Corn hybrid stacks and pyramids aren’t the same
If you refer to Bt corn hybrid stacks and pyramids as if the terms are interchangeable, you aren’t alone. However, there are important distinctions between the two terms. Here is some information to bring you up-to-speed on terminology.
Bt corn hybrids got their start with a single protein made from Bacillus thuringiensis (Bt). This naturally occurring, soil-borne bacterium produces crystal-like proteins (Cry proteins) and the vegetative insecticidal protein Vip3A, essentially toxins, that selectively kill insects. Introduced in the mid-1990s, the first Bt protein was primarily aimed at controlling European corn borer populations.
Today, there are many Bt genes in the marketplace with various Cry proteins (more than 60) and the single Vip protein to control a variety of insects. When you purchase "stacked" Bt seed, you are buying a hybrid that contains at least two unique proteins, and each of them is designed to control a different insect pest. The seed is also likely to have herbicide tolerance.
"A hybrid that contains a protein for above-ground protection against European corn borer and a different protein for below-ground protection against western corn rootworm, in addition to providing herbicide tolerance, is a stacked hybrid," explains Mike Gray, University of Illinois Extension entomologist.
One way to think about a stack is to liken it to weed-control measures. When farmers use a tank mix with two or more different herbicide active ingredients to control two or more weed species that is a stack.
Hybrids stacked with Bt proteins and herbicide tolerance are used extensively in the U.S. In 2013, farmers planted them on more than 80% of their corn acres. In total, 90% of corn acres were planted to genetically modified corn hybrids this year.
Focused on one pest. Bt hybrids that are classified as a pyramid contain two or more Cry proteins to control a single pest. Vip3A is not used in any pyramid. For instance, two Cry proteins could be targeted at western corn rootworm and two other Cry proteins aimed at above-ground insects, such as the European corn borer. The main point is pyramided Bt hybrids aim two or more Cry proteins at one target insect. Examples of corn genetics that have a pyramid include Genuity SmartStax RIB Complete, Refuge Advanced Powered by SmartStax and Optimum Intrasect Xtreme.
The benefit of using a pyramided Bt hybrid is that if one of its Cry proteins fails to adequately work, the second one should still function and control the pest. This scenario is playing out today in some cornfields where rootworm has developed resistance to the Cry3Bb1 protein. In those cases, much of the heavy lifting is left to the other two proteins currently available for rootworm control, Cry34/35Ab1 and mCry3A.
Some of the newer pyramided hybrids, such as the refuge-in-a-bag (RIB) or single-bag products, are planted under a reduced refuge, often just a 5% or 10% refuge instead of the historic 20% structured refuge. "The concern is the reduced refuge will hasten the resistance problem," Gray says.
However, he acknowledges that the upside to the technology, giving farmers easier ways to be refuge compliant, is a significant benefit that farmers, seed companies and the Environmental Protection Agency all value.
When Rotation Fails
The need for integrated pest management is increasingly clear for Illinois corn farmers. In late August, significant western corn rootworm larval injury was confirmed in first-year cornfields planted to Bt rootworm hybrids containing the single Cry protein, Cry3Bb1, in Kankakee and Livingston counties, says Mike Gray, University of Illinois Extension entomologist. Joe Spencer, entomologist with the Illinois
Natural History Survey, collected adult rootworms from the infected fields. Bioassays will be conducted on the adults’ offspring to determine if the rotation-resistant corn rootworms are also
resistant to the Cry3Bb1 protein.
You can e-mail Rhonda Brooks at firstname.lastname@example.org.
- October 2013