Scientists Discover How Farmers Can Fight Pests With Crops

A new study shows that varying the nutrient levels in a monoculture can reduce the need for pesticides.
Rows of various lettuces. (Photo: Alvis Upitis/Getty Images)
Oct 16, 2016· 2 MIN READ
Tove Danovich is a journalist based in Portland, Oregon.

It’s easy to spend a lot of time thinking about how to get a balanced diet. There are almost certainly some fruits and vegetables we’re neglecting to eat enough of, not to mention finding a healthy balance of fiber, sugar, and salt. These problems don’t exist for most insects—and not just because they don’t shop at grocery stores. Most are so-called specialists, meaning they’re healthiest munching on one plant species over and over again.

This tendency—which can lead to the devastation of farmland planted with one crop over many acres—is one of the reasons agroecologists have championed the idea of intercropping and polycultures. Not only can having a variety of plants growing on one farm reduce pest problems, but it can also improve soil health and yields. Yet on large mechanized farms that rely on specialized machines, such an approach to agriculture isn’t practical. To increase diversity, some farmers plant strips of wildflowers or weeds along the edges of their fields to act as a barrier for “bad” pests and an attractive food source for beneficial insects. But a new study published Wednesday in Nature has found that farms might be equally well served by growing a slightly different type of monoculture made up of different varieties of the same crop that have varying nutrient levels.

“For a long time, scientists and farmers have noticed that when farms have greater plant diversity, they have fewer problems with insect pests,” said Michigan State University’s William Wetzel, one of the study’s authors. He compared monocultures with sticking an insect “inside a jelly doughnut,” where it can eat to its heart’s delight. To combat this effect, farmers have often found boosting biodiversity with flowers and other nonedible plants to be the most practical solution—but increasing diversity around the edges of a farm can only do so much. Ultimately, farmers want to grow plants they can sell.

Wetzel explained that while the phenomenon of plant diversity leading to fewer pests is recognized, there hadn’t been as much research on the physiological reasons why diversity could influence insect populations. “What we found was that all these insects have a very specific nutrient level on which they thrive, grow fast, and have high survival,” Wetzel said. “When they get the wrong level of nutrients, they do very poorly.”

This might seem obvious—eating junk food all day isn’t as good for people as eating a balanced diet either. What was surprising was just how badly insects did on a non-optimal diet. “A key result from our study was that the negative of eating plants with the wrong nutrient level outweighs the benefit of eating plants that are the right nutrient level,” Wetzel explained. Putting plants with the “wrong” nutrient levels into a field can be actively harmful to pests. Most insect species have relatively similar nutritional needs, which makes it easy to target harmful insects.

“What our study suggests is that you don’t have to add new non-crop species to increase the diversity pool on the farm,” said Wetzel. “You can just increase the diversity within one crop.” What this means is that the problems monocultures cause aren’t from a farmer growing corn—it’s that each stalk of corn is a genetic replica of the others. A 2000 study published in Nature suggested this solution could work for noninsect problems too, with the authors noting that “crop heterogeneity is a possible solution to the vulnerability of monocultured crops to disease.” The researchers found that when they planted disease-susceptible and -resistant rice varieties together, yields were 89 percent higher, and levels of fungus were “94 percent less severe than when they were grown in monoculture.” After the two-year test program was finished, farmers no longer needed to apply fungicidal sprays.

Yet, thus far, varying crops by genetics rather than species has not become popular with farmers, at least not in the United States. While many researchers are focused on big-picture diversity—planting different crops on a plot of land each year, polyculture, or crop biodiversity—there’s little mention of the benefits of tweaking various members of a crop just enough that insects no longer see monocultures as an endless buffet.