July EcoQuest - Milkweeds and Monarchs
Milkweeds (Asclepias sp.) are one of the most common, showy flowers now blooming in the greater metro area. Milkweeds are easily identifiable – they have sepals and petals, but they also have an elaborate corona, usually comprised of a “horn” and “hood.” Milkweeds also have opposite leaves and milky sap. There are 10 species of Asclepias found in the greater metro area, but Asclepias speciosa (showy milkweed) is our most common species. The Xerces Society has a great guide to the milkweeds of Colorado that you can use to recognize the most common native species of milkweeds in our region.
Milkweeds are the sole food source for monarch butterfly (Danaus plexippus) caterpillars. Although milkweeds are toxic to most insects, monarch caterpillars can eat the leaves and store the toxins in their bodies, in turn making them toxic as well. Once these caterpillars have developed into butterflies, they then drink the nectar from the milkweed flowers for food. And in drinking this nectar, the butterfly’s foot sometimes slips into a structure of the corona called the stigmatic slit, within which lies a ball of sticky pollen called pollinia. This pollinia then becomes attached to the butterfly, traveling with it as it moves on to the next flower, where it is again deposited into another stigmatic slit, thus completing the act of pollination. And pollination ensures that the milkweed will produce fruit and seeds for the next generation. It’s a win-win for milkweed and monarchs!
Monarch butterflies migrate an astounding 6,000 miles each year, roundtrip from Mexico to Canada, through successive generations (it will take 3-4 generations before they reach Canada from Mexico). And as they migrate, monarchs lay eggs on milkweeds before dying. Migrating monarchs are divided into two populations – with the eastern one east of the Rocky Mountains and the western one west of the Rocky Mountains. In Colorado, our monarchs are part of the eastern population. You can help the monarchs along on their journey by planting a milkweed or two in your own garden!
Help Denver Botanic Gardens document monarchs and milkweed in the greater metro area by photographing as many plants and caterpillars (or even monarch butterflies!) as possible in the month of July. Post your findings to iNaturalist so they will automatically be added to the Denver EcoFlora Project.
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