Danaus plexippus

The familiar monarch hardly needs an introduction or description--it is a large orange-red butterfly with black wing veins and black wing margins with two rows of white spots. The hindwing beneath is tan but with the same pattern as above. There is one widespread similar species, the viceroy which mimics the often toxic monarch. Viceroys are smaller, have a very different flight pattern, have a single row of white spots on the black wing margins, and almost always have a prominent curved black line a bit beyond the middle of the hindwing. In Florida and other extreme southern areas, the gulf fritillaries, soldier and queen butterflies could be confused in flight. The soldier and queen are smaller, darker and with the wing veins not nearly as prominent. Gulf fritillaries are not similar except if far away, they have silvery spots beneath and differ in many pattern details. Monarch caterpillars are as distinctive as the adults: they are about two inches long at maturity, ringed along the length of their body with a combination of black, white, and yellow bands and have two long thin filaments on the thorax and one on the abdomen. Only caterpillars of the queen, which also occur on milkweeds in the far south, are similar, but these have four thoracic filaments and broader stripes. Caterpillars of the black swallowtail, which occur on plants in the carrot family, are similarly banded, but have no filaments and the yellow occurs as spots within the black bands.

In North America this species occurs in summer in all of the lower 48 states and southern Canada from Newfoundland to British Columbia. Monarchs now occur naturally or unnaturally in many other parts of the world and some populations, such as those in Hawaii, do not migrate.

Monarchs occur almost anywhere that milkweeds occur, including old fields, meadows, marshes, prairies, savannas, pastures, roadsides, and gardens. The most critical conservation feature for monarchs is the overwintering habitats, which are certain high altitude Mexican conifer forests and coastal California conifer or Eucalyptus groves. It appears that virtually all North American monarchs overwinter in one of these two areas.

Most North American monarchs are strongly migratory, although there are non-migratory populations in southern Florida. Overwintering areas are a few conifer groves in coastal California and certain locations in mountain fir forests in southern Mexico. Migrants leaving Mexico in late February reach the southernmost U.S. in March and their progeny reach as far north as New Jersey and much of the Midwest by the end of April. By midsummer monarchs are often common in southern Canada. If milkweeds are available, caterpillars are present within a week or two of the first arrivals and then occur all summer. The often spectacular southward migrations are triggered by shortening days and start in late August in Canada, later southward. Some adults leave too late or fail to migrate and they or their offspring get stranded far north of where they can survive the winter in any stage. Doomed adults are not rare into December as far north as New Jersey. Milkweeds (Asclepias species) are the usual larval foodplants.