Corn Rootworm Remains a Lurking Threat

For the most part, there's good news about corn rootworm in 2019.

Wyffels Hybrids had an extensive rootworm-trapping network this last summer for adult rootworm beetles in order to assess next year's larval (root damaging) populations.

"We have not seen a huge spike in numbers," says Mitch Heisler, marketing manager for Wyffels Hybrids. "There are some hot pockets, including northwestern Illinois and eastern Iowa. But there is nothing drastic."

Still, corn rootworm isn't called the billion-dollar bug for nothing. There have been times when corn rootworm has been flat on its back only to return and shred corn roots and corn yields. It's estimated corn rootworm costs corn growers $1 billion annually.

Two Rebels

Up until the mid-1990s, farmers had a built-in rootworm control system with the corn-soybean rotation. The rotation's soybean component foiled any feast for corn rootworm larvae that hatched from eggs laid the previous year by adult female beetles.

Over time, though, corn rootworm nixed this strategy in two ways.

The western corn rootworm variant initially emerged in east-central Illinois and northern Indiana in the mid-1990s. That's when severe root injury and accompanying yield losses in first-year corn stunned farmers in that region. Mike Gray, a University of Illinois (U of I) Extension entomologist who's now a professor emeritus, recalled that farmers had unknowingly selected for a rootworm population where females laid at least a portion of their eggs on nearby soybean fields.

Some root-damaging larvae hatched out of these eggs on corn planted in the previous year's soybeans. In 2012, Gray noted U of I researchers found the variant western corn rootworm had three to four times more of a key digestive enzyme (cathepsin L-like protease) that enabled them to feed on soybean foliage longer and spend more time in soybean fields laying eggs.

In more northern areas, extended diapause occurs when eggs laid by predominant northern corn rootworm beetles remain dormant in soil through the next year when soybeans are planted. They then hatch the following year when corn is again planted in the field.

Some good news: The western corn rootworm does not show this characteristic.

Still, first-year corn in areas of Minnesota, Iowa, South Dakota, and Wisconsin have all been damaged by rootworms exhibiting extended diapause.

What to Do?

• Consider your cropping sequence, says Tim Dahl, a Syngenta agronomy service representative based in Minnesota. "If you are in a corn and soybean rotation and you haven't seen any extended diapause or any western corn rootworm variant, then you have a different management decision than if you are corn-on-corn and have heavy rootworm pressure," he says. In areas where extended diapause and the western corn variant aren't present, crop rotation is still a management tool, he adds.
• Apply rootworm control (whether in the form of a Bt hybrid or a soil insecticide) only where it is economically justified, say University of Illinois entomologists Nick Seiter and Joe Spencer, and Kelly Estes, state survey coordinator for the Illinois Cooperative Agricultural Pest Survey Program. Base this decision by sampling rootworm adults the previous year, they point out. For yellow sticky traps, the economic threshold is two rootworm beetles per trap per day in corn following corn. For rotated corn, the economic threshold is 1.5 western corn rootworm beetles per trap per day in soybeans. (These thresholds are based on a recent study in Iowa, which recalculated economic thresholds for corn rootworm based on updated crop values and control costs).
• If monitoring indicates that control is justified in corn, rotate control measures used from year to year, Seiter, Spencer, and Estes say. This means rotating among Bt hybrids with different trait combinations and non-Bt hybrids treated with a soil insecticide.
• Follow all refuge requirements for all Bt corn hybrids. In many cases, the refuge in a bag or RIB approach is now used, but check with your seed distributor on specific requirements for your hybrids, they advise.
• Monitor your control methods. "Checking your fields is the only way to know what condition they're in," says Mark Bernard, a New Richland, Minnesota, crop consultant.

Sometimes, rootworm damage isn't apparent. The U of I scientists point out that corn can withstand much root damage without lodging, depending on soil type and weather conditions.

"Insects feeding on a restricted root system can have a greater impact than those feeding on a healthy root system," says Sean Evans, technology development manager for Bayer Crop Science.

Numerous other factors besides rootworm damage can lead plants to lodge. The best way to assess rootworm damage is via root digs in late July. The U of I scientists recommend planting a small area or a portion of a row with a non-Bt/untreated hybrid as a check strip. This allows you to compare the efficacy of a management tactic vs. the background level of damage where no rootworm protection was used.