Milkweeds are unwilling hosts to monarchs

Monarch butterflies and milkweed plants are entangled in a struggle for domination.

Milkweeds, which are poisonous to most animals, are the only thing monarch larvae eat.

The butterfly thrives and survives on milkweed. The plant would rather it didn't. Monarchs evolve a tolerance for milkweed. Milkweed in turn evolves a new defense strategy.

Milkweed's first defense is the sticky white material that leaks from the plant when you break its skin. It is called latex, according to Anurag Agrawal, professor of biology at Cornell University.

He has written a book about the opponents in this battle. "Monarchs and Milkweed: A migrating butterfly, a poisonous plant, and their remarkable story of coevolution." (Princeton Press, $29.95)

Milkweed latex contains cardenolide, a potent toxic chemical. It has been used as rat poison. Aboriginals have used it as the dipping sauce for poison arrows. It will make your eyes sting and your lips numb, Agrawal says, if you are careless with the smear on your finger.

The butterfly larva can choke on the initial burst of latex when it bites a milkweed leaf. Mortality is about 60 percent. A successful caterpillar/butterfly deals with cardenolide by sequestration. It isolates and stores the chemical in its weapons locker.

For humans, however, cardenolide can save lives — in small doses. It is commonly prescribed for treatment of congestive heart failure, according to Agrawal. The daily dose would be 0.25 mg.

The butterfly routinely ingests and stores more than that, even though we humans are about 68,000 times heavier.

If as an insect you eat plant parts and drink plant nectar, you usually return the favor by assisting with pollination. The monarch eats and drinks but does not pollinate. From the plant's perspective, Agrawal says, the butterfly is no more than a pest.

Considering its life cycle, however, it is an amazing pest.

Let's begin 10,000 feet up on a mountain in central Mexico. Our butterfly spends four months there with millions of other monarchs each winter. It rests, clinging to a pine tree, in a suitably cool temperature.

The butterfly heads north into Texas. There, it finds its preferred young milkweed plant, lays one egg per leaf, and dies. The egg is eaten or becomes a new monarch that continues north, where it repeats the process: Find plant, lay egg, die. That cycle could happen at least one more time before the insect reaches Minnesota.

I saw my first monarch this year in June. That's early. Late July and August are peak monarch months here.

When fall arrives, the current monarch generation reacts to air temp and day length. Hormones are activated, according to Agrawal. This time, the butterfly turns south.

Its previous generations lived a month. This butterfly becomes Supermon-arch. (Sorry.) It will live eight months, flying to Mexico, spending the winter, then breeding before beginning the new cycle on a milkweed plant in Texas.

Monarchs, like birds, navigate using the Earth's magnetic field and sun position. The insect, weighing about as much as a dollar bill, can calculate sun position even as day length changes. Monarchs, using air thermals and riding southerly winds as do birds, always know where they are going.

Read more about monarchs and other butterflies and how they compare with birds at Jim Williams' birding blog at startribune.com/wingnut.