Discovery of root feeding and lodging in first-year Cry3Bb1-protected corn in central and east-central Illinois is only one of several scenarios adding to the challenge of corn rootworm (CRW) control in areas of the Corn Belt. The increasing complexities are leading more growers to consider a layered approach.

"We started having problems with rotation-resistant western corn rootworms in the mid-1990s," recalls Michael Gray, Illinois Extension entomologist. "The first response was to ramp up the use of soil insecticides. With the introduction of Cry3Bb1, growers began to transition away from insecticides, but we advised to continue rotating as a second line of defense. Now it appears, or is suspicious, that we have a subset of the CRW that is not only rotation-resistant, but also resistant to the Cry3Bb1 protein."

Gray is quick to point out that using an insecticide overlay will not delay resistance development, and some research suggests it may speed it. Unfortunately, maximizing profits for the short term may lead to more problems long term.

The best way to delay resistance among rootworms is to “more fully embrace integrated pest management, including scouting," says Gray. "Use soil insecticides in combination with Bt and non-Bt hybrids. Mix it up from time to time. Be careful how we deploy these tools."

Steve Pitstick agrees with Gray on the importance of scouting, and he certainly "mixes it up." His fields have alternated between continuous corn in the past to mostly 50:50 corn/soybean rotation in the late 1990s and early 2000s, and back to mostly continuous corn since the introduction of Bt traits. This year the Maple Park, Ill., corn and soybean grower added pyramided Bt hybrids with multiple CRW Bt traits, to an every-other-year rotation of the three available single CRW traits on his 2,600 acres.

After running some plots in 2012, Pitstick also applied Aztec soil insecticide as a first line of defense on all single-trait hybrids except where rotating into 2012 soybean fields or in check plots in select fields. Even with two modes of action, he reports root feeding, regardless of treatment. However in the checks, lodging and root feeding were extensive. In one field where he used only a single-trait hybrid with no overlay, 25% of the field is lodged. Failures in checks ranged from second year corn with a Cry34/35Ab1 trait that followed an mCry3A trait in 2012 to various trait failures  (all due to heavy pressure, not resistance), on fields in continuous corn for 3 to 14 years.

Crop rotation and scouting defense

Other than crop rotation, Steve Pitstick agrees with Gray on the importance of scouting, and he certainly "mixes it up." This year the Maple Park, Ill. corn and soybean grower added pyramid-stacked traits to an every-other-year rotation of the three available single CRW traits on his 2,600 acres. After running some plots in 2012, Pitstick also applied Aztec soil insecticide as a first line of defense on all single-trait hybrids except where rotating into 2012 soybean fields or in check plots in select fields. Even with two modes of action, he reports root feeding, regardless of treatment. However in the checks, lodging and root feeding were extensive. In one field where he used only a single-trait hybrid with no overlay, 25% of the field is flat.

"The rootworms just overwhelmed the traits in the checks," says Pitstick. "This was especially true in areas that were high yielding in 2012. I think those areas were more succulent and attractive last year and became a trap crop for beetles laying eggs."

He also suspects last year's beetles laid eggs deeper due to the drought, resulting in a prolonged hatch this past spring. Unlike many growers, Pitstick did not have delayed planting. As a result, later emerging larvae missed the first nodal roots with the highest expression of Bt protein. Scouting fields throughout the season, Pitstick noted feeding was primarily on later emerging, higher nodal roots with less protein expression.

Even with late season feeding, the difference between insecticide overlay with single traits and no insecticide was noticeable. "Leaving the check showed we did the right thing," says Pitstick. "You could see the difference with a significantly larger root mass where an insecticide was used. We also noticed that hybrids with a naturally large root mass were better able to handle the late feeding than those with a smaller root mass."

He warns growers where delayed planting produced late, succulent "feeding traps" this year to expect similar pressures. Noting extremely heavy adult populations feeding on his family's pumpkin patch as late as early September, Pitstick expects equally high rootworm pressure in 2014 and will continue and perhaps expand his overlay protection.

"Next year we may use an overlay with pyramid-trait hybrids too," says Pitstick. "I want the trait as backup in case it is too dry to activate the insecticide beyond the furrow so we get control outside the zone."

Insecticide overlay

Ken Ostlie, University of Minnesota professor and Extension entomologist, says a number of factors in 2012 and spring of 2013 likely lowered CRW populations in that state's previous hot spots. He credits aggressive rootworm management by growers, including a switch away from hybrids with the Cry3Bb1 CRW trait alone to pyramids or other traits not yet shown susceptible to resistance.

Management was aided by weather. Ostlie suggests the 2012 drought created competition for food, which resulted in smaller adults that in turn may have resulted in fewer eggs being laid. A dry soil profile over winter with a couple extreme cold snaps may have contributed to egg mortality, while an extremely wet spring with saturated soils likely caused newly hatched larvae to drown in some areas. Add to all of that was the considerable growth in the use of insecticide overlays in 2012 and 2013.

"At winter meetings with ag professionals, they indicated more than 50 percent of corn-on-corn would end up with an insecticide overlay of traits," says Ostlie. "About 25 to 30 percent of rotated corn with no threat of resistance was expected to receive an overlay. I suspect final percentages of both were higher."

Ostlie can see benefits of an insecticide overlay on traits or using pyramid traits in problem areas. However, where traits are working, he suggests an overlay doesn't do much beyond giving a zone of protection against lodging. While pyramid traits are an option, Ostlie worries about their long-term effectiveness, especially in situations where one trait has proven resistance. While it will deliver some control, it no longer offers true second mode of action where resistance has developed.

While Pitstick is satisfied with the CRW control he got this year with two modes of action, he knows that it won't last. He recalls watching his dad spread Aldrin with a Gandy applicator in the 1960s in an attempt to control corn rootworm.

"Two modes of action will be a temporary treatment," he says, with a note of resignation. "We've been battling this constantly changing insect for more than 50 years. They will adjust, and then we'll be on to the next thing. In the meantime, we need to protect the technology by rotating traits, using soil-applied insecticides and hybrids with the most robust root systems."

Control Tips

The one thing certain with CRW is that nothing is certain. While western CRW resistance to the Cry3Bb1 trait has been established in multiple areas within the Corn Belt, it continues to be spotty. Even in those areas, other traits appear to provide control, though the low dose rate (offers less than 99.9% control) of CRW traits, including pyramid traits, suggests resistance will eventually develop. Careful use of these tools in conjunction with integrated pest management techniques and best management practices can extend their useful life.