Monarchs find milkweed in summer heat

The air hangs thick near Faubourg St. John this morning, heavy with the promise of another summer day. Chinese tallow trees bloom in clusters along the water's edge, their small white flowers releasing a faint sweetness into the humid air. If you're inside, step out now and let the weight of this Louisiana summer settle on your shoulders.

Among the elderberry and hackberry, where the invasive brown anoles dart between branches, a different drama unfolds. Monarch butterflies move through this landscape with purpose, their orange and black wings catching the early light. These threatened travelers carry an ancient urgency: they must find milkweed. The monarchs that pass through here in summer are part of the northern breeding generation, females heavy with eggs that can only develop on one type of plant. They taste leaves with their feet, searching for the bitter compounds that signal safety for their young. When a monarch caterpillar hatches, it will eat nothing but milkweed. The plant's toxic sap flows into the caterpillar's tissues, making both larva and adult butterfly poisonous to most predators.

Milkweed plants grow quietly in the spaces between the more obvious trees. Their broad leaves collect morning dew that will burn off as the day heats up. The relationship between monarch and milkweed spans the butterfly's entire life cycle. Adult monarchs sip nectar from milkweed flowers when they bloom, but the real dependence runs deeper. Female monarchs lay single eggs on the undersides of milkweed leaves, one per plant when possible. The eggs are tiny, cream-colored domes barely visible against the leaf surface. Within days, a caterpillar emerges and begins eating the leaf that held its egg. It will consume nothing else for the next two weeks, growing through five distinct stages, each requiring a larger leaf surface. The caterpillar's black, white, and yellow stripes advertise its toxicity to birds and other predators. When ready to pupate, it wanders away from the milkweed to form its jade-green chrysalis, often on nearby elderberry or hackberry branches. The adult that emerges carries the milkweed's chemical protection in its wing scales and body tissues.

This summer generation faces particular challenges. The heat that makes morning air shimmer above the pavement also stresses both butterfly and plant. Milkweed leaves can wilt in the afternoon sun, reducing their nutritional value just when caterpillars need them most. The monarchs that successfully breed here will produce the generation that attempts the long flight south to Mexico in fall. Their success depends on finding enough healthy milkweed plants to support egg-laying through the summer months. Each female may lay several hundred eggs over her lifetime, but only a fraction will survive to adulthood. The summer heat presses down on leaves and wings alike, creating the conditions where this ancient partnership either flourishes or fails.

The Chinese tallow flowers release their scent into air that barely moves. Somewhere above, a monarch tests the wind with wings that carry the chemical signature of the milkweed it knew as a caterpillar.