Winter Road Salt Is Wreaking Havoc On Summer Butterflies

April Flowers for – Your Universe Online

In the winter months, road salt is just one of those things we take for granted. It makes driving easier in icy and snowy conditions.

We already know that road salt takes a toll on lakes and rivers, but what is it doing to the organisms that live and forage at the edge of our roadways? Very little is understood about consequences for the development and evolution of wild animals created by how the use of road salt has altered patterns of sodium availability.

Emilie Snell-Rood, a behavioral and evolutionary biologist with the U of M’s College of Biological Sciences, led a new study that suggests the availability of the micronutrient sodium might alter selection on foraging behavior for butterflies and other roadside developing invertebrates. Small, sparing amounts of micronutrients such as sodium and iron are required by all living things, but such nutrients can play a large role in development.

“Salt is normally limited in availability and sodium plays an important role in development” says Snell-Rood. “After experiencing my first Minnesota winter, I began wondering how road salt might be affecting the development of organisms along roadsides.”

Milkweed, the main food of monarch butterflies, grows in abundance along the country roads near Cedar Creek Ecosystem Science Reserve in East Bethel, MN, where the team collected their samples. This made the monarch a perfect model organism for the study, which has been published in the Proceedings of the National Academy of Sciences.

“We compared monarchs reared on roadside-collected and prairie-collected host plants,” says Snell-Rood. “Monarchs were chosen because milkweed is a common roadside plant and investment in sodium-rich muscle should be important for a migratory species like monarchs.”

Sodium concentrations were found to be elevated in the tissue of some plants by as much as 30 times the normal rate. Butterflies feeding on these plants experience a similar elevation. The effects of this rise in sodium differ according to the sex of the butterfly, the team found, and can even have opposing effects on the same tissue. For instance, females saw gains in brain size, while males did not. Likewise, males had an increased investment in flight muscle while females exhibited the opposite pattern.

Moderation is the key for health in most organisms, and the same is true for butterflies with sodium. The study found that a slight rise in levels showed some benefits, but too much became toxic to the insects. In the control group, excessive sodium was found to lead to a significant rise in mortality. Snell-Rood would like to see further research in urban areas where road salt concentration would be higher. She believes the results of this future research would underscore the downsides for roadside dwelling organisms.

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