Diagnosis: The Monarch is one of the largest Canadian butterflies (wingspan: 93 to 105 mm). The upperside is bright orange with heavy black veins, and a wide black border containing a double row of white spots. There is a large black area near the wing tip containing several pale orange or white spots. The underside is similar except that the hindwing is much paler orange. Males have a sex patch, a wider area of black scales on a vein just below the centre of the hindwing.
Subspecies: There are several non-migratory subspecies in Central America, but only the nominate D. plexippus plexippus is found in North America and the rest of the world.
Range: The centre of the Monarch's vast range is in North America, but it occurs south to Argentina and has spread to Bermuda, the Azores, the Canary Islands, Hawaii, and even India, New Zealand, and Australia. It strays regularly to Europe, but has not become permanently established there because of lack of suitable foodplants. In Canada it occurs from Newfoundland to Vancouver Island, at times well beyond the northern limits of its foodplants, with one old record from the Northwest Territories at Fort Providence (Cary, 1907).
Early Stages: The larvae are banded in white, black, and yellow. There are two pairs of fleshy black filaments, one just behind the head and the other near the end of the body. They feed on many species of milkweeds, in eastern Canada most often Common Milkweed (Asclepias syriaca), but also utilizing Swamp Milkweed (A. incarnata) and Butterfly Weed (A. tuberosa). In addition, Green Milkweed (A. viridiflora) is used in Manitoba (Klassen et al., 1989) and Showy Milkweed (A. speciosa) and Low Milkweed (A. ovalifolia) in Alberta (Bird et al., 1995). All milkweeds contain poisons, cardiac glycosides, in varying amounts, that protect the plants from most herbivores. Monarch larvae are not affected by the poisons but store them in their bodies and pass them on to the adults. Most birds that attempt to eat adults or larvae vomit and learn to associate this unpleasant experience with the bright patterns of the adults and larvae and thus soon learn to avoid them. The Viceroy (Limenitis archippus) mimics the pattern of the adult Monarchs and gains some protection by the resemblance; recent evidence suggests that the Viceroy itself might be somewhat distasteful to birds, so the mimicry could benefit both species.
Abundance: The Monarch varies greatly in abundance from year to year, depending on the weather conditions, which affect both reproduction and migration, as well as on their survival rate over the winter. The populations usually build up through the summer through several generations. In the early fall they can be seen in the hundreds or thousands at places where the southward migration is concentrated, such as at Point Pelee, in southwestern Ontario, and Presqu'ile Provincial Park on the north shore of Lake Ontario.
Flight Season: Occasionally the Monarch arrives in Canada towards the end of May, but the bulk of immigrants arrive in June. They then pass through two or three generations before the bulk of the population leaves in September, although stragglers will be seen well into October and even November.
Habits: The Monarch is the best known of all migratory butterflies, and the only one with a well-documented return migration. Its migratory tendencies enabled it to spread over most of North America in the 1800s, probably coinciding with the clearing of the forests of eastern North America and a vast increase in the population of both Common Milkweed (Ascleptas syriaca) and Monarchs. All Monarchs from west of the Rockies fly to a few small areas of California, a fact that has been known for more than a century; Pacific Grove, near Monterey, California, is famous for its congregating Monarchs. All other North American Monarchs, from Alberta to Newfoundland, travel even farther, to about 30 tiny overwintering sites in a small area of the Transverse Neovolcanic Belt of Mexico, between 70 and 170 km west of Mexico City (Brower, 1995). All the sites are in forests of Oyamel Fir (Abies religiosa, Pinaceae); the Oyamel Fir forest is a specialized high-altitude ecosystem on the higher peaks of Mexico. In these sites, at altitudes of around 3000 metres, where the temperature usually stays just a few degrees above freezing all winter, the Monarchs pass the winter hanging on the trees in such numbers that they sometimes break large branches by their weight. Estimates as high as 14 million butterflies have been made in the few acres of one such site.
Remarks: Although their migratory tendencies were well known, the Mexican overwintering sites were discovered only in 1975, as a result of a lifetime's study by Dr. Fred Urquhart, of the University of Toronto. Dr. Urquhart pioneered a method of tagging and releasing Monarchs, then plotting their distribution when a tiny proportion of the tagged butterflies were caught and returned to him. When the overwintering sites were finally discovered, a few marked butterflies were found that had been tagged in the northern U.S. Since then, others tagged in Mexico have been found in Canada, proving that at least some individuals make a round trip. However, more recent work by Malcolm et al. (1993) has shown that these are the exceptions; they demonstrated that at least 90 per cent of the Monarchs recolonizing the northern U.S. in June developed from larvae that had fed on species of milkweed common in the Gulf states and Florida (i.e., they were the offspring of females that had returned from Mexico to the Gulf States and laid their eggs on the local milkweeds there).
The concentration of the entire eastern North American population in just a few sites creates a serious risk, and some sites, when discovered, were already under pressure from local development and logging. Since then the Mexican government has taken steps to protect them, but effective enforcement is still a serious problem. Vulnerability of the overwintering sites and the elimination of milkweeds and nectar sources through the use of herbicides in the vast agricultural areas of the American Midwest continue to threaten the future of this species.
Anyone interested in the development of the current state of knowledge of the migratory habits of the Monarch, as well as the surprisingly large gaps that still exist in our knowledge, should read Brower, 1995.
© 2002. This material is reproduced with permission from The Butterflies of Canada by Ross A. Layberry, Peter W. Hall, and J. Donald Lafontaine. University of Toronto Press; 1998. Specimen photos courtesy of John T. Fowler.
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Date Modified: 2010-05-31