New Resistance

Scientists have found populations of European corn borer in Canada that are fully resistant to the Bt trait Cry1F, sold under the brand name Herculex I.

So far, the discoveries are limited to the Maritimes region, specifically Nova Scotia, where single-trait Cry1F hybrids were used. But, all corn growers who still rely on single-trait corn hybrids, such as Herculex I and Herculex XTRA, are at risk, scientists report.

Farmers in both the U.S. and Canada should check with their seed dealers to see if they are using or have access to pyramided corn hybrids, which add additional corn borer traits such as Cry1Ab (YieldGard Corn Borer) or Cry1A.105 + Cry2Ab2 (YieldGard VT Pro).

Representatives from Corteva Agriscience, which owns the Cry1F trait, say single-trait Cry1F hybrids will not be sold under any Corteva seed brands in Canada by the 2020 season. They could not speak for other seed companies.

In the U.S., use of single-trait Cry1F hybrids is still occurring in “minimal quantities” in “scattered pockets” of the country, says Clint Pilcher, global biology leader for Corteva.

Growers can use the Handy Bt Trait Table
from Michigan State University to check which Bt traits they are using. The Cry1F trait is sold under multiple brands by many seed companies through licensing agreements.

Corn hybrids known to contain only Cry1F to control corn borer include Herculex 1, Herculex XTRA, AcreMax 1 and TRIsect.

Producers using a single-trait hybrid are required to plant a structured 20% non-Bt refuge.

Corteva believes Cry1F remains effective against corn borer beyond the Maritimes region of Canada, as the company monitors for Bt resistance each year across corn-growing regions of North America.

However, scientists from the University of Guelph, who confirmed the Bt-resistant corn borer populations in Nova Scotia, are investigating another case of unexpected corn borer injury outside of this region, says Tracey Baute, an entomologist with the Ontario Ministry of Agriculture, Food and Rural Affairs.

OLD-SCHOOL SCOUTING

The Canadian scientists urge all growers to be on the lookout for corn borer injury this year, Baute says. Even corn hybrids that have two corn borer traits could be compromised if one of the traits is Cry1F, she says, because scientists don’t yet know how widespread this type of corn borer resistance is.

Baute notes most farmers are not always watching for corn borer injury. Decades ago, corn borer was a major economic pest of corn--and it still is for non-GM corn growers. But, for the past 20 years, the pest has been so well-controlled by Bt traits, a generation of farmers has grown up not having to scout for it.

“We’ve all relaxed our monitoring for this pest, and now we have to go back to the drawing board,” she stresses.

Corn borer populations are on the rise in parts of the Corn Belt where non-GM corn production is increasing, notes Michigan State University Extension entomologist Christina DiFonzo.

“I don’t get the impression that a lot of people are walking through their corn scouting for it or know exactly what traits they are planting for it, so [resistance] could be going under the radar pretty easily,” she says.

Growers may need to reacquaint themselves with signs of corn borer injury in corn, which include shot holes in corn leaves, “frass” (insectexcrement) from larva tunneling holes into the stalk, broken tassels, stalks falling over and ears dropping on the ground before harvest.

Corn borer is the second corn pest to develop resistance to the Cry1F trait in recent years. Two years ago, scientists from Indiana, Michigan, New York and Ohio reported Cry1F-resistant populations of western bean cutworm. That pest has been removed from Cry1F’s marketing language, and the Handy Bt Trait table notes what regions growers should expect to use non-Bt controls for this pest. That resource has also been updated with the confirmed corn borer resistance to Cry1F, DiFonzo says. Entomologists are urging growers to add the pest to their scouting regimen once again and notify university Extension offices if they find unexpected damage.

FOR MORE INFORMATION:

> The Handy Bt Trait Table

> Purdue University IPM Guide

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