Introduction

The monarch is one of the best known butterfly species. Although native to North and South America, it has expanded its distribution to Australia, many Pacific islands, and a few places in Europe. Monarch larvae, like those of many other Danaini, feed on milkweeds of the family Asclepiadaceae, from which they sequester cardiac glycosides that often render them unpalatable to potential predators. The species is perhaps best known for its annual migrations, from eastern North America to the mountains of central Mexico, and from western North America to the coast of California. The butterflies rest for the cool winter months in large aggregations which may contain tens of millions of individuals.

Left: Monarch larva feeding on milkweed host plant at Poco Sol (Caribbean slope below Monteverde), Costa Rica. Right: Overwintering monarchs covering a tree branch at Alpha Monarch research site, Mexico. Images © Greg and Marybeth Dimijian