This is Scientific Americans 60 Second Science. I'm Jason Goldman.

Monarch butterflies depends on milkweed.

They lay their eggs on milkweed and their caterpillars eat only the leaves of the plant. No milkweed means no monarchs.

So the best way to help declining monarch populations and to preserve their epic multi-generational migration is to plant milkweed.

Seems simple, right?

But the reality is far more complicated.

Milkweed is slightly toxic.

The plant evolved its noxious substances to keep herbivores from

towing down on the leaves. But monarch butterflies evolved tolerance. In fact, they arm themselves with the stuff.

Monarchs sequester these toxins right as an anti-predator defense and sort of an anti-parasite defense.

Louisiana State University biologist Matt Feldin. So by ingesting the

toxin the caterpillars become toxic themselves. That keeps them safe as long

as they don't ingest too much of the poison. The problem is there are different types of milkweed, and

one that's native to the tropics is now growing in the southern U.S. As these plants sense

warming temperatures, they produce more of the toxin, so much more that the monarch butterflies begin to suffer.

To gauge the threat, Phaldin and his team raised monarchs on either the non-native tropical milkweed or on a native milkweed.

And they also tested the effects of current environmental conditions,

as well as temperatures expected for the southern U.S. by the year 2080.

Monarchs that ate native milkweed had comparable survival rates at both current and higher temperatures.

But under those future warmer conditions, monarchs raised on the tropical milkweed survived

at only one-fifth the rate of butterflies raised under current conditions.

The monarchs could thus find themselves in what's known as an ecological trap.

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