Production Blog

Can Rootworms Do The Backstroke?

Pam Smith
By  Pam Smith , Crops Technology Editor
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Root digs help assess potential rootworm pressure and are used to predict how to manage the pest in the coming year. Monanto agronomist Dave Shenaut and his crew assess plots in central Illinois. (DTN photo by Pamela Smith)

WARRENSBURG, Ill. (DTN) -- Dave Shenaut has learned never to underestimate the western corn rootworm (WCR). The early rains that plagued most of the Midwest this summer should have washed away most of the pests, but the Monsanto agronomist is still finding adult beetles this fall.

"I've seen fewer adult beetles, but they are still here and will cause issues for growers if we don't continue to manage them," said Shenaut.

University of Illinois entomologist Mike Gray said in a recent issue of The Bulletin (…) that summer surveys found WCR densities far below average in Illinois corn and soybean fields. However, he also noted that the survey data doesn't necessarily mean the incidence will be lower in fields next year.

"Keep in mind there are no effective rescue treatments available for corn rootworms (larval feeding) in most field situations across Illinois," Gray wrote.

"Secondly, corn rootworm management decisions need to be made on a field-by-field basis. Unless you've scouted your fields this season, don't assume that western corn rootworm densities in your field mirror those described as regional averages in this article," he said.


Shenaut said two of the three rootworm plots he planted this summer had significant pressure in the untreated check plots. "The damage trend was similar to what we've seen the past several years," he added. "Single-trait products didn't perform as well as stacked (or pyramided) products such as SmartStax. Control with single-trait products was generally improved with the addition of a soil-applied insecticide at planting."

Preston Schrader, Insect Development Representative for Monsanto, agreed that overall rootworm densities are lower. "But given the saturated soils in June, I'm surprised at the adult numbers I have observed. I'm seeing more beetles later," said Schrader.

The rootworm question is important as growers head into seed selection time for the coming year. "Recommendations for specific fields should be based on scouting individual fields, not region-wide observations," Schrader added. "Once again we have lower populations, but there are enough areas with adults flying around that I can't recommend reducing management recommendations for corn rootworm unless a field was adequately scouted." He notes the importance of scouting soybeans for the pest too -- particularly in areas where rotational-resistant rootworm has become an issue.

He also reminds growers that reduced management plans can lead to heavy corn rootworm densities the following year since each female beetle can lay 500 to 1,000 eggs. "I have some concern regarding replant acres as they remain green longer and adult corn rootworm will seek out these areas late in the growing season, which can result in elevated pressure next year," he added.

An alphabet soup of trait stacks and pyramids meets growers as they contemplate seed catalogs. That complexity can be daunting as growers attempt to limit costs and match needed traits to actual problems. A good reference to sort through those traits is the Handy Bt Trait Table (…). The chart can also help double check insect refuge requirements.

In Gray's studies across four sampling regions of Illinois (northwest, northeast, west-central and east-central), 81% of the leaf samples tested positive for the expression of at least one Cry (Bt) protein targeted at corn rootworms. Of these positive leaf samples, 75% expressed more than one Cry (Bt) protein for corn rootworms and are often referred to as pyramided hybrids. In southeastern and southwestern Illinois, only 25% of leaf samples tested positive for the expression of a corn rootworm Cry (Bt) protein.

The use of pyramided Bt rootworm hybrids is taking a toll on corn rootworm larval survival. The use of planting-time soil insecticide on top of Bt hybrids is also having an effect in overall suppression of the corn rootworm population in Illinois, Gray noted in his report. Heavy spring rains that led to saturated soils at the time of larval hatch (late May and early June) took a toll too.

All of those factors don't rule out the need to continue to factor this pest into management plans. Reports of lodging continue to mount as combines roll. Shallow roots from prolonged wet weather throughout much of the Corn Belt are contributing to stalk lodging. However, it's important for seed selection to know if that lodging is occurring because of hybrid differences, weather or the increasingly wily rootworm. For a good image gallery to help sort out late-season lodging differences, go to….

Pamela Smith can be reached at



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