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The monarch, Danaus plexippus, is the large orange and black butterfly of the Americas, best known for flying south each autumn to spend the winter in crowded roosts. It belongs to the family Nymphalidae, and its caterpillars feed on milkweeds. The species ranges from North and South America and the Caribbean to Australia, New Zealand, the oceanic islands of the Pacific, Mauritius, the Canary Islands and, most recently, Western Europe. In the Americas it breeds from southern Canada through the United States and Central America into most of South America.
Identification
This is a big butterfly, with a wingspan of 8.6 to 12.4 cm (3 3/8 to 4 7/8 inches). The upperside is orange crossed by black veins, framed by a wide black border, and both sexes have white spots on the borders and the wing tips. The sexes can be told apart from above. The male is bright orange with black veins and a patch of scent scales on each hindwing; the female is a duller orange-brown with blurred black veins. The veins on a female are thicker than those of a male, and the male’s scent patch shows as a swollen pouch on both hindwings.
The colours are a warning. A predator that has tasted a monarch may link the bright colours of the adult or the caterpillar with an unpleasant meal and avoid monarchs afterwards.
Range and habitat
In the breeding season the monarch is a butterfly of open habitats such as fields, meadows, weedy areas, marshes and roadsides. It is frost intolerant, and where it breeds depends greatly on the presence of milkweeds. Winter is a different story. The overwintering butterflies need dense tree cover, and most of the present sites in California are in groves of blue gum (Eucalyptus globulus), a tree introduced from Australia.
Life cycle and host plants
Caterpillars eat milkweeds, including common milkweed (Asclepias syriaca), swamp milkweed (A. incarnata) and showy milkweed (A. speciosa), with milkweed vine used in the tropics. Most milkweeds contain cardiac glycosides, which caterpillar and adult store in their bodies and which make them distasteful and emetic to birds and other vertebrate predators.
Females lay their eggs singly on the underside of host leaves, and the caterpillars eat both leaves and flowers. The egg and caterpillar stages together take about 2 weeks, depending on temperature, and the adult emerges from the chrysalis after 9 to 15 days. In North America there are 1 to 3 broods a year in the north and 4 to 6 in the south, and the species can breed all year in Florida, South Texas and southeastern California.
Adults drink nectar. Early in the season, before milkweeds flower, they visit dogbane, lilac, red clover, lantana and thistles; in autumn they turn to goldenrods, blazing stars, ironweed and tickseed sunflower.
Behaviour and flight period
The migration sets the monarch apart. Adults leave their breeding grounds from August to October and fly thousands of miles south to spend the winter along the California coast and in central Mexico, with a few staying along the Gulf coast or the south Atlantic coast. The eastern North American population winters in the volcanic mountains of eastern Michoacán, while the western population winters along the California coast. At the Mexican sites the butterflies roost in trees in huge clusters that may hold millions of individuals.
The butterfly spends the winter as an adult, and on warm winter days it may take moisture and nectar. Mating takes place in spring, just before the butterflies leave the overwintering sites, and females lay eggs along the way as they fly north. Not every monarch migrates: residents of tropical areas stay put, though they appear to move up or down in altitude during the dry season.
On cool mornings a monarch warms up by basking with its wings open toward the sun.
Conservation
The species as a whole is listed by the IUCN under the category shown in the facts box, taken from its Wikidata record. The migration is the fragile part. The IUCN considers the annual monarch migration a threatened phenomenon, and the governments of the United States and Mexico, together with private groups, have taken steps to protect the overwintering sites. Butterfly specialists call for conservation and management plans for all wintering sites, migration corridors and the main breeding areas.
Similar species
- Viceroy (Limenitis archippus): the classic look-alike. It has similar colours and is also distasteful, a case of mimicry that benefits both species.
- Southern monarch (Danaus erippus): its South American counterpart, found in tropical and subtropical South America, mainly in Brazil, Uruguay, Paraguay, Argentina, Bolivia, Chile and southern Peru. The two may once have been a single species.