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.