Monarchs (A Post for Cheryl)


Ta-da!  Here it is, the current darling of conservation, the monarch butterfly (Danaus plexippus).  I promised a friend I’d write about them if I ever actually saw one.  So here you go, Cheryl, this post is for you!

This one was in the Smithsonian butterfly garden on the National Mall, feeding on butterfly milkweed (Asclepias tuberosa).

The monarch is easily mistaken for another butterfly, the viceroy.  The two look a lot alike; the major difference is a dark lateral stripe toward the rear of the viceroy’s hindwings.  I don’t have a picture to illustrate that but have a look at the Maryland Butterflies website.

I’m no entomologist so I’ll stick with a few basic facts.

There are two populations of monarchs, one east of the Rocky Mountains and one to the west.

Most of the eastern population overwinters in Mexico, though some will hibernate on the Gulf and southern Atlantic coasts.  Adults already in the tropics do not migrate.

Monarchs lay eggs on milkweeds (Asclepias species, and two other species in the Apocynaceae).  The caterpillars go through five instars before pupating and emerging as an adult butterfly.  The adults will feed on a variety of plants, including species in the aster, carrot, pea, and mustard families.

Monarchs will produce one to six broods during the summer.  Most of the butterflies live only a few weeks; it’s the last brood of the year that migrates south to overwinter, mate, and return north, laying eggs on the way.

The eastern population has declined by more than 90% since 1995, which is about one billion individuals.

The major threat to monarchs is habitat loss, which takes two forms: destruction of the forests in which the monarchs overwinter, and destruction of milkweeds, the sole larval host plant.  Milkweeds are considered nuisance plants by farmers and many homeowners.  Another threat – and another reason to hate invasive species – is the increasing presence of two species of Cynanchum, exotic alien plants that look like the native Cynanchum laeve (which is a larval host plant) but are poisonous to the larvae.

Other species of butterflies migrate, but the monarch is the only one that migrates both ways.

For more detailed information check out any of the following sites:
USDA Forest Service
Butterflies and Moths of North America
Monarch Watch
The Xerces Society


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