Western corn rootworm behavior in soybeans offers clues to understanding rotation resistance

CHAMPAIGN, Ill. — Illinois farmers’ concerns about increasing western corn rootworm populations and plant damage in rotated cornfields have University of Illinois researchers taking a closer look at how rootworm diets affect the beetles’ flights from soybeans to cornfields.

A corn-soybean crop rotation has long been a technique used for corn rootworm control because larvae emerging in soybean fields can’t survive. With increasing resistance to the toxins expressed in Bt corn hybrids in the past few years, rootworms are getting out of hand again, especially in northern Illinois. Rotating a field to soybean is the best way to knock down a high rootworm population.

“We are back to where we started before commercialization of Bt corn,” said Joseph Spencer, insect behaviorist at the Illinois Natural History Survey. “Egg laying in soybean fields by crop rotation-resistant rootworm beetles was one of the drivers behind Bt corn commercialization. Before Bt, growers had few options to protect rotated cornfields from rootworm larvae emerging from eggs laid in the soil when the fields were planted to soybean. Now that we’re nearing the end of the Bt era, the study of egg-laying rootworm beetle movement in and out of soybean has become relevant again.”

In previous studies conducted in the late 1990s, Spencer and colleagues collected feeding female western corn rootworm beetles from soybeans and cornfields to study the effects of the two diets.

As expected, the soybean-feeding beetles died sooner than beetles feeding on a corn diet, 5.05 versus 6.26 days. However, they found the ill effects of a soybean diet could be minimized if beetles later ate corn tissues.

Recent findings from that study showed that beetles fed soybean tissue were more active and likely to take flight in a wind tunnel than the corn-fed beetles.

“The nutritionally poor soybean diets made the beetles more active in their cages and more likely to lay eggs because they are under nutritional stress,” Spencer said. “As a result, they are more likely to fly. In Illinois, if they start flying, they’re eventually going to run into a cornfield where they can recover from the poor diet.”

Corn rootworms in rotated corn systems are “rewarded” when they lay their eggs in soybean fields because when the larvae emerge the next year, the soybean field will have likely been rotated to corn, and the next generation of corn rootworms will find the corn roots they need to survive.

In addition, rotation-resistant beetles have developed soybean-adapted gut microbiota and physiological adaptations so they can stay in soybeans for longer periods before suffering ill effects. Those that are poorly adapted are likely to respond faster to negative effects of the diet and will fly off in search of corn faster.

If the adverse effects from a soybean diet cause beetles to fly back to cornfields, then studying beetle activity in soybeans may be the key in understanding the behavioral mechanism of corn rootworm rotation resistance.

“The options for farmers are dwindling,” Spencer said. “Bt corn resistance is leaving us with an insect population that is difficult to control. Dealing with rotation-resistant rootworms again may add insult to injury.”