Monarchs laying eggs on midsummer milkweed

Along the open edges of Sandy Creek Park, where the tree canopy gives way to sun and the ground stays dry enough for forbs to take hold, common milkweed grows in loose stands. The broad, paired leaves are fully out now, thick and waxy, and the pink-purple flower clusters are either open or just finishing. This is the plant a female monarch is looking for.

Monarchs are in the middle of their breeding season here in the Durham Piedmont. The females moving through these open areas are not wandering. A female monarch locates milkweed partly by sight and partly by contact: she lands on a leaf and drums it with her forelegs, which carry chemoreceptors sensitive to the compounds in the plant's surface. If the leaf registers correctly, she curves her abdomen and deposits a single egg on the underside, pale green and ribbed, about the size of a pinhead. Then she moves on. She may lay several hundred eggs over her lifetime, almost never more than one per leaf, distributing them across multiple plants. This spreading matters. A caterpillar that exhausts one plant's leaves before it finishes developing will not survive, and there are no other options. Milkweed is the only plant monarch caterpillars can eat.

The reason for that exclusivity runs through the plant's chemistry. Common milkweed produces cardenolides, a group of compounds toxic enough to stop the heart of most vertebrates that ingest the leaves in quantity. Monarch caterpillars have a version of the cellular pump that cardenolides normally disrupt, modified just enough to tolerate the toxin. They do more than tolerate it: they store it. A caterpillar that feeds on milkweed accumulates cardenolides in its tissues, and that toxicity carries through metamorphosis into the adult butterfly. A blue jay that catches and eats a monarch typically vomits within minutes and avoids the orange-and-black pattern afterward. The monarch's coloration is not incidental to this. The pattern is a signal, and it works because predators learn it. Pearl crescents and silver-spotted skippers also move through these open areas right now, but they draw no such avoidance. They are palatable, and birds take them. The monarch's relationship with milkweed is what separates it from those other butterflies.

Milkweed does not simply sit still for all of this. The plant invests heavily in its cardenolides, and some individuals produce more than others. Milkweed also has latex, a sticky white sap that runs through channels just under the leaf surface. A small caterpillar that bites directly into a leaf vein can be immobilized by the flow. Experienced caterpillars, even early instars, often cut a circle of veins before feeding, severing the latex supply to a patch of leaf before eating it. This is not learning in the way we typically use that word; it is a behavior that appears early and reliably. The plant and the insect have been working against each other for a long time, and the marks of that are in what each one does.

Monarchs are a threatened species, and the pressures on them are real: habitat loss along the migration corridor, declining milkweed in agricultural landscapes where herbicide use has reduced it sharply, and climate variability affecting the timing of migration and breeding. What happens in patches like this one, at the weedy margins of parks and roadsides in the Piedmont, is not trivial. A female laying eggs here in midsummer is contributing to the late-summer generation that will eventually make the long flight to Mexico. That generation is not born yet. Right now it is an egg smaller than a sesame seed, tucked under a milkweed leaf in the full heat of the afternoon.

If there is milkweed near you, look at the undersides of the leaves. Run your eye along the midrib and out toward the edges. The egg, if it is there, is cream-colored and faintly ridged, sitting upright on the leaf surface. The leaf itself is warm from the sun.