The promise and peril of milkweed

Two weeks ago I described butterflies that visit milkweed flowers. Insect species that actively feed on the leaves include milkweed bugs, with piercing mouthparts, and milkweed longhorn beetles, with small jaws (“mandibles”) that work from side-to-side. When aphid colonies appear, ladybug beetles visit to prey on them.

The most common predators include shield bugs, various spider species and harvestmen (better know as “daddy-long-legs”).

Among the insects that become meals, monarch butterflies plus their eggs and caterpillars are at great risk. Monarch females may be captured as they lay eggs on milkweed leaves, and I’ve seen the eggs victimized by microscopic spiders, to say nothing of larger insect predators. It’s estimated that a mere one percent of a female’s 200 or 300 eggs will become butterflies.

Monarch caterpillars are also vulnerable—especially after hatching, as in the picture, and at four points in their life cycles when they have to molt their outer “skins.” Molting can take a day, even in mid-summer, and a monarch caterpillar is immobile and helpless on the milkweed plant during the process.

The tiny caterpillar in the picture is eating its eggshell, which supplies useful protein. Grit from the alley surrounds the egg, which weighed about 0.54 milligrams when deposited. And in this place, as in many others, human actors will determine the future of the caterpillar and its milkweed host.

A summary of the monarch’s life cycle is available on our Butterfly Barn website, at butterflybarn.org/eggto.html.