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Monarchs on the move

October 30, 2009

Monarch on Seep-willow

All across the Southwest, millions of migrants are winging their way toward the tropics. Birds are the most conspicuous and well studied travelers, but many insects, including butterflies and dragonflies, also fly south to escape certain death in the northern winter.

The best known of all migrating insects is the magnificent Monarch. Northern populations of this widespread butterfly don’t overwinter as caterpillars or pupae as most temperate-zone butterflies do. Instead, adults that emerge as the days grow shorter in late summer head south to warmer climates instead of mating. Those that survive will become the first of up to five generations that will gradually recolonize their summer range.

Until recently, it was widely believed that North American Monarchs hatched east of the Continental Divide wintered in Mexico, and western populations wintered in California. Thanks to the efforts of biologists and citizen scientists marking Monarchs with tiny numbered wing tags, we now know that it’s not that simple. Recoveries of tagged Monarchs indicate those that hatch east of the Rockies winter primarily in Mexico while at least some individuals from northwestern populations (northern Rockies and Pacific Northwest) migrate to coastal California. Observations of Monarchs migrating through the Southwest on a southeasterly heading suggest that they’ll eventually join the flocks in Mexico, but individuals tagged in southeastern Arizona have been found both in Mexico and California. Even such a familiar creature still has a few secrets left to unravel!

Like migrating hummingbirds, Monarchs must stop every now and then to refuel. The sugars in flower nectar will be converted to fat that will help them survive the winter and begin their spring migration. Rubber Rabbitbrush or Chamisa (Chrysothamnus nauseosus) and Seep-Willow or Mule-fat (Baccharis salicifolia) are important nectar sources for southbound Monarchs as well as a wide variety of other insects. When you encounter these shrubs in bloom, be sure to look for Monarchs (especially ones with tags).

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