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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.
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.