In 2011, Monsanto realized there may be a problem with Cry3Bb1.
As Sean Evans, Illinois insect technology development representative, explains, they received a "flare-up" of calls from areas of Iowa and northern Illinois. That flare-up amounted to 379 growers submitting 458 performance inquiries on a total of 75,000 acres across the Corn Belt.
"These traits can be overwhelmed by heavy corn rootworm populations," Evans explains. "It's a low-dose event."
Monsanto responded to grower complaints by launching a full-scale investigation. They found in most instances, trait failures were consistent with high corn rootworm pressure in continuous corn areas where one mode of action was being employed.
Later that year, Iowa State University's Aaron Gassmann confirmed populations of western corn rootworm had developed resistance to Monsanto's Cry3Bb1 trait. The following year, University of Illinois entomologist Mike Gray, in cooperation with Iowa State, confirmed the same thing in Henry and Whiteside Counties in northwestern Illinois.
In 2012, Monsanto worked with farmers to implement best management practices. Performance inquiries dropped from 75,000 acres to 45,000 acres.
Three management tips
After slogging through one of the worst droughts on record, it appears rootworm resistance to Bt traits is poised to once again take the agronomic center stage.
Does that mean growers should steer clear of Cry3Bb1 at all costs? Not necessarily, say both Evans and Gray. Monsanto has developed three best management practices to help steward this technology into the future.
Evans says the first BMP is relatively simple: rotate to soybeans. As previously mentioned, continuous corn places extreme selection pressure on corn rootworm populations. When faced with a single low-dose Bt event, populations are capable of overwhelming fields. Evans notes rotating to soybeans does have the added risk of potentially developing Cry3Bb1-resistant corn rootworm variants (rootworms that lay their eggs in soybean fields, rather than corn fields). However, the risk of typical Cry3Bb1 resistance is much more immediate.
The second BMP: employ two stages of defense against rootworms. For many, this means planting a hybrid with two pyramided Bt traits that protect against rootworms.
If rotation or planting a pyramided Bt hybrid are not options, Evans suggests a third BMP – using a soil- or foliar-applied insecticide in combination with a single-Bt trait hybrid.
In Gray's research, a pyramided hybrid has typically garnered better root ratings than a soil insecticide plus a single Bt trait. To date, research does not indicate that adding a soil insecticide to a pyramided hybrid, one expressing more than a single Cry protein targeting corn rootworms, makes economic sense.
Lastly, Evans urges growers to work harder at scouting for and controlling adult populations. This means getting in the field in early August and counting gravid female beetles. Those who find more than two pregnant females per plant should consider a late-season application to knock down the population for next year's growing season, he recommends.
"More than 90% of the performance-inquiry fields where best management practices were adopted in 2012 saw minimal feeding damage, and farmers who planted soybeans were able to reset the corn rootworm populations in those fields," Evans notes.
Gray says last year's test plots highlighted just how devastating an active rootworm population can be in hot and dry conditions. In DeKalb County plots, Gray and his Department of Crop Sciences colleagues, Ron Estes and Nick Tinsley, planted numerous hybrids, some with Bt rootworm protection, some without. They also applied planting-time soil insecticides to some Bt hybrids in plots and left others untreated.
The biggest yield difference cropped up between non-Bt hybrids (without a planting-time soil insecticide) and pyramided hybrids. The plot with no protection saw average root injury ratings of 2.32 (over 2 nodes of roots destroyed) with a yield of 23.2 bushels. In the same experiment, a pyramided hybrid, also without soil insecticide, had root injury ratings of only 0.33 (one-third of a node destroyed) and yielded an average of 145.4 bushels.
"You don't want to make a mistake with this insect, especially in hot and dry conditions," Gray notes.
Gray's recent surveys at the Corn and Soybean Classics suggest most folks are already protecting against rootworm resistance by planting a pyramided corn hybrid, i.e. one that includes two Bt traits to protect against rootworms.
More than 70% of the participants said they will plant a refuge-in-the-bag hybrid in 2013. Gray conducted the poll in five locations (Champaign, Malta, Moline, Peoria and Springfield). More than 550 farmers were polled. It shows this technology has caught on perhaps faster than many anticipated.