Last year Mike Gray reported on severe cases of rootworm damage to Bt hybrids expressing the Cry3Bb1 protein in some producers' fields in northwestern and north-central Illinois. The fields with severe root pruning and lodging had been in continuous corn production systems for many years. In addition, the producers had relied consistently on the use of a single trait (Cry3Bb1). These problem fields fit a pattern similar to one described by Dr. Aaron Gassmann, Iowa State University, for Iowa fields in which resistance to the Cry3Bb1 protein was confirmed in 2011.
As indicated in the 2011 Bulletin articles, the presence of severe root pruning (1 to 2 nodes destroyed), lodging, and confirmation of the expression of the Cry3Bb1 protein from injured roots does not prove resistance. Bioassays are required to understand the mechanism behind these Bt failures in producers' fields. Gray and associates collected adults from the Illinois problem fields and sent them to Dr. Gassmann. Bioassays were conducted on the offspring (larvae) from mated adults from these problem fields. Control western corn rootworm larvae (never exposed to Bt proteins) also were used in the bioassays.
The results from Dr. Gassmann's laboratory were provided to Gray in early August, and he recently shared them with audiences at the 2012 Agronomy Day on campus. The findings from Illinois mirror those published by Dr. Gassmann last year concerning problem fields in Iowa. Survivorship of the Illinois larvae on Cry3Bb1 root tissue was not different from the isoline tissue (no Cry3Bb1 expression). The larvae for the Illinois bioassays were offspring reared from adult western corn rootworms collected in Henry and Whiteside counties last year (2011). Larvae collected from these problem Illinois fields remained more susceptible to the Cry34/35Ab1 protein. The control larvae (never exposed to Bt proteins) remained susceptible to both Cry3Bb1 and Cry34/35Ab1 proteins.
In light of these results, growers who have experienced less-than-satisfactory performance with a Bt hybrid should consider the following recommendations. In addition, growers who want to avoid a future problem with a Bt hybrid and, more importantly, to prolong the usefulness of this technology should think through these recommendations as well.
- Consider rotation to soybeans or another non-host crop.
- Consider using a corn rootworm soil insecticide at planting along with a non-Bt hybrid.
- Consider using a Bt hybrid that expresses a different corn rootworm Cry protein than one that may have performed poorly in your fields in 2012 or has been in use for several consecutive years.
- Consider usng a pyramided Bt hybrid that expresses multiple Cry proteins targeted against corn rootworms.
- Most importantly, consider a long-term integrated approach to corn rootworm management that includes multiple tactics.
Gray is particularly concerned about the escalation of soil insecticide usage with Bt hybrids for corn rootworm protection. This practice is seemingly becoming very common in many Corn Belt states. Many will recall that a primary benefit touted when rootworm Bt hybrids entered the marketplace (2003) was the reduction or potential elimination of soil insecticides. Even though commodity prices have increased, so have seed costs, and now many producers are adding yet another input cost--a soil insecticide.
Another very strong concern that Gray and many of his entomology colleagues hold is the increased selection pressure being placed on the Cry34/35Ab1 protein in areas of the Corn Belt where resistance to the Cry3Bb1 protein has been confirmed. Pyramided hybrids that are being used in these areas continue to work reasonably well; however, in effect, one protein (Cry34/35Ab1) is providing the primary control. And the required refuge for these pyramided Bt hybrids has been reduced from 20% to a 5% seed blend. Even though a seed blend (refuge-in-a-bag) is a preferred resistance management strategy for corn rootworms, the reduction in refuge is a lingering concern for Gray.
The western corn rootworm remains a versatile insect foe. It has adapted to many classes of insecticides, to crop rotation, and to this relatively new transgenic technology. Producers will need to employ a well-conceived integrated pest management approach to stay one step ahead of this insect.