point where it’s toxic?”