The Monarch (Danaus plexippus) is surely the best known of North American butterflies. Occurring all across the continent, as well as on several other continents, this big orange butterfly with a vivid black-lined pattern delights all who see it. Like the swallowtails, it is the epitome of butterflyhood, bouncing across meadows and sipping nectar from a wide variety of flowers.
At this time of year, Monarchs all across North America head south for the winter. The vast majority acome from the eastern two-thirds of the continent. They fly south in trickles, then streams, then rivers converging on just a few areas in the mountains of central Mexico, wintering grounds not discovered by scientists until 1975. Many fewer (about 5% of the total population) head for the Pacific coast to winter in a few coastal groves in California.
Although many insects are migratory, this one is the most noteworthy in the US because of its large size, bright coloration, and great numbers of individuals on the move at once.
If you live near milkweed plants (Asclepias species), you may be very familiar with Monarchs. The beautiful flowers of milkweeds give no hint of their toxic nature. The plants have a milky sap full of cardiac glycosides, chemicals that affect the sodium-potassium pump in vertebrate cell membranes and can cause tachycardia and ventricular fibrillation, both very stressful to the human heart. The sap is also viscous and bitter tasting and caustic to our mucous membranes.
Monarchs breed on milkweeds, and their caterpillars are among the small number of insects that have evolved a resistance to the harmful effects of milkweed sap. They munch it with no worries throughout their development. The sap confers unpalatability and distastefulness on the larvae, and their bright aposematic coloration serves as a warning flag to potential predators.
The poisonous qualities of the milkweed are carried through metamorphosis, and the butterflies are also distasteful, unpalatable, and probably warningly colored. They are left alone by vertebrate predators, although invertebrates such as dragonflies can handle them.
Monarchs are large and conspicuous, not especially fast flying, and if they weren't distasteful they would probably be the object of much predation during their migrations and certainly on their communal roosts. In fact, they probably roost communally to concentrate the warning to predators that they are not to be touched.
Monarchs are nowhere nearly as common in the West as in the East, and in Washington state they are downright uncommon. It's always thrilling to see an adult or larval Monarch in the Columbia Basin, where they are associated with the big milkweed Asclepias speciosa. Presumably our Monarchs head south to the California roosts.
After overwintering, Monarchs take off for the North. They move back into southern US and breed where they stop. Their larvae develop quickly in the spring and, as soon as they are out of the pupa, the adults head north like the previous generation. They are one-way migrants, and they will stop somewhere in the northern US or even southern Canada to breed. There may be as many as three northbound migrant generations, before their offspring head off in the other direction, back to the wintering ground.