Warning: Western Corn Rootworms Had an Easy Winter – DTN

Western Corn Rootworm Photo: Purdue University

After a tough couple of seasons, western corn rootworm populations are heading into the spring with high hopes.

“It’s a rebuilding year for Team Rootworm,” said Joe Spencer, an entomologist with the Illinois Natural History Survey.

Heavy rains decimated rootworm populations during their egg hatch period in 2015. Farmers have received a reprieve on rootworm control ever since, entomologists told DTN.

But rootworm populations edged higher in 2016, and most eggs probably survived the winter, Monsanto technology development manager Sean Evans said.

“Last fall, we started to see an uptick in adult rootworm numbers in areas where we traditionally have a lot of rootworm pressure: northeastern Iowa, southern Minnesota, northern Iowa, central Illinois and eastern Nebraska,” Evans said.


Performance problems with Bt traits have seen a marked decrease since the Great Rootworm Drowning of June 2015, Evans told DTN.

In addition to weather, increased crop rotation and greater adoption of pyramided corn hybrids have also probably suppressed their populations the last two years, Evans said. (Pyramided hybrids contain more than one Bt trait targeting the rootworm).

This year, Mother Nature gave the rootworm a winter reprieve, however.

Typically, frigid temperatures and the cycle of thawing and freezing will destroy a portion of the egg population buried in corn fields. Most Midwesterners didn’t see those conditions this winter.

“This is probably a winter where many rootworms survived,” Spencer said. “But we also know we went into the winter with low populations.”


Due to warmer-than-average temperatures this spring, egg hatch could be a little earlier than normal in Illinois this year, Spencer said. Rootworm eggs usually hatch around May 31 and June 1.

The weather during egg hatch will be the biggest factor in determining rootworm populations this summer, Evans and Spencer agreed.

Not only can plentiful summer rains drown emerging larvae, but plants are better able to cope with root feeding in favorable moisture conditions, Evans said.

“With rootworm pressure, if we have really dry conditions, then corn roots are at a premium,” he pointed out. “When you have drought stress present, that’s really when we start to see the benefit of [Bt] traits from a yield standpoint.”


Low rootworm pressure might sound like a good reason to pass on digging roots, right? Not at all, Spencer and Evans said.

Assessing root damage can help a grower decide if rootworm pressure even warrants the use of pricey Bt pyramids next year, Spencer said.

“If we turn the clock back 20 years, and farmers had to decide on soil insecticide when facing historically low populations, it would be a no brainer not to apply,” he said.

Now that rootworm control is built into corn hybrids, it can be harder for growers to figure out how to scale back on control costs. Use the Handy Bt Trait Table from Michigan State to figure out which Bt proteins you are using.

On the flip side, cutting Bt traits without scouting is a risky step, too, Evans said.

“Are you doing it based on scouting information you have about your operation or just sort of rolling the dice?” he asked. “If you have scouting information — no beetles, no issues with dug roots — then that’s a great way to make a decision, but how many folks actually have that information to make those choices?”

Monsanto has designed a website called Trait Answers to help growers sort through just these types of Bt trait decisions, Evans said. See it here.

Emily Unglesbee can be reached at Emily.Unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

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