Rootworm Countdown

Beetle Numbers Indicate Need for Caution in 2018

Those who did root digs last summer have the best shot at predicting corn rootworm pressure next summer. (DTN photo by Pamela Smith)

Corn growers who noticed a rootworm beetle invasion last summer take note. It may mean a need for some management changes in 2018.

The Iowa Soybean Association (ISA) On-Farm Network placed sticky traps in 176 cornfields across Iowa to collect weekly adult corn rootworm beetle counts. With four traps per field, 50 to 60 beetles were collected in each field over the course of a week. According to Iowa State University, the threshold for CRW beetles is two beetles per trap per day.

Most of the fields in the trapping program had been using the same corn rootworm control methods for several years. "But that was not always the case," said Rich Stessman, ISA operations manager. "We also saw high numbers in fields that used a variety of practices, from rotation to traits to insecticides."

Higher rootworm numbers were expected based on conditions last winter and spring, but they didn't show up in the fields where they were anticipated, Stessman explained. "Fields with higher numbers did not necessarily correspond with trapping numbers we saw in 2015 and 2016. And we saw the most severe infestations in areas where we also had drought, meaning that larvae survival likely played a factor in the counts," he said.


Erin Hodgson, Iowa State University extension entomologist, told DTN this year's trapping was highly variable, with some fields seeing little to no activity on the sticky traps, while a field nearby would see a nearly filled card. "Not every field had the same amount of activity, even at a local level," she said.

The extended 2017 planting season may be part of the cause. "There tends to be more adults in late-planted fields because they are feeding on silks," Hodgson said. "But the adult beetles are mobile, so just because you see a lot of trappings in a field, that doesn't necessarily mean that you have rootworm injury."

While traps are a good way to identify beetle activity, they don't tell the whole story. Digging and assessing root injury in-season is the best way to evaluate if current rootworm products are holding up, Hodgson noted.

"Seeing adult beetles in a field would more likely point to possible increased numbers next year, because the beetles are laying eggs," she added.

Joseph Spencer, University of Illinois research program leader in insect behavior, also urges growers not to jump to conclusions. "It is possible to have a rootworm beetle population and not have significant economic injury," he said.

Illinois research from the 1980s showed that use of soil-applied insecticide (SAI) to protect corn roots can actually result in higher rootworm populations than observed on unprotected corn roots. "By protecting the area around the seed, the roots have a chance to grow and proliferate so that by the time they extend beyond the protected zone, the mass of roots available to feeding larvae is greater than what is able to grow on an unprotected root. Thus, the SAI-treated plants can produce more beetles than untreated, they have good root protection and don't lodge," Spencer said.

Spencer said the first choice for growers who saw Western corn rootworm pressure last year should be to rotate to soybeans. "I'd then suggest follow-up beetle monitoring in soybeans using sticky traps (2 CRW/trap/day threshold) to determine whether soil insecticide would be needed on first-year non-Bt corn," Spencer said.

"If sticky trap rootworm counts in rotated corn indicate a potential problem, then it might be a good time to use a pyramided Bt hybrid that expresses Bt traits that have worked on your farm or go back to soybeans," he added.


"We are now working with partners [in Iowa] to continue the On-Farm Network trapping survey into 2018," Stessman said. "And the more data that we can collect, the more information we have available to help producers make more informed management decisions." For more information contact the Iowa Soybean Association at or

The Handy Bt Trait Table can help sort through traits while making seed selections. Find it here

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