NPS/J. Ratchford
General DescriptionA flash of orange catches the corner of my eye. It’s a monarch! This handsome butterfly is relatively large (9–12 cm; 3.5–4.8 in), beats its wings slowly, and sports rich orange-colored, black-veined wings, edged in white spots. Its larval form—a caterpillar—is also brightly colored, banded in yellow, black, and white stripes. Males and females are similar, though males are slightly larger and he two roundish black spots on their hindwings that are specialized chemical producing scales. The North American monarch (Danaus plexippus) breeds in the U.S. and is one of three species of monarchs—“milkweed” butterflies—found worldwide in the brush-footed butterfly family (Nymphalidae). While butterflies and moths are together in the order, Lepidoptera, they he differences. One is that butterflies typically he slender antennae ending in a rounded club, whereas moth antennae tend to be threadlike (filiform) or feathery (pectinate) without rounded tips.
Distribution and HabitatThe western U.S. migratory population of monarchs, which is separated from the eastern U.S. population by the Rocky Mountains, breeds in most western U.S. states and winters largely along the California coast, down into Baja California, Mexico. The much larger eastern migratory population breeds throughout the eastern U.S. and winters in the central highlands of Mexico. Monarchs forage in open habitat, such as fields, meadows, weedy areas, marshes, and roadsides in search of nectar. Breeding habitat is strictly tied to patches of milkweed (Asclepias spp.), whose lees exclusively nourish their larvae. In winter, great clumps of migratory monarchs settle in moist forested groves with a mild climate, such as in coastal California or central Mexico. Here, they cluster for warmth and protection from wind and snow.
Behior and DietButterflies sense the world very differently than we do. Through chemoreceptors on their feet, butterflies can “taste” important molecules, like dissolved sugar or salt, on a plant. In this way, monarch females can detect the correct plant species on which to lay their eggs. Their antennae, along with fleshy labial palpi (sensory organs) on the head, are densely packed with chemoreceptors to detect the odor of nectar or male butterfly pheromones. Adult butterflies see color, including the special ultriolet markings visible on some flowers and other butterflies, through compound eyes. Each eye aims at a slightly different angle to view up, down, forward, backward, and both sides at the same time. Two other senses partly explain the marvel of how monarchs nigate long-distance migration: sensing earth’s magnetism and sensing the direction of sunlight. Through their straw-like proboscis (tongue), adult monarchs drink water and sip nectar from a variety of flowers, including thistles, daisies, sunflowers, clover, and more. Their caterpillar young he a clever dietary defense against predators. They eat only milkweed lees, which contain toxic steroids called cardenolides. These steroids are distasteful and even toxic to predators like birds and mice. The toxins transfer to the adult during metamorphosis, protecting both stages of life.
EcologySpreading pollen as they sip nectar from flowers, monarchs are important pollinators. They are also food for birds, like jays. Most birds, however, oid their bitter taste, warned off by the bright orange wings that signal poison. This bright color warning defense is known as aposematism. Black-headed grosbeaks (Pheucticus melanocephalus) he some tolerance for cardenolides and prey heily on monarchs during the winter. It helps the birds that cardenolide loads decrease with monarch age, making the long-lived winter migrants more vulnerable. Monarch larvae are much more vulnerable to predators. Beetles, true bugs, ants, spiders, and mice eat them, and fly and wasp parasitoids lay their eggs in the larva body, eventually killing it.