Plant diversity could be a natural repellent for crop pests

Wheatfield_vertical_blogScientists and farmers have long been puzzled over why a field with a variety of plants seems to attract fewer plant-eating insects than farm land with just one type of crop. Now, a new study is shedding light on this interaction and shows that much of the discrepancy may have to do with the nutritional needs of insects. Returning plant diversity to farmland could be a key step toward sustainable pest control.

After studying 53 species of insects, researchers from Michigan State University found that bugs have narrow ranges of nutrient levels where they flourish. If the plants being fed on are too nutrient rich or poor, the insects are less likely to thrive. Bugs surrounded by diverse plants, however, are harmed much more by low-quality plants with the wrong nutrient levels than they are benefited by high-quality plants with high nutrient levels.

“Insects have a perfect nutrient level that they really like,” William Wetzel, Michigan State University entomologist and the study’s lead author, said in an article. “When it’s too high or too low, they do poorly.”

The problem with monocultures, Wetzel explains, is if an insect likes the crop, that insect has a large food supply to draw from all in one place. Conversely, a field containing a variety of plants does not offer a large block of food for the insect, so it will not get the nutrients it needs to survive and thrive.

“A monoculture is like a buffet for plant-eating insects where every dish is delicious,” Wetzel said. “A variable crop is like a buffet where every other dish is nasty.”

Many small farms around the world already include a diverse mixture of plants. But in most monocultures, the plants are bred to be as identical as possible. How can larger growers introduce more diversity while maintaining their same level of production? Well, according to the research, crop varieties could be bred with variable nutrient levels in the parts eaten by insects, for example, the leaves or roots, while the parts ate by consumers could be consistent. Or farmers could plant new mixtures of crop varieties or genotypes that differ in nutrient levels.

Next, Wetzel said he intends to take this research to the next level by using modern genetic resources to develop a model system for manipulating plant trait diversity in field populations and measuring the effects on insect populations and plant damage. Wetzel said this sort of genotype mixing for plants is already being done on some rice and wheat fields to reduce the spread of disease among the crops.

“So far people haven’t done that in ways to reduce insects,” Wetzel said. “But it shows that it’s possible to mix varieties and genotypes together. Now, we need to think about how to do that to control insects.”

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