How salt used to melt road ice affects butterflies

Study shows survival rates of caterpillars fed on leaves of plants growing near the road which had more sodium was lower

By Elga Morrison
Published: Monday 09 June 2014

The researchers began by documenting the effects of roadside salt run-off on sodium availability in butterfly's host plants

Those living in places that receive loads of snow and ice have probably seen city maintenance departments scattering salt on the road to melt the ice.

While this salt helps people to get rid of unwanted ice, very little is known about how this road salt affects the organisms in the neighbourhood. An investigation was carried out by researchers at University of Minnesota and University of Nevada, and they came across interesting outcomes in relation to the influence of road salt on the development of butterflies.

Sodium is essential for survival. It drives the way the organism searches for food. Also, the amount of sodium in the animal's diet affects the development of the brain and muscle tissue. This makes it important that the amount to which the organism is exposed to is regulated.

To study the alteration caused by varying sodium levels, butterflies were used. The team began by documenting the effects of roadside salt run-off on sodium availability in butterfly's host plants. The testing comprised of two rearing experiments, one being roadside collected and control host plants and the other using controlled artificial diet manipulation.

The team found that plants near the road had higher sodium levels than the plants collected from the control site, a hundred metres away. Significant difference was seen between the sites for milkweed and oak species but not the grass and mustard.

Effects on development

The survival rates of the monarch caterpillars were substantially lower on the leaves of plants growing near the road than on the leaves collected from a distance. In adults, an increase in the thoracic muscle of the males was seen on leaves taken from plants growing near the road. Females reared on roadside milkweed had larger eyes than those fed milkweed collected at a distance from the road. Nevertheless, elevated sodium levels were seen in their abdomens regardless of sex.

The survival of cabbage white butterfly was lower on the high sodium diet than those on medium or low sodium dietFor the second trial, the researchers grew the plants in three concentrations of salt– low, medium and high—and studied the growth of cabbage white butterflies on these plants. The results showed that development time was longer on medium sodium diet than the low sodium diet. The survival of cabbage white butterfly was lower on the high sodium diet than on medium or low sodium diet. While the males and females on a medium sodium diet showed a significant increase in thoracic protein, the others didn’t exhibit any such trait. Additionally, the females fed a low sodium diet  had an undersized brain.

The study was published in the June 9 edition of the Proceedings of the National Academy of Sciences.

It is not very clear whether the salt is having a net positive or negative effect at the population level says  Emilie C Snell-Rood, the lead author of the paper from Department of Ecology, Evolution and Behaviour at the University of Minnesota. She also added that from other butterfly and moth studies it is known that males donate sodium to females when they mate and females deposit some sodium in their eggs; it is speculated that slightly higher sodium could lead to higher fitness, which needs to be addressed in future studies.

Other insects, herbivores may also be affected

Moreover, she said that the effect is not restricted to butterflies and that any herbivore that feeds on plants along the side of the road has the potential to be affected. This includes all animals from deer to mice to insects. A species that moves around less like immature insects, would likely be more affected as far as development is concerned.

Subscribe to Daily Newsletter :

Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.