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Resistance to Bt corn identified: What does it mean?Resistance to Bt corn identified: What does it mean?

Case in Canada serves as a reminder to Nebraska growers to follow resistance management practices.

June 5, 2019

3 Min Read
Close-up of European corn borer on ear of corn
CAUSAL FACTORS: Several factors likely played a role in the development of Cry1F resistance by the European corn borer in Canada. Adam Sisson, Iowa State University, Bugwood.org

The first case of field-evolved resistance of European corn borer to Bt transgenic corn has been reported in Canada.

Tracey Baute of the Ontario Ministry of Agriculture, Food and Rural Affairs reported that Jocelyn Smith and Art Schaafsma of the University of Guelph Ridgetown in Ontario, Canada, confirmed the resistance of ECB to the Cry1F protein in Bt corn hybrids in Nova Scotia.

The Cry1F protein targets lepidopteran pests — such as ECB, black cutworm, fall armyworm and common stalk borer — with its efficacy against western bean cutworm recently declining.

Although the Cry1F protein can be found in many Bt trait packages (see the Handy Bt Trait Table for a full listing), it may be most familiar to Nebraska growers as the above-ground Herculex trait.

European corn borer management through the use of Bt transgenic technology has been a great success, both with respect to efficacy and durability. Widely available since the late 1990s, Bt corn hybrids targeting ECB are grown throughout the U.S. Corn Belt and into Canada.

The high-dose refuge insect resistance management strategy developed for ECB and Bt corn has been effective. Current Bt proteins (Cry1Ab, Cry1A.105, Cry1F and Cry2Ab2) kill more than 99.9% of the ECB larvae, and the non-Bt refuge plantings supply Bt-susceptible moths to mate with any resistant moths that may occur, diluting resistance genes.

How did it happen?

Several factors likely played a role in the development of Cry1F resistance by ECB in Canada. Nova Scotia is a low heat-unit region, so short-season hybrids are required. Choice of Bt corn hybrids is limited, and single-trait Cry1F Bt corn hybrids were among those used.

Single-trait hybrids are being phased out, given the increased risk of resistance they pose, but they still are available and being planted. Refuge compliance in Nova Scotia is unknown, but it is suspected that structured refuge was not always planted, and some farmers may have been relying on other ECB hosts as refuge.

What does it mean for farmers?

Corn hybrids containing Cry1F protein still are effective against ECB in Nebraska and surrounding states, although the Nova Scotia case should remind growers to always comply with the required resistance management practices, to be vigilant to unexpected damage (Bt corn still should be scouted), and if possible, to not rely on the same Bt proteins year after year.

University of Nebraska-Lincoln insect toxicologist Ana Vélez has been monitoring ECB susceptibility to Bt proteins for years in several Corn Belt states, including Nebraska, South Dakota and Minnesota. Although she has never observed a case of field-evolved resistance, she has found Cry1F resistant genes are present in some ECB populations.

This means Nebraska populations have the capacity to develop resistance if selected for it. It appears ECB resistance management is working in the region, and it is the responsibility of growers to keep it working.

Recommendations for resistance management

Here are three things growers should do:

• Follow the non-Bt refuge requirements listed on seed bag tags.

• Treat any non-Bt structured refuge the same as Bt fields: Keep agronomic practices such as planting date and irrigation as similar as possible.

• Avoid planting hybrids that express only one lepidopteran-targeting Bt protein, and avoid using the same Bt proteins repetitively each year.

Source: UNL CropWatch, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.

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