Butterflies do not alter oviposition or larval foraging in response to anthropogenic increases in sodium

Humans have drastically increased the availability of many once-limited nutrients, resulting in potential attraction of animals to now toxic, novel nutritional conditions. For instance, herbivores are attracted to sodium in many terrestrial ecosystems because sodium occurs in much lower concentrations in plants than in animals. However, sodium availability has increased due to crop irrigation or distribution of de-icing salts along roads. While moderate increases in once-limited sodium may be beneficial, large increases are potentially toxic. Here, we investigate whether increased sodium along roadsides is functioning as an ecological trap, drawing butterflies to oviposit and forage on plants with toxic levels of sodium. As in many other animals, butterflies exhibit sodium-specific foraging behaviour – males will ‘puddle’ for minerals on mud, dung or carrion; yet sodium-seeking behaviours at other life stages remain unexplored. We present data from a series of field- and laboratory-based sodium preference trials using both caterpillars and ovipositing adult females. Female monarch and cabbage white butterflies did not show any preference for laying eggs on sodium-enriched plants, either at moderate levels that are likely to be physiologically beneficial, or at potentially toxic levels typical of high-traffic roads. Monarch caterpillars somewhat avoided the plants highest in sodium, but this behaviour is unlikely to compensate for the failure of ovipositing females to avoid toxic high-sodium plants. These results suggest that while butterflies frequently use sodium-enriched roadside habitats, they are unlikely to be seeking out or avoiding these areas due to elevated sodium.