Get to Know Your Corn

Bt Corn Guide Breaks Down Traits for Growers

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Save your corn bag tags this year and make sure the Bt traits you got are the ones you ordered, with help from Michigan State University's Bt Trait Table. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- Michigan State entomologist Christina DiFonzo has spent hours poring over seed company offerings this spring and sorting out which Bt traits each one holds, and the result should make farmers' lives easier.

You can find DiFonzo's 2016 Handy Bt Trait Table here:…. The table breaks down all the various Bt corn hybrids by trait name (Agrisure, Herculex, Optimum, YieldGard/Genuity, and Powercore and SmartStax). Within each trait, DiFonzo lists every available offering and what Bt traits they hold, which pests those traits target, and which herbicide-tolerant traits are included.

The table can help you avoid common pitfalls such as thinking you don't need to plant a refuge, planting a trait that isn't approved in international markets, planting traits that have been compromised by insect resistance in your region, or thinking you have a wide variety of choices.

(Note: The table is geared toward Midwestern growers, but DiFonzo has also designed one for Southern growers, which you can find here:….)


"On this table, it looks like a lot of choices, but they are all the same Cry proteins repackaged," DiFonzo noted. A lot of cross-licensing has occurred between seed companies, so the same proteins often appear under different product names.

Many of the Bt rootworm traits have been compromised by insect resistance in certain parts of the U.S. Resistance to Cry3Bb1 (found in YieldGard, SmartStax and Genuity VT Triple products) has been well documented in Iowa, Illinois, Minnesota and Nebraska, and suspected resistance has been reported in Colorado, Kansas, Missouri, South Dakota and Wisconsin as well as distant Corn Belt fringe states like Michigan and Pennsylvania, DiFonzo noted.

Recently, Iowa State has confirmed cross resistance between three of the four available Bt rootworm traits -- CryBb1, mCry3A (found in Acremax, Trisect, and all Agrisure rootworm products), and eCry3.1Ab (found only in Agrisure Duracade). That means that rootworm populations resistant to Cry3Bb1 are likely also resistant to Bt products that rely only on mCry3A and eCry3.1Ab, Iowa State University entomologist Aaron Gassman explained in a March 2016 university article.

The fourth available Bt trait, Cry34/35Ab1 (AcreMax RW and Herculex RW) -- which has been added to all pyramided rootworm products on the market -- is also showing signs of weakness in Iowa, Gassman reported.

Finally, don't assume that because your hybrid has multiple Bt traits, it's not a single-trait rootworm product. Bt traits such as Cry1Ab, Vip3A, Cry1Fa2, Cry1A.105 and Cry2Ab2 all target above-ground pests, not rootworm.


Many Bt products are sold as refuge-in-a-bag (RIB) to help farmers automatically comply with refuge requirements designed to slow resistance.

Southern growers may not realize that they are still required by the EPA to plant block or strip refuges even if they're using RIB products, warned Texas A&M entomologist Pat Porter.

Refuge compliance hovers around 30% in the South, and this misconception is likely to make it worse, Porter explained.

"Seed blends [RIB] can be planted down here, and more people are doing that, but what the growers don't know is none of that non-Bt seed in the blend counts as a refuge here," he said of Southern growers. "You still have to plant the appropriate size block or strip refuge."

Many corn hybrids on the market still require planted refuges of 20% in the Midwest, DiFonzo added. Check your product requirements carefully.


Agrisure Duracade still doesn't have import approval in China and the EU, so farmers must keep it out of the commercial grain stream at harvest time.

DiFonzo's chart includes the "event name" of each Bt trait, which allows growers to look up regulatory and international approvals for that trait in the International Service for the Acquisition of Agri-biotech Applications GM approval database here:….

The National Corn Growers Association also maintains a website, called Know Before You Grow, which keeps growers informed of corn hybrids that don't have all the crucial international approvals for safe trade:….

Finally, DiFonzo recommends that Bt growers save the tags from a few of the bags they plant and keep careful planting records. "There have been a number of times where the seed [a farmer] was sent was not what he thought he had ordered," she said. "Save the tags and cross reference with what you ordered."

For more information on the state of Bt rootworm resistance, see Aaron Gassman's most recent update from Iowa State University here:….

You can find new rules from the EPA on Bt corn planting, which mostly affect the companies selling it, here:….

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Emily Unglesbee