Corteva confirmed second report of western corn rootworm resistance to the Herculex RW trait that is the underpinning of most pyramided rootworm hybrids

EXCERPT: Rootworm populations that are resistant to one Cry3 Bt protein, such as Cry3Bb1, can also tolerate another, such as mCry3A, even if they never encountered that trait in the field.
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Beware rootworms lurking

Emily Unglesbee
DTN Progressive Farmer, Oct 12 2020
https://www.dtnpf.com/agriculture/web/ag/crops/article/2020/10/12/corn-rootworms-poised-stage-comeback
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* Corn rootworms poised to stage a comeback as Bt resistance spreads

After several years of low corn rootworm populations, the "billion-dollar bug" is catching eyes again in the Corn Belt.

Populations of both western and northern corn rootworm are rebounding after a mild winter and favorable egg hatch conditions this year, and they're bringing some bad habits along: growing Bt resistance.

Corteva Agriscience recently confirmed their second report of western corn rootworm resistance to Cry34/35Ab1, the Herculex RW trait that is the underpinning of most pyramided rootworm hybrids, this summer. The trait is usually offered in pyramids that cross many brand platforms, with names such as SmartStax, QROME, Intrasect Xtreme, AcreMax Xtreme and Agrisure 3122.

With resistance already established against the other three rootworm Bt traits, growers in intensive corn-growing regions may need to be more innovative than ever when it comes to managing this pest, entomologists told DTN.

Here are four big takeaways for the 2021 season.

1. RESISTANCE IS GROWING

Corteva has now confirmed western corn rootworm resistance to Cry34/35Ab1 in Iowa County, Wisconsin, in 2020 and Delaware County, Iowa, in 2018. Academic scientists have also confirmed varying levels of resistance to this trait in Iowa, Minnesota and North Dakota, as well. (See more here: https://www.dtnpf.com/agriculture/web/ag/crops/article/2018/11/15/rootworm-resistance-bt-pyramids-rise-2). The other three traits -- Cry3Bb1 (YieldGard Rootworm), mCry3A (Agrisure RW) and eCry3.1Ab (Agrisure Duracade) -- also have documented resistant populations throughout the Corn Belt.

As a result, the actual state of resistance to Cry34/35Ab1 (Herculex RW) is likely more widespread, noted Bryan Jensen, University of Wisconsin entomologist. He uses the term "field resistance" to describe the unexpected damage growers see in their fields that never gets officially confirmed with time-consuming laboratory tests.

"We have been dealing with field resistance to the other traits [Cry3Bb1 and mCry3A] for a while now," Jensen said of Wisconsin corn growers. "My feeling is that we have been relying on that Cry34/35Ab1 protein and not diversifying management practices."

That's true of many corn-growing regions, added Ken Ostlie, University of Minnesota entomologist. "The three Cry3 traits have taken the brunt of it, because of cross-resistance between them," he noted in a university webinar in July.

That means that rootworm populations that are resistant to one Cry3 Bt protein, such as Cry3Bb1, can also tolerate another, such as mCry3A, even if they never encountered that trait in the field. That's why researchers rapidly discovered resistance to eCry3.1Ab (Duracade) in 2016, even though the trait had not yet been widely commercialized. See more here: https://www.dtnpf.com/agriculture/web/ag/news/article/2016/08/10/rootworm-resistance-pyramided-bt
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