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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.
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."