An emerging threat

Kevin Moore hoped he was wrong.

In July 2016, the MFA Crop-Trak consultant found an excessive number of northern corn rootworm beetles in a northwest Missouri field. They weren’t supposed to be there. The corn field had been planted in soybeans the previous year, and crop rotation is traditionally the cure for rootworm infestations.

“It was definitely cause for alarm,” Moore said. “Rootworm is usually only an issue in continuous corn. If the rootworm beetles lay their eggs in a corn field that will be rotated, they don’t have anything to eat when it’s planted in soybeans the next year, and they die.”

His discovery indicated otherwise. Moore suspected a phenomenon called “extended diapause,” a genetic adaptation that allows rootworm eggs to survive through two winters. When this occurs, the rootworm beetles lay eggs during a corn season, those eggs lie dormant in the subsequent soybean crop and then hatch in the next corn rotation. They feed on corn roots as larvae and emerge as adult beetles, and the cycle starts again. Left unchecked, rootworms can cause substantial yield loss and standability issues.

“Extended diapause rootworms have been found to the north and west of us, but it’s never been confirmed in Missouri until now,” Moore said. “Adult beetles can fly, so we believe some of those with that genetic trait have moved this way.”

Two years later, his suspicions have been validated through the work of MFA’s Agronomy team in partnership with the USDA Agricultural Research Service.

“We collected 4,000 beetles, brought them back to the lab, fed them well and gave them a place to lay eggs,” said Bruce Hibbard, USDA-ARS research entomologist in Columbia, Mo. “We gradually cooled down those eggs over time and over-wintered them at 47 degrees Fahrenheit—just a little bit warmer than refrigerator temperature. It’s genetically pre-determined that they have to experience cold before they can hatch.”

A typical diapause, or period before eggs hatch, is one winter. In an extended-diapause situation, rootworm eggs must endure two or more winters before hatching. After the eggs went through one lab-simulated “winter,” 44 percent hatched. The remaining eggs were subjected to a second “winter” in 2017, with a hatch of 12.4 percent. Hibbard said those results prove that a significant number of the sampled beetles exhibited an extended-diapause trait.

“The genetics are complex, so it doesn’t mean that every egg will have this trait,” he explained, “but enough eggs hatched after two winters to confirm that this problem is here, at least in the location where we collected those beetles.”

Now that those initial corn fields have been rotated through soybeans and back to corn, Moore and other Crop-Trak consultants are methodically scouting this summer to see if there are more rootworms with the extended-diapause trait. They’re focusing on Atchison, Nodaway, Holt and Worth counties in Missouri as well as Page County, Iowa.

“We’re taking root samples in the areas that had a large amount of beetles two years ago, and then we’re also taking samples from bordering counties to track their movement,” Moore explained. “We also set up traps for adult beetles to get a better understanding of their density and movement. It’s something we need to keep monitoring so we can help protect our crops. If we see numbers that are alarming, we’re certainly going to let growers know that they need to take action.”

In samples taken this summer, Moore and the Crop-Trak team have additional evidence of extended diapause in northern corn rootworms. MFA and USDA are working together to confirm that assumption and determine the severity of the problem.

“If our lab work shows higher percentage of eggs start hatching after two winters, then Missouri corn growers are going to have to consider managing for northern corn rootworm,” Hibbard said. “With 12.4 percent, maybe they can tolerate what those rootworms eat and not have to do anything. But if they start to see their fields laying flat, they’re going to have to treat all their corn in the future—but maybe not yet.”  

If crop rotation is no longer effective against corn rootworm, there are other options, said MFA Senior Staff Agronomist Jason Worthington. However, there is no “rescue” treatment.

“The most effective plans must be proactively developed along with hybrid selection,” he said. “The two real options a grower has to control rootworm are Bt corn traits with multiple effective proteins such as SmartStax, and granular in-furrow soil insecticides such as Force or Aztec. Both of these control options require planning ahead.”

Hybrids with Bt traits utilize a gene from naturally occurring soil bacteria, Bacillus thuringiensis, that kills larvae of rootworm. Just like weeds have developed resistance to herbicides, however, the western corn rootworm has developed resistance to the Bt trait, which was introduced in 2003. MFA agronomists are watching for pests that exhibit Bt tolerance in the field, and Hibbard and his team are testing the rootworm beetle samples for susceptibility to Bt in the USDA-ARS lab.

“We suspect that Bt resistance to northern corn rootworm doesn’t exist in Missouri, but we want to document whether that’s the case or not,” Hibbard said.

For growers in Missouri and adjoining states, extended diapause is a daunting discovery. Corn rootworm is the most damaging corn pest in North America. Before the adoption of Bt corn, the USDA estimates growers spent an estimated $200 million in control measures and suffered $800 million in lost yield. Until now, Missouri growers have seen fewer losses from rootworm because of the prevalence of rotational practices.

This summer’s sampling shows that extended-diapause rootworms have not yet reached an economically damaging threshold, Worthington said, but he and his Crop-Trak team remain vigilant and encourage growers to do the same.

“The good news is that it still looks like it’s not to the level where we have to change our practices, but it’s something we need to take seriously,” Worthington said. “If the extended-diapause population continues to increase, rootworms will have to become a focus of our integrated pest management.”