A new study answers a question that has baffled researchers for more than 15 years: How does the western corn rootworm – an insect that thrives on corn but dies on soybeans – persist in fields that alternate between corn and soybeans? The answer, researchers say, has to do with enzyme production in the rootworm gut. Their findings are described in a paper in Ecology and Evolution.

Crop rotation declined in the middle of the 20th century as the use of insecticides and fertilizers expanded in the U.S. Then in the 1950s and 1960s, when some insecticides began to fail, growers again turned to crop rotation to kill off the rootworms that fed on corn. The method was effective for decades, but by 1995 some growers started seeing rootworm damage even in rotated fields. Today rotation-resistant rootworms are widespread in the Midwest Corn Belt, where corn and soybeans dominate the landscape.

Crop rotation in east-central Illinois imposed intense selection pressure on rootworms, a key to the emergence of insect resistance to crop rotation, says University of Illinois Crop Sciences Professor Manfredo Seufferheld, who led the new study.

“In Champaign County, Illinois, where you see a lot of rotation-resistant rootworms, 84% of the total land area is corn or soybeans,” he says. “But as you go to Missouri, which has only wild-type (non-resistant) rootworms, almost 50% of the land area is not corn or soy.”

Rootworm larvae live on the roots of corn plants, so it makes no sense for a rootworm beetle to deposit its eggs in a soybean field, Seufferheld says. “But with crop rotation, we’re making special conditions that allow those crazy insects to survive.”

Previous studies focused primarily on the behavioral changes that led rootworm beetles into soybean fields, but Seufferheld and his colleagues focused instead on the rootworm gut.

Their focus was prompted by observations made by Jorge Zavala, a former postdoctoral student at Illinois and a co-author on this work. Zavala, now a visiting scholar at Illinois from the University of Buenos Aires, knew from previous research that levels of protein-degrading enzymes in the insect gut, called proteinases, rise and fall in response to chemical defenses in soybean leaves. He saw that rotation-resistant rootworms survived longer on soybeans and inflicted more damage on soybean leaves than their non-resistant peers. He also detected differences in levels of proteinases in rotation-resistant and non-resistant (wild-type) rootworms.