The Evolution is On

ON A STRIP OF YELLOW ADHESIVE TACKED TO A WOODEN STAKE in the middle of a Wisconsin soybean field, a beetle crouches in an eternal pose, frozen by a last, fatal landing. She’ll never know how close she came to immortality.

Here is how life might have played out for this beetle, a brown, winged speck known as a western corn rootworm. Had the trap not snared her, she might have buzzed around the soybean field for a few days, searching for soft soil to house her eggs. Finding a suitable spot, she would have burrowed in, laying her eggs deep enough to shelter them from the winter freeze. The eggs would hatch the following spring, yielding dozens of tiny, hungry larvae. Being corn rootworms, they would have looked around for some corn roots to munch on, giving them energy for their pupal transition to adult beetles, but potentially crippling the plant in the process. And that’s where this beetle was particularly clever.

She was about to lay her eggs in a soybean field, somehow knowing that the following year corn would be planted there,a crop rotation designed in part to ward off pests like her. Staying in the cornfield where she was born—a field that next spring would rotate into another crop—would have meant certain death for her brood. It was a brilliant strategy, spoiled only by a moment of misfortune that landed her in Sarah Schramm’s trap.

Dealing with corn rootworms is nothing new for farmers, explains Schramm, an entomology researcher who has monitored the insects on Wisconsin farms for the past three summers. According to the U.S. Department of Agriculture, rootworms infest 30 million acres of U.S. corn each summer, causing nearly $1 billion in crop losses and control measures. But dealing with them in soybeans is something no one expected. “It’s definitely surprising, because crop rotation usually controls these beetles,” she says, hip-deep in soybeans as she wades through the field to retrieve another of her traps. “But this variant seems to have figured it out.” First discovered about 10 years ago in east central Illinois, the new, rotation-savvy beetles have expanded into Iowa and Wisconsin, where they have now been identified in six counties. At the same time, the closely related northern corn rootworm beetle appears to be working out its own strategy to defeat rotation: Some northern beetles are now laying eggs that rest dormant in the soil for two years before hatching, essentially waiting out a rotation cycle until corn plants return.

One might admire the beetles’ spunk, but of course spunk has nothing to do with it. As the project’s lead investigator, entomologist Eileen Cullen, points out, the beetles aren’t really learning anything. What they’ve done is evolved, the oldest trick in the book.

Darwin had pigeons. Mendel bred pea plants. For farmers and agricultural researchers, the most immediate illustration of evolution is the constant tussle between crops and the pests that prey on them. Every season, plants wage a game of can-you-top-this with their enemies—a legion of weeds, insects and disease-causing pathogens that can weaken or outright kill cultivated crops—to settle who holds dominance in the field.

Because we depend on them for food and energy, we side with the plants. We breed them to have genetic superiority over their foes, and, when that isn’t enough, we assist them with chemical and cultural aids, among them crop rotation, fertilizers, insecticides and herbicides that cut down competitors and nurture vigorous development. But our efforts are fleeting. No matter how clever the technique to kill them, a few pests manage to survive, sometimes by quirk of circumstance, but sometimes with the benefit of superior genes. Deploying a single insecticide or herbicide repeatedly over wide areas only serves to eliminate competition for those gifted few, allowing them to pass their genes on to huge numbers of descendants.

“There’s really no way around the fact that if you expose an insect population to one suppressive method over time it will develop resistance,” says Cullen, an associate professor of entomology for CALS and UW-Extension. “Most insects reproduce quickly, and that means that they have more opportunity for exchanging their genes and adapting to management practices.”

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