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By Jason Bittel
Most people know that monarch butterflies can't exist without milkweed. As caterpillars, the monarchs feed on milkweed plants exclusively, absorbing the milkweed's poisons in order to ward off birds and other predators. On their epic migration across the North American continent, the butterflies also lay their eggs on these plants, relying on the noxious taste of the leaves to keep their brood safe from grazers while simultaneously providing a buffet for the next generation when it hatches.
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The Migration and Importance of Monarchs
Monarch butterflies, which pollinate many different kinds of wildflowers, are among nature's great wonders. Their annual migration is one of the most awe-inspiring on Earth: Each fall, millions of these striking black-and-orange butterflies take flight on a 3,000-mile journey across the U.S. and Canada to wintering grounds in Mexico's Sierra Madre mountains.
Wikipedia / CC BY-SA 3.0
Dicamba—a drift-prone herbicide linked to millions of acres of off-target crop damage across in 17 states—destroys mostly everything in its path except the crops that are genetically engineered to resist it. It's so damaging that several states, including Arkansas, Tennessee and Missouri have introduced temporary bans on the weedkiller.
There's now another reason to worry about the controversial chemical. It's particularly harmful to milkweed, the only host plant for the iconic and already at-risk monarch buttery.