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Summer 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

June 26, 2012

Common milkweed: Bloom and doom

By late June, common milkweed blooms have reached their fragrant peak. People who find them on sunny roadsides and
lean in to inhale a deep draft of perfume from the nose-high, flower-crowded umbels will find the blossoms covered with a multitude of insects.

Bees, flies and butterflies sipping the abundant nectar can sometimes get caught in a tricky pollen-dispersal mechanism that snags insect legs if they step into slits in the side of each flower. If the insect is strong enough, its leg will yank out a pollinarium, a set of tiny saddlebags filled with pollen. Weaker insects can lose a leg or, if they can't pull free, will die stuck to the flower.

Although numerous insects are visitors, several species are dedicated exclusively to milkweeds, including the larvae of monarch butterflies, red milkweed beetles, and large and small milkweed bugs, all of which concentrate bitter milkweed toxins in their tissue — and all of which are conspicuously colored in black-and-orange or black-and-yellow patterns.

The loud colors say to would-be predators, "WARNING: Eat me at your peril."

But that potent threat could evolve into a bluff as atmospheric carbon dioxide levels continue to climb, altering the physiology of milkweed.

Scientists at the University of Michigan discovered that, as levels rise, milkweed tends to react by boosting its physical defenses while reducing the manufacture of chemical defenses. Rather than making itself less palatable, milkweed generally uses excess greenhouse gas to grow more biomass (outrunning insect damage) and to develop tougher leaves (barricading against insect attack). The plant sometimes even steps up production of the sticky white latex that bleeds from the plant when an insect bites, prompting the bug to eat elsewhere.

But there are exceptions. Milkweed is genetically diverse, and some populations buck the trend, raising production of toxins when CO2 rises. That genetic flexibility should help milkweed — and the organisms that depend on it — adapt to a rapidly changing environment.

Sources: Rachel L. Vannette, Stanford University; Global Change Biology; Craig Holdrege, The Nature Institute

Common Milkweed, Asclepias syriaca

Asclepias syriaca

Flower

Pollinarium

Small milkweed bug

Umbel