Menu

Inside the battle for survival in the field

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.”

That ability to adapt has turned our chemical assault on bugs and weeds into a long and frustrating arms race. Despite the millions of pounds of pesticides now applied to food crops around the world, research by German plant pathologist Erich-Christian Oerke has shown that crop losses due to pests really haven’t changed all that much during the past 40 years. That’s not to say we shouldn’t be using some form of intervention. Oerke’s studies estimate that if we let pests have their way, the world could lose more than a quarter of its soybeans, nearly 40 percent of its potatoes and corn, and half of its wheat. We may not be winning the war on weeds and bugs, but without an uninterrupted stream of innovative ways to keep them under control, we might not even be holding our own.

A striking example is the creeping vulnerability of glyphosate, the most widely used herbicide in the United States. More commonly known by its original trade name, Roundup, glyphosate has been a potent assassin of weeds since it was first introduced by Monsanto in the 1970s. Homeowners spray nearly 8 million pounds of the herbicide around lawns and gardens annually, but its real dominance is in agriculture. In the mid-1990s, Monsanto pioneered corn and soybean crops that were genetically engineered to withstand glyphosate, allowing farmers to spray Roundup broadly across fields to combat weeds. Farmers planted millions of acres of Roundup Ready crops, dramatically increasing the application of glyphosate. Recent estimates say 90 million pounds of the herbicide are applied to crops in the United States each year.

But the widespread use also increased the selection pressure for weeds that aren’t felled by glyphosate, says CALS weed specialist Chris Boerboom. As early as 1998, ryegrass in California had adapted to survive the herbicide, and now 20 states and 13 countries have identified glyphosate-resistant weeds. Wisconsin isn’t on the list yet, but officials are investigating a suspicious case that arose this summer.

Boerhoom says it’s no accident that resistance has been slow to arise in Wisconsin. “We’ve spent a lot of time with Wisconsin corn and soybean growers discussing resistance and practices to reduce the risk of glyphosate-resistant weeds,” he says. A survey conducted by CALS colleague and agricultural economist Paul Mitchell found that Wisconsin growers were more aware of the dangers of overusing Roundup and more likely to rotate herbicides than farmers in any other state.

Does that mean that Wisconsin farmers are more friendly to Darwin and his theory of evolution? Not necessarily, says Cullen. “I don’t think we often talk about it in terms of evolution. I, for instance, certainly don’t think of myself as an evolutionary biologist,” she says. “But really, it is evolution. The concept is there in many of our conversations on the farm.”

In the case of the western corn rootworm beetle, for instance, the insects were pressured to adapt by the common practice of upper Midwestern corn farmers to rotate fields between corn and soybeans in alternate years. With an unbroken cycle of corn, beetles would never have developed an urge to leave the cornfield behind because staying put would have worked for generations of ancestors. But a rootworm employing the same strategy in a field that rotates between crops would find its lineage cut short. The advantage would go to the beetle that, for whatever reason, liked the look of a nearby soybean field and thought it would make a nice place to raise a family. Researchers don’t yet know what genes may have been passed along to drive that instinct, but the growing size of the soybean-nesting beetles tells them that it’s more than chance. Why did the beetle cross the road? To survive.

Not all western corn rootworm beetles know the trick, and that’s one reason the variant population hasn’t spread uniformly across the corn belt. In southern Wisconsin, where the emerging beetle was first discovered in 2003, the variants intermingle and mate with beetles that are still prone to crop rotations, helping to dilute the transfer of their genes to future generations. But rotations are already failing to keep the insects under control in several counties in Illinois, and farmers near the border are beginning to ask whether they should change tactics.

Cullen tries to help them answer that question using integrated pest management, an approach that employs a spectrum of management options, from cultural to biological to chemical. One of the central dogmas of IPM is to do nothing that you don’t have to do, especially when it comes to spraying pesticides. Schramm’s traps are essentially testing whether farmers should worry about these new beetles at all. During weekly visits to farms in August, she was looking for an average of five beetles per trap per day. Anything less and the tenets of IPM suggest that it would be more expensive for a farmer to fight the beetle than to just live with it. And while that appeals to farmers’ economic interests, it is also a strategy rooted in an understanding of evolution.

Pea aphids climb the stem of a bean plant.

“We don’t want to constantly expose (pests) to one particular pressure to adapt,” says Cullen. And that desire is putting farmers in charge of another process on their fields that they may not often think about. They’re now running the show when it comes to the evolution of the plants and animals that live under their domain.

Perhaps its no revelation that humans are now turning the gears of evolution. We’ve dabbled in the field for some 10,000 years. Since the birth of agriculture, we have commandeered the natural evolution of plants and animals and directed them toward human uses. Without human-directed selection, we would have few of the crops and animals we depend on: no dairy cows, no donkeys, no woolly sheep and no ears of corn. The family pet would be a wild menace.

But things have gotten considerably more complicated than crossing plants or gathering desirable seed. Scientific advancement and an exploding human population have combined to give us unparalleled influence on our environment. Our use of the planet’s resources has profoundly reshaped the species who share space with us. Some, like the dodo, couldn’t compete and went away. Others, like cod, which mature faster and smaller than before we fished them, look considerably different because of us. As the evolutionary biologist Julian Huxley wrote more than 50 years ago, “It is as if man had been appointed managing director of the biggest business of all, the business of evolution … and the sooner he realizes it and starts believing it, the better for all concerned.”

David Baum, a professor of botany who leads UW-Madison’s Evolution Initiative, says Huxley’s sentiment is even more apt today, when disease-causing microbes are developing resistance to antibiotics and invasive species are altering the landscape. “We’re really coming to appreciate how many human problems are evolutionary problems,” he says. “There are situations in which we want to stop, slow down or direct evolutionary processes, and in those situations, we can use what we understand about evolution in nature to provide us with insights about how to do that.”

One of the pressing questions in the current influenza pandemic, for instance, is how and where the H1N1 virus evolved. Its heritage matters to people such as Christopher Olsen, a professor in the School of Veterinary Medicine who studies flu viruses, because they can learn a lot about a virus by understanding to whom it’s related. An evolutionary link can help public-health officials more quickly design vaccines and identify what populations are most vulnerable to the virus. By tracking and probing the evolution of emergent viruses, Olsen says, “we can get ahead of the curve when we see a virus begin to spread or change in a way that concerns us.”

But evolution matters to Olsen for other reasons. He’s particularly interested in how viruses make the leap from infecting birds to other species, namely pigs and humans. It’s a complicated transition that involves far more than a simple genetic tweak, he says. But many viruses do it. And that’s partly because there are just so many viruses. In his lab, where Olsen’s research team grows flu viruses in chicken eggs for experiments, a single egg can produce several million viruses in a day.

“For something like a flu virus, the evolutionary time scale is so much shorter than it is for us,” says Olsen. “It’s a matter of hours, not decades or centuries.”

It is true about most of the things that make us sick: They change faster than we can. The same dynamic plays out in agricultural fields, where pests turn over one or more generations each season while engineering resistant crops and new crop treatments can take decades of research. Our best defense is to use something that can adapt more quickly than our genes: our understanding of how evolution works.

One difficulty is that evolutionary processes don’t always behave in ways that we might expect. Take the case of the pea aphid, a ubiquitous and occasionally troublesome guest in pea and alfalfa fields across the United States and Canada. Pea aphids prefer cool temperatures, and so one might guess that global warming would portend bad things for their kind. But when UW-Madison postdoctoral researcher Jason Harmon and ecologist Anthony Ives set up an experiment to test that hypothesis, they found a more complex web of interactions at work.

“The reason I started working with pea aphids is that they are a really great example of biological control. They don’t die natural deaths,” says Ives, a professor of zoology and entomology. Instead, a suite of predators, including ladybugs and parasitoid wasps, keep the aphids in check. When Harmon and Ives covered alfalfa plots with shrink-wrapped plastic boxes to simulate the temperature spikes that might come from global warming, they found that some aphids did suffer. But aphids that harbored particular bacteria suffered less, suggesting that they might evolve symbiotically as temperatures climb. The researchers also found that one of the aphids’ main predators gave up when there weren’t as many of the bugs around, which means that in a real warming scenario, aphids might actually get a reprieve from predator pressure.

The bottom line, says Ives, is that “you can’t simply add the effects together and come up with a conclusion. There are layers of interactions involved, and when you throw evolution on top of that, it produces something that’s quite complicated. But it’s not entirely unknowable.”

That kind of thinking increasingly is finding its way into agricultural policy decisions. “Really, the U.S. government regulation on insecticidal crops is all about resistance management,” says Ives, who has modeled the effects of various regulation schemes in his research. When the U.S. Environmental Protection Agency approved commercial use of Bt corn—a transgenic crop modified to produce the bacterial toxin Bt to deter plant-eating insects—the agency imposed rules that require farmers using Bt corn to plant at least 20 percent of their fields with non-Bt crops to stall the evolution of Bt-resistant bugs. And while farmers and environmentalists have far from settled their differences on the use of Bt-producing crops, Bt resistance has not yet become an issue.

As for corn rootworm beetles, evolutionary biologists are on the case. Scientists are actively searching for the genes that allow western and northern corn rootworm beetles to independently outwit crop rotation. If those genes are identified, they could help biologists devise new strategies to keep the beetle from spreading or forestall its evolutionary development.

But in Eileen Cullen’s lab, the mysteries of evolutionary biology can’t compete with the present demands of economic reality. “Farmers can’t really wait to find that out,” she says. “We know that the insect is changing, and that means we need to change, too.”

That immediacy was apparent on the day that I joined Sarah Schramm to check insect traps in a soybean field south of Darlington. We arrived in late morning, under a blanket of heavy gray clouds that were just beginning to burn off to reveal the high August sun. As we worked our way through the fields, a cacophony of life buzzed around us: grasshoppers, ladybugs and beetles of various colors and sizes. Schramm pointed out a western corn beetle on a nearby leaf, easily distinguishable with its yellow-and-brown striped wings. I wondered if the beetle was getting ready to lay eggs, a small, solitary act of rebellion against farm management practices that might trigger a series of reactions. To alter crop rotations. To treat or not to. The game is on, and it’s our move.

Comments are closed.