Efficacy of Cry1F trait on western bean cutworm
Entomologists in the Great Lakes region address the issue of efficacy of the Cry1F trait on western bean cutworm with an open letter.
Back in August, I alerted Michigan producers to elevated western bean cutworm damage in corn, particularly on traited corn expressing the Cry1F Bt protein. This protein was never perfect against western bean cutworm, but for at least a decade, brochures from multiple seed companies included it on lists of insects controlled by Bt trait packages with Cry1F. For the last few years, however, there has been growing evidence that Cry1F was not performing as expected.
This fall, the wheels have fallen off the cart. Through the month of September, extension entomologists in the Great Lakes region fielded numerous phone calls and emails, and visited Cry1F fields with unacceptable levels of damage. We collected photos, efficacy data and personal communications. The groundswell of frustration was not limited to growers, but also bubbled up from folks with agribusinesses, seed companies and local extension offices. These people work at the end of the line, so to speak, dealing directly with field-level problems and taking the brunt when things go wrong.
After hearing from many voices from the field, we have taken the unusual step as a group to recommend, in writing, that seed companies take a serious look at the failure of the Cry1F Bt trait on western bean cutworm, before sales ramp up for 2017. Our open letter is shared below.
Open letter regarding efficacy of Cry1F trait on western bean cutworm
This open letter was prepared by the following extension entomologists from the Great Lakes region: Chris DiFonzo, Michigan State University; Christian Krupke, Purdue University; Andy Michel, The Ohio State University; Elson Shields, Cornell University; Kelley Tilmon, The Ohio State University; and John Tooker, Pennsylvania State University. It’s regarding the efficacy of the Cry1F (Herculex 1, TC1507) trait on western bean cutworm (Striacosta albicosta). We strongly urge seed companies to remove the designation of “control” for this pest with regard to this toxin.
At the time Cry1F received regulatory approval in 2001, western bean cutworm was found in the far western Corn Belt (Colorado, Idaho, Nebraska and Wyoming), with occasional movement into western Iowa. Indeed, EPA’s original Biopesticide Registration Action Document (BRAD) for Cry1F Bt corn, published in August 2001, did not even mention western bean cutworm. Instead, the following language was used: “The registrant-submitted data indicate that Cry1F protected corn offers excellent control of European corn borer, southwestern corn borer, fall armyworm, black cutworm and suppression for the corn earworm.”
References to Cry1F giving “excellent protection” against western bean cutworm began to appear in marketing literature only after Iowa State University entomologists documented its eastward range expansion and the first economic damage in that state. Presumably, this rating was based on a limited number of lab assays and field trials done in pure Bt stands, not Refuge-in-a-Bag hybrids.
The rapid eastward range expansion of western bean cutworm across the central Corn Belt into the Great Lakes region resulted in a dramatic increase in the number of western bean cutworm-infested acres in a short period. This created a large-scale “efficacy test” of Cry1F hybrids to (as stated in the BRAD) “provide highly efficacious control of key Lepidopteran pests, reduce the use of more toxic chemical insecticides and reduce levels of mycotoxin in corn.” In all these regards, Cry1F has failed in our states. This 2016 season in particular, the level of larval infestation and damage is troubling in single and pyramided Refuge-in-a-Bag hybrids from multiple seed companies.
Wherever Cry1F is challenged by western bean cutworm, it fails to provide observable benefit to producers. We have collectively fielded dozens of phone calls and emails, and visited numerous fields; we know our agribusiness contacts and seed industry agronomists have responded to many more, and corn acres were sprayed with insecticides and fungicides (most too late and with little hope of benefit). People are frustrated and angry and, more importantly, yield was lost. Growers purchased Cry1F hybrids with the understanding that the trait provides “control,” thus negating the need to scout for egg masses or larvae in those fields. When the visible manifestations of damage became apparent late in the season, such as the intense ear-feeding we witnessed, it was far too late for rescue treatments. As the fall progresses and damaged corn is harvested, additional issues are sure to arise regarding quality and mycotoxin levels. The severity of the latter will largely be dependent on weather conditions favorable for ear mold development. What is certain is that many damaged ears are primed for fungal colonization and quality loss.
As extension educators and specialists, we can no longer refer to Cry1F as providing western bean cutworm control. In fact, the opposite is true, and our extension recommendations (including the Handy Bt Trait Table) will be changing to classify Cry1F hybrids for western bean cutworm the same as non-Bt, Cry1Ab or double/triple pro hybrids, all of which provide no control. In other words, we believe Cry1F fields must be scouted for egg masses and sprayed with foliar insecticides if needed, the same as a non-Bt corn. Western bean cutworm is now the PRIMARY Lepidopteran ear pest in many parts of the Great Lakes region. For growers in our states, the costs of scouting and spraying Cry1F corn nullifies a major reason they purchased and planted a hybrid with the trait in the first place.
Before growers make seed choices for 2017, we again urge the seed industry to acknowledge the reality of what is happening in the field, and to reclassify Cry1F in hybrid fact sheets, technical use agreements, and other educational materials. This would reduce grower expectations of Cry1F and allow local agricultural professionals to deal with their customers in a more truthful manner, in a way that allows for protection against yield loss. We also urge the industry to regard western bean cutworm as a primary, not a secondary, pest. Doing nothing risks alienating those close to the situation, including field agronomists, consultants, university extension staff and, most importantly, corn growers themselves who have a vested interest in finding effective pest management solutions for a growing world.