Experts across the Corn Belt are warning farmers about more documented cases of resistance to Bt corn by the western corn rootworm (WCR).

A cold, wet winter and late corn planting in some Northern states could spell trouble for many cornfields, especially those planted with single-trait Bt corn year after year.

Of the four Bt proteins registered in the U.S. for corn — Cry3Bb1, Cry34/35Ab1, mCry3A and eCry3.1Ab — field-evolved resistance has been documented for both Cry3Bb1 and mCry3A traits, and cross-resistance has also been confirmed between those two traits.

Cry3Bb1 is the protein found in YieldGard VT3 and YieldGard VT3 Pro hybrids, while mCry3A is the protein used in some Agrisure hybrids.

Five Illinois counties have reported field-evolved WCR resistance to Cry3Bb1 corn, and researchers in Nebraska, Minnesota, Wisconsin and Iowa have also reported WCR resistance issues.

During a recent webinar, several university experts shared tips on how farmers can improve long-term management of rootworms and keep resistance at bay.

Evolution Of Resistance 

Diversified techniques are needed to manage corn rootworms effectively while reducing the risk of resistance developing. But that’s a difficult task due to the strong adaptation ability of WCRs, says Iowa State University entomologist Aaron Gassmann.

Gassmann says his laboratory began studying Bt resistance in corn rootworms in 2009 after getting complaints from farmers about fields with severe larval feed injury to Bt corn.

His research group began by studying the effectiveness of Cry3Bb1 (YieldGard VT3 and VT3Pro); Cry34/35Ab1 (Herculex); and mCry3A (Agrisure).

Researchers visited problem fields and sampled WCR adults, bringing them back to the lab to obtain eggs and use bioassays to check survival on Bt and non-Bt corn.

The researchers also examined larvae survival rates by comparing problem fields — with a history of continuous corn planted to a single Cry3Bb1 hybrid — to control fields with a corn-soybean rotation and a variety of transgenics and insecticides used.

They rated root injury on a scale of 0 to 3 by examining damage to the nodes of corn roots. A node is a ring of roots around the base of corn plants, and each node of injury represents a 15% to 17% reduction in yield.

Researchers that year found higher rates of larvae survival for corn with Cry3Bb1 traits than with the other traits or non-Bt corn. The more consecutive years farmers grew Cry3Bb1 corn, the more resistant some populations were, Gassmann says. Corn with the Cry34/35Ab1 trait showed no resistance or cross-resistance with other proteins.

In 2011, WCRs from problem fields planted to either Cry3Bb1 corn or mCry3A corn were compared by researchers to strains of WCR that hadn’t been exposed to Bt strains. These problem fields had a history of 6 years of continuous corn, on average, and 3.6 years of either Cry3Bb1 corn or mCry3A corn, on average.

They found that these populations were resistant to both Cry3Bb1 corn and mCry3A corn, and there was cross-resistance between the traits. But there was no cross-resistance between Cry34/35Ab1 and either Cry3Bb1 or mCry3A.

In 2012 and 2013, researchers took 3 million eggs obtained from a USDA-ARS lab in South Dakota and artificially infested research plots, then compared these fields to grower fields with history of injury on Cry3Bb1 corn.

They found much higher rates of survival and root injury in fields planted to Cry3Bb1 and mCry3A in the grower fields compared with control plots. This illustrates the need for growers to rotate tactics to manage rootworm larvae instead of layering them, Gassmann says.

“Susceptibility of corn rootworms to these tools can be a non-renewable resource,” Gassmann says. “Use a pyramided Bt event with more than one toxin, rotate Bt events, try crop rotation and plant some non-Bt corn with a soil insecticide.

“If you’re in continuous corn, monitor the performance of the strategy by digging up the roots and rating the root injury,” he adds. “If there are a high number of adult beetles in a field planted to a Bt hybrid, your management tactics aren’t working.”

Controlling Adults 

One tool to help reduce corn rootworm damage in future crops is spraying foliar insecticides to reduce populations of adult beetles.

But growers must understand the biology and behavior of beetles to optimally use adult control, says University of Nebraska entomologist Lance Meinke.

Corn rootworm beetles are strongly attracted to corn pollen, silks and young ear tissues as feeding sites. Depending on the population density of beetles and environmental conditions, beetle feeding can lead to high levels of silk clipping, which may impact yields if it happens during pollination, Meinke says.

Late-planted and late-pollinating cornfields surrounded by fields that were planted on time may also become a magnet for beetles and silk-clipping problems.

The first step with adult control, Meinke says, is to monitor fields and determine if enough beetles are feeding on plants to justify an insecticide application.

Often adult control to prevent silk clipping isn’t justified in commercial cornfields, he says, unless growers anticipate having a late-planted crop or high beetle populations that may get ahead of corn.

Meinke says thresholds commonly used to determine if adult control is needed are up to 20 beetles per plant in commercial corn, or silks clipped back to within ½ inch of the husk prior to 25% to 50% pollen shed.

To control beetles, foliar applications of insecticides can be effective, but he notes there’s a limited arsenal of insecticides available. Pyrethroid insecticides are a popular choice, and some adjuvants containing rootworm-feeding stimulants can be tankmixed with insecticides for greater beetle control.

“Most products will knock populations down so pollination can be completed. After that, corn rootworms can eat external silks without negatively impacting yield,” Meinke says.

Time Your Spraying

The goal with spraying insecticides after beetle emergence is to reduce female beetle populations so egg laying is reduced and management plans work better the following season, Meinke says.

Understanding beetle behavior and development, and the factors that influence it, are important because spraying at the wrong time may result in mostly males being killed and long-term control being ineffective, Meinke says.

Grower choices at planting can have an effect on beetle emergence patterns. Meinke says there’s some evidence that sub-lethal exposure to Bt corn delays development of corn rootworms at each growth stage.

The mechanism ins’t clearly understood, but the repellent effect of some Bt hybrids can change larval feeding behavior and possibly contribute to delays in development. Adding an insecticide on top of planting Bt traits may cause additional delays in beetle emergence and push emergence curves closer to fall in some years.

Variability in egg-hatch dates can also impact when adults will be present in a field. Thirty years of data collected by Purdue University shows rootworm eggs begin hatching anywhere from early May to mid-June, although early June is the typical timeframe.

Last year, weather was cooler and beetles were emerging all the way into September, Meinke says.

“When you see beetles with a distended abdomen and they’re becoming gravid, the abdomen can be squeezed to see if eggs have developed,” he says. “They have to feed on high-quality food sources like corn pollen, silks or tassels, and then it’s 10 to 14 days before females can lay eggs.

“If you’re trying to get by with one insecticide application to lower populations for the next year, target your spray window after you start seeing initial females that can lay eggs. A week or two after is the best time to try to spray.”

Caution On Insecticides

Granular or liquid insecticides applied in a band or in furrow at planting can improve control of corn rootworms.

But there are limitations with these products that no-tillers should be aware of, says University of Nebraska Extension entomologist Bob Wright.

Insecticides will protect the central root system to reduce lodging and yield loss, but they may not necessarily control a high percentage of larvae because the entire root zone may not be protected, Wright says.

Early corn planting can also challenge the efficacy of insecticides due to the longer period the products must work, he says.

In his university trials, Wright says he hasn’t seen a large performance gap between banded or in-furrow insecticide applications, but if a soil insecticide is needed to target other insects like cutworms, banding may work best.

Wright says trials of liquid and granular insecticides in Nebraska haven’t shown a big winner in terms of yield bumps, although granular products seem to work better with heavy rootworm pressure.

Wright feels insecticide applications used in combination with Bt corn are most profitable in specific fields where damage has occurred in the past with treated corn or where high rootworm densities are expected.

The Nebraska trials found that fields with the Cry3Bb1 trait and an insecticide such as Aztec greatly reduced corn injury, but where insecticide was added to corn with an effective pyramid of Bt genes, the extra product wasn’t always needed.