As final seed purchases are being made in the Texas Panhandle, here is a brief update on the status of Bt corn. But before your eyes glaze over from the discussion below of new Bts and resistance to older Bts, I want to highlight a publication that makes it easy to tell which toxin packages and herbicide traits are in which type of corn. Dr. Chris DiFonzo at Michigan State University publishes the annual "Handy Bt Trait Table for U.S. Corn Production", and the 2018 version was posted online today. (I am a contributor to this publication, but Dr. DiFonzo does the heavy lifting.) In just two pages she lists the types of Bt present in all commercialized corn in the U.S.A., and the table presents the trade names for traits, Bt event, protein(s) expressed, targeted insects and herbicide traits.
The 2018 Trait Table also lists the insect x Bt combinations with documented field-failures, confirmed resistance, or cross-resistance. These statements are based on published lab assays and/or field research. The resistance column is intended to alert growers and consultants to potential management problems, influence seed selection, and encourage field scouting. It is important to note that the Trait Table is a national publication, so check with your local seed company or extension personnel for the types of Bt resistance present in your area.
For those looking at a printed version of this newsletter, the Handy Bt Trait Table can be found here: https://www.texasinsects.org/bt-corn-trait-table.html .
And now to the Bt corn update.
Monsanto is marketing Trecepta, its version of Vip3a pyramided with other toxins. I worked with Trecepta in 2017 and was impressed with its insect resistance and yield. For Vip3a corn we have Monsanto's Trecepta, DuPont Pioneer's Leptra, and Syngenta's Viptera. This type of corn is virtually immune to caterpillar damage whether the pest is western bean cutworm, fall armyworm or corn earworm. I have worked with Leptra for six years and Trecepta for one and, in that time, have seen only two live caterpillars in many thousands of ears I have examined.
On the resistance front, laboratory studies conducted primarily in Canada by Dr. Jocelyn Smith (with coauthors from academia and seed companies) have shown that western bean cutworm (WBC) is now resistant to Cry1F (often sold as Herculex). Control with Cry1F, even in the Texas Panhandle, has been slipping for several years, and it is now conclusive that resistance is a big part of the reason. All of the Cry1F registrants and licensees have removed WBC from the list of insects their non-Vip3a Bt corn will control. All of the companies correctly point out that the only Bt corn that will control WBC are the hybrids with Vip3a (combined with other ineffective toxins which vary by company). Bt hybrids that do not contain Vip3a should be scouted and treated according to extension recommendations.
Southwestern corn borer is known to be resistant to Cry1F in southeastern Arizona and southwestern New Mexico. The good news is that we have no indication that this resistance has come to Texas. DuPont Pioneer made significant efforts in 2017 to determine where the resistance does and does not occur. As part of this they funded John David Gonzales, IPM Agent in Bailey, Parmer and Castro counties, to make SWCB collections, and that project will be ongoing in 2018. The other good news is that in places where the resistance does exist, planting Bt corn with two toxins rather than just Cry1F has eliminated economic yield losses.
In the less than good news category, Dr. Ed Bynum, Extension Entomologist in Amarillo, and I have been in continuous corn fields in the Dalhart area that had unusually high amounts of corn rootworm damage in mCry3a hybrids. There have been many instances of western corn rootworm resistance to Bt toxins in other parts of the country, and is known that there is cross resistance between Cry3Bb1, mCry3a and eCry3.1Ab. It looks to me like we have resistance to these toxins in the Panhandle. However, resistance should be determined through laboratory analysis of the offspring of adults collected from fields, and that is why it is important to contact extension personnel or your seed dealer if you are seeing indicators of resistance. These indicators include fields that have been planted to the same Bt toxins for three or more years, the need to use soil applied insecticides on top of the Bt, high levels of root pruning, goosenecking or lodging of plants, or consistently high numbers of adult beetles in July or August. It is important to report suspected resistance while there are still plenty of adult beetles in the field to collect. Dr. Bynum and I would be glad to discuss resistance with anyone who thinks they might have a problem.
