Meeting Abstract
Sodium occurs in much lower concentrations in plants than in animals and is often limiting in herbivore diets. As a consequence, herbivores are attracted to sodium in many terrestrial ecosystems. Human activity, however, has drastically increased the availability of sodium in certain habitats where it was once rare. For example, we distribute tons of salts on roads during winter for de-icing, and these activities elevate the concentration of sodium in roadside soils and plants. This scenario is particularly interesting in the context of monarch butterfly (Danaus plexxipus) conservation, where roadsides are being restored as pollinator habitat, but the ecological consequences of such restoration is unclear. Prior research has shown that small increases of sodium in caterpillar diets can have positive effects on adult performance, but large increases are toxic. Butterflies may indeed be attracted to high sodium roadsides given that many species display “puddling” behaviors as adults, where they seek sodium and other minerals from carrion, dung or mud. Here we present data from an observational field study showing sodium levels in roadside milkweed plants are elevated in plants closer to the roadside, and along higher traffic roads. We also present data from a series of field-and lab-based sodium preference trials using both larvae and adults. Monarchs did not show significant preferences for sodium-enriched plants at levels that are physiologically beneficial. They also failed to avoid sodium-treated plants with toxic sodium levels typical of high traffic roads. Though monarchs frequently use salty roadside habitats and this influences their phenotypes, they are unlikely seeking out roadside habitats because of this elevated sodium. However, our observations of both adult and larval behavior suggest they may indeed consume toxic levels of sodium in some sodium-enriched areas.
