MIGRATORY MONARCHS

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.

Dennis Paulson

BUTTERFLIES IN WINTER

The majority of butterfly species in our region pass the winter in the larval stage (caterpillars), a smaller number as pupae (chrysalides), and rather few as eggs. But some of our common butterflies overwinter as adults.

They are the last generation of summer, and instead of just dying, as the great majority of adult butterflies and other insects do in autumn, they seek out a protected spot, for example under loose tree bark or in a rock pile, close their wings, and become dormant for the cold season. This can be as long as 4-5 months in the Pacific Northwest.

Adult winter dormancy is characteristic of the genus Nymphalis, butterflies with showy upperwings but very cryptic underwings. These adults are then the first butterflies of spring, even as early as February, as those with a hibernaculum in a sunny spot warm up enough to emerge and fly out to explore the world and, presumably, find a mate and breed.

The Mourning Cloak (Nymphalis antiopa) is the best-known among these butterflies, as its upperside is both spectacularly colored and unmistakable. Breeding on willows and many other common trees and shrubs, it is almost ubiquitous, although absent from dense forest. A freshly emerged Mourning Cloak is often the first sign of spring for a Northwest naturalist, rocketing through a clearing or resting on a sun-drenched log. Don't look for this species on flowers; it prefers tree sap, rotting fruit, and fresh dung!

Milbert’s Tortoiseshell (Nymphalis milberti) is another common species all over the Pacific Northwest. Its upperside is no less beautiful. This species breeds only on stinging nettle, but that plant is sufficiently common that Milbert’s are everywhere. They overwinter in the same sorts of places as Mourning Cloaks and emerge similarly early. Fresh individuals of both species have yellow bands on their wings that fade to white with age. That difference can be seen in the two photos here.

The California Tortoiseshell (Nymphalis californica), also known as Cal Tort to the butterfly cognoscenti, is similar in shape to the other two Nymphalis species but with still another dorsal pattern. It looks rather similar to other butterflies, for example several species of anglewings (Polygonia). The host plants are shrubs of the genus Ceanothus (deer bush, mountain balm), and these are most common from the Cascades east.

This species is of even more interest biologically than the other Nymphalis because of its population cycles. It is absent some years but present in prodigious numbers in others. If you ever drive on a mountain road in late summer and are dazzled by the sheer density of butterflies flying off the road and swirling around the car, they are likely to be this species.

Cal Torts that build up to these numbers undergo massive emigrations, pouring through the mountains and down to the coast, where they are seen flying south with other butterflies and dragonflies. The points of origin and destination of these migrating butterflies are unknown, but the movements may be a consequence of host plant availability, population pressures, and/or weather conditions. This would be a good species to study in depth.

Dennis Paulson