Corn rootworm scouting, management strategies

GENESEO, Ill. — Corn rootworm management is no longer simply a “plug and play” strategy.

“It’s to the point where we can’t just rely on broad-scale recommendations. We need to take our operations and manage them on a field-by-field basis,” said Eric Wilson, Wyffels Hybrids agronomy manager, in a recent webinar.

“In order to do that we have to know our corn rootworm pressure. We can’t just assume that we know what’s going on out there. We’ve got to get traps out, do root digs to verify that trait performance, because in a lot of instances it’s just not holding up to those expectations that we have of those trait platforms.

“You need to assume at some level that there is resistance in your field, and you want to think about that going forward as we introduce new technologies, new trait technologies especially, for CRW management.

“We want to manage those for longevity. We want to keep those around and operating to the best of their ability. We want manage those as well as we can for the future.”

Species Comparisons

Western CRW variant lays eggs in soybean fields. It tends to be the dominant species in continuous corn and has known resistance to all of the Bt traits for CRW that are currently in the market.

Northern CRW can exhibit extended diapause where a portion of its eggs stay dormant during the soybean years and is very difficult to predict. Traditionally that’s been about 40% to 50% of that population.

“In some of our conversations with universities, in reality the number that stays dormant is probably increasing just because how used to that rotation they have got, but it’s very difficult to predict,” Wilson said.

“Just because you’ve had extended diapause on your fields before doesn’t necessarily guarantee that you’re going to see it this year or in the future. The reason it’s difficult to predict is you’re really trying to go over a span of two years — two growing seasons and two winters that are affecting that population.”

Northern CRW is typically seen in first-year corn, which was especially the case in 2021, and there are populations in the Midwest that have known resistance to all of the Bt traits in the market.

Widespread

CRW pressure is increasing, and it’s no longer a localized problem.

Wilson said CRW beetle pressure, based on traps, increase yearly from 2019 to 2021, particularly in continuous corn, but also in corn after soybeans.

The increased pressure in 2021 was attributed to drought conditions that began in 2020.

“We were very dry in the region of northern Iowa that I cover. That allows very good survival of CRW larvae to feed on the roots, reproduce and lay more eggs going into 2021. So, it was really no surprise that we saw increased pressure,” Wilson said.

“In central Illinois we saw some of the lowest pressure in 2021, and that was tied to the above-departure seasonal rainfall in June of 2021. The peak egg hatch is late May to early June. If we get heavy rainfall during that period, what it’s effectively doing is drowning a lot of the larvae out so they can’t survive and that coincides to where we saw the lowest pressure in 2021.”

Winter Kill?

Western CRW egg hatch starts to become affected when soil temperatures go below 19.4 degrees.

According to data provided by Bruce Potter, University of Minnesota integrated pest management specialist, researchers have found at a soil temperature of 14 degrees, the western CRW mortality is 47% to 50% in one week, 77% mortality after two weeks and 100% after four weeks. The mortality of western corn rootworm is 100% at 0.5 degrees.

Northern CRW eggs are more tolerant to cold.

At a soil temperature of 0 degrees, northern corn rootworm has 50% mortality and increases to 90% at 6.7 degrees.

“We’re not so much concerned with air temperature. Soil temperature is not always going to line up with air temperature,” Wilson noted.

CRW generally lay their eggs in moisture and have been documented to lay eggs 12 inches deep. The deeper the egg deposition, the more buffered they are to colder temperatures, therefore increasing survivability.

Management

“The question has been asked is if it would be OK to going to a Double Pro with a good quality soil-applied insecticide in continuous corn. Data in trait and insecticide trials in moderate to heavy pressure last year at the Urbana site supports that,” Wilson said.

He noted Wyffels will be conducting additional field trials of its new SmartStax Pro and will be introducing it in 2023.

“SmartStax Pro is a very good trait and we expect it to be highly effected at killing CRW larvae and to significantly reduce corn rootworm populations,” he said.

“However, it does work slightly different than our current Bt traits. Current Bt traits poison corn rootworm while SmartStax Pro is essentially starving the larvae to death, which requires time. So, some root feeding will occur, especially in environments with high corn rootworm populations, but overall we do expect to see a lot less root injury damage on these SmartStax Pro hybrids.”

Heavy Pressure

According to Wyffels trials, a soil-applied insecticide combined with a Bt trait provides better residual protection than relying on seed-coated insecticides in fields with high CRW pressure.

“The performance of dry insecticides is consistently better than liquid formulations across research sources,” Wilson said.

“A good quality soil-applied insecticide on average provides 60 days of residual. So, you have to think about how that coincides with primary egg hatch from the end of May into early June, especially as it pertains to early planted corn.”

Management strategy plans begin the previous year.

“We’ve got to know our rootworm pressure levels in order to be confident with management recommendations. I know putting out traps is a lot of work, it’s usually the hottest time of the year with the most humidity, but it answers a lot of questions for us, especially in terms of what we’re going to be doing that next year,” Wilson said.