Bay Weekly: Earth Journal

Monarch on Milkweed
On sunny summer days, the black-and-orange monarch floats on the air, seeking nectar from milkweeds and other flowering plants. Its graceful flight pattern is just one characteristic that distinguishes the monarch from others of its kind. Unlike most butterflies, monarchs are bitter tasting and poisonous to birds that prey on them. They are also North America’s only migratory insect.

The midsummer cohort of monarchs are a non-migratory generation. In late summer, their offspring will begin a round-trip journey that is remarkable not just because of the distance they will attempt but also because they are now two or three generations removed from the last migratory generation. Somehow, millions of monarchs will travel hundreds of miles to a single location with no obvious means of navigation.

Monarchs travel on paper-thin wings from as far north as Canada, southward across rivers, mountains and prairies to the Mexican border and beyond to the mountains called Sierra Transvolcanica in the state of Michoacan. Huge numbers, 40 million or more, concentrate there, seeking shelter in the pines from wind and cold, cloaking the trees with their royal colors. In spring, they will retrace their southward journey. Along the way, some females will stop to lay their eggs on the leaves of milkweed.

The various species of Asclepias, or milkweed, occur throughout the monarch’s range, and these plants are key to both the butterfly’s toxicity and migratory nature. Milkweed is the host plant for monarch caterpillars and the preferred nectar source for the adult butterfly. Milkweed sap contains a heart poison that the caterpillars consume but are immune to, while would-be predators are not. The widespread presence of milkweeds enables monarchs to feed and lay eggs at convenient places along their migratory trail.