Road Salt Creating Beefier Butterflies

Road salt used to clear snow-covered roads in the United States is creating beefier monarch butterflies, a new study says.

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. (Related: “The Surprising History of Road Salt.”)

A monarch butterfly rests in Minnesota. Photograph by Jim Brandenburg, Minden Pictures, Corbis

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. (Related: “U.S. Cold Snap Inspires Climate Change Denial, While Scientists See Little Room for Doubt.”)

Salty Diets

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.

Watch a related video: Where Does Road Salt Come From?

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. (Watch video: “Growing Up Butterfly.”)

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.

Monarch Impacts

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. (See National Geographic’s butterfly pictures.)

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. (Related: “Migrating Monarch Butterflies in ‘Grave Danger,’ Hit New Low.”)

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?”

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Danielle Elliot is a multimedia producer and writer who earned her chops reporting and producing for networks, start-ups, and everything in between. A graduate of the University of Maryland, she covered tennis and Olympic figure skating for a few years before earning an M.A. in Science and Health Journalism at Columbia University. Follow her on Twitter @daniellelliot.
  • Dwayne LaGrou

    I think that the hard copy of the map should have a web site address where the viewer could see what the CURRENT conditions look like RIGHT NOW, Or as close to the actual view as possible. There are a number of Weather Satellites that follow a polar orbit that could be used to show the nearest thing to a “Live View” as possible. That way the general public could actually see the extent of global warming and it’s effects on the polar ice caps.
    Just a thought! Anybody else have a better idea?!

  • Bob Aitken

    Not sure it would be what you consider a “better idea”. I sure would like to see what you propose alsot. But, what if the polar ice caps begin expanding?? Then what?? Would that also be Global Warming??
    Science is no longer science when it becomes an ideology. Just ask Copernicus.

  • Bob Aitken

    Road salt has been used on roads for nearly 75 years now.
    Is the effect only now being seen in Monarchs– or only now being noticed??

  • Grandpa

    “Road salt has been used on roads for nearly 75 years now.
    Is the effect only now being seen in Monarchs– or only now being noticed??”

    Its only now being studied- by science. I guess no one else has the time, patience, or competence to conduct large scale analysis of such trivial things, especially not religions, and not corporations. They have much to fear from science, which exposes their lies and the damage done.

    People dumped salt on the roads for 75 years, yes. With no idea what the effects could be. Now that they are being discovered, we’ll hear form ignorant folks that want to shut up the truth tellers- and cover up the damage. Neither did anyone guess what else might be in those salt formulations. That would take curiosity and research, things in short supply in the land of milk & honey, aka God’s country. So close up your ears, folks. Maybe hit “back” on the browser, and visit some Jesus-freak revival website. Otherwise hold on to your pants:

  • Philip Biggins

    As a planet owner my thoughts is purely bio- chemistry
    micro organisms trapped and dormant still existing and come to life with the warmth of spring when unearthed in the mining process. Insects where larger in their particular time line and combined with current genetic codes and past will produce such abnormal growth do to microorganisms time line gaping.
    Thanks for the epiphany.

  • Sheba J

    While researchers, writers and activists work long and hard, often with only ridicule for reward, to warn us against the dangers of our mindless lifestyles and our habit of misusing conveniences, all that the self-important ‘denier’ has to do is lean back in his armchair, turn up the corners of his mouth and dismiss, dismiss, dismiss, demanding ‘PROOF’ that the earth does not enjoy being force-fed chemical cocktails.

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