Covering icy roads with salt can save the lives of drivers but US researchers say it may be killing butterflies.
Sodium chloride, the cheapest salt, is the most common used to melt ice and snow on slippery winter roads.
Previous studies have already shown that salting roads can have an impact on plant and animal life in nearby lakes and rivers.
But this latest study, published in the Proceedings of the National Academy of Science on Monday, said pouring the substance over roads in winter could also affect the lives of butterflies and other invertebrates that forage for food near the asphalt.
"Salt is normally limited in availability, and sodium plays an important role in development," said lead author Emilie Snell-Rood, a biologist at the University of Minnesota.
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She focused in particular on the monarch butterfly, a migratory insect that feeds on the milkweed plants that grow both along the roadside and in the open prairie.
Her team found that road-side milkweed had up to 30 times more sodium than normal in their tissues.
When butterflies eat those leaves, their sodium levels go up as well.
Among males, the increased sodium led to increased development in muscles used for flight, while among females, the opposite was true. And in female butterflies, the excess sodium led to gains in brain size, but that was not the case for males.
Although increased sodium levels can have beneficial impacts, too much can be deadly - and Snell-Rood's research found a markedly higher mortality rate among butterflies exposed to excessive sodium.
Likewise, the survival rate for monarch caterpillars - the immature form of the butterfly - along roadsides with high sodium concentrations was just 40.5 per cent, compared to nearly 60 per cent for caterpillars feeding on plants in the prairie.
Snell-Rood said similar studies should be conducted in urban areas where even more salt is used during the winter than on the county roads where she conducted her study, in order to test the impact on other creatures.
Other research has already shown that road salt can damage leaves, dry out roots, and change soil properties.
