Milkweed flowers open as monarch caterpillars emerge

The air carries a faint sweetness across the Potomac Hills neighborhood this morning, where clusters of pink and orange flowers have begun to open on the milkweed plants scattered through yards and along fence lines. Common milkweed stands waist-high with its broad oval leaves, while the lower butterfly milkweed shows off bright orange blooms that seem to glow in the early light. The timing is no accident.

Monarch caterpillars are hatching now from eggs laid on these same plants just days ago. The tiny larvae, smaller than a grain of rice, emerge with an immediate need. They eat their own eggshells first, then begin the work that will define their entire existence: consuming milkweed leaves. Nothing else will do. The chemical compounds in milkweed, toxic to most creatures, become the monarch's protection. The caterpillars concentrate these toxins in their bodies, making them unpalatable to birds and other predators.

This relationship runs deeper than simple feeding. The milkweed plants are now entering their peak flowering period, their blooms attracting not just the adult monarchs that will lay the next generation of eggs, but dozens of other pollinators. Bees work the flowers methodically, their legs heavy with pollen. The monarch adults, when they visit, are less efficient pollinators than the bees, but they serve the plants in a different way. By laying their eggs exclusively on milkweed, they ensure a steady population of leaf-eating caterpillars that never fully defoliate their host plants. The caterpillars consume enough to stimulate new growth but leave the root systems intact.

As the caterpillars grow over the coming weeks, they will shed their skin five times, each molt revealing a larger, more boldly striped larva. The final instar caterpillars, thick as a thumb and marked with yellow, black, and white bands, can strip a milkweed plant of most of its leaves before wandering off to pupate. But the plants recover. Their deep taproots, sometimes extending six feet down, store enough energy to send up new shoots. By late summer, many of these same milkweed plants will bloom again, providing nectar for the monarchs beginning their long migration south.

The butterfly milkweed, with its more compact form and brilliant orange flowers, follows a slightly different schedule. Its blooms last longer than those of common milkweed, extending the nectar season well into summer. Both species contain the cardiac glycosides that make monarch caterpillars toxic to predators, but butterfly milkweed produces less of the milky latex that gives common milkweed its name. The caterpillars show no preference between the two species, moving from plant to plant as they outgrow their current food source.

In yards where homeowners have planted native milkweeds, this ancient cycle plays out just as it does in prairies and meadows. The plants ask for little: decent drainage, full sun, and patience as they establish their deep root systems. In return, they anchor one of North America's most remarkable migrations. The caterpillars feeding on milkweed leaves today will become the butterflies that travel thousands of miles to overwinter in Mexico, guided by magnetic fields and celestial cues that scientists are still working to understand.

Step outside if you can, and look for the milkweeds beginning to bloom in your area. The flowers release their fragrance most strongly in the warm afternoon sun, sweet and slightly cloying. If you find a plant, check the undersides of the leaves. The monarch eggs, when present, appear as tiny white dots, usually laid singly on the youngest, most tender foliage. The whole relationship begins there, with that small white sphere and the vast genetic memory it contains.