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MANAGING CORN ROOTWORM IN THE ERA OF BT RESISTANCE

04/15/2015
MANAGING CORN ROOTWORM IN THE ERA OF BT RESISTANCE
  • Researchers confirm Corn Rootworm (CRW) resistance to Bt traits Cry3Bb1 and mCry3A in the Midwest
  • EPA has responded by proposing new rules for Insect Resistance Management
  • Stewardship of new pyramid stacks is essential to preserve the technology and avoid regulation
  • A voluntary Integrated Pest Management (IPM) systems approach is the right answer

What is the nature of the Bt resistance problem?  Corn root worm resistance to the Bt trait Cry3Bb1 has been confirmed by researchers in the four Midwestern US states of IllinoisIowaMinnesota, and Nebraska.  Resistance has also been documented for the mCry3A Bt event, and now cross-resistance has been documented for both traits.  The first detection of a resistant population took 8 years from the introduction of Cry3Bb1 in 2003, and only 7 years for mCry3A, which suggests that the resistance problem may be accelerating.  If this pattern continues, we are at risk of ‘burning out’ our best transgenic technologies before new ones can be developed.  Bt technology is a tool that must be protected through sound stewardship. 

To manage the problem, we need to understand how and why it happened.  Any resistant pest population develops through a combination of natural selection and selective pressure.  Natural selection is a brilliant biological mechanism to ensure survival of species – in this case, an insect mutation aids survival in the face of a control like Bt, survivors then mate, offspring are resistant to the control, and the cycle repeats.  Refuges are based on the principle that Bt resistance is a recessive trait and that resistant survivors would breed with susceptible insects to create susceptible crosses.  The pace of resistance development suggests that resistance may be dominant rather than recessive, as first believed.  The second component is selective pressure that we apply through our controllable management practices:  continuous corn rotations, repeated use of the same single trait in the same field, over-reliance on traits as a single mode of action, and poor refuge compliance.  The initial CRW traits also expressed low dose concentrations of Bt toxins in the plant roots, and certain growing conditions resulted in declining dose expression over the course of the growing season – both enabling survival and development of resistance.

How has EPA responded?  EPA has issued a proposal that imposes new requirements on Bt registrants for CRW resistance management.  If adopted, the proposed rules will definitely impact the farm operation.  Complete proposed framework.    Rules would require Bt registrants to:

1.       Establish targets for adoption of IPM by users in the resistance focus zone, with practices to include:

a.       Mandatory crop rotation.

b.      Use of stacked pyramid traits.  No single trait hybrids.

c.       Use of conventional hybrids plus soil applied insecticides.

2.       Change labels to PROHIBIT using soil applied insecticides in combination with Bt traits.  In fact, EPA openly refers to this point as 

a ‘non-starter’ or “Not effective or productive”

3.       In cases of unexpected damage in Bt fields, report to EPA, test for resistance with on-plant assays, and build remedial action (RA)

plans that incorporate IPM.

Voluntary stewardship is in agriculture’s best interest. 

To combat selective pressure for resistance, we must build better tools and change our practices to preserve the technology traits.  Trait stewardship is grounded in a year-round IPM system that uses multiple practices in combination to keep pest populations below economic thresholds.  Plan to implement a system that includes the following practices:

Crop Rotation.  In high pressure situations, rotate to soybeans and plant no more than three consecutive years in corn.  While CRW soybean and diapause variants have weakened this cultural practice in the variant areas, it remains viable elsewhere.

Compliance with refuge requirements.  Refuge-in-bag products have made compliance easier in recent years, but make sure you understand and follow label requirements specific to the hybrids being planted and to your geography.

Trait rotation and pyramid stacks.  The premise here is to simultaneously deploy multiple modes of action (MOA) and to rotate toxins to vary the selective pressure being applied to the CRW populations.  If a resistant population is established, even using a pyramid can put continued pressure on the compromised trait while applying accelerated pressure to the effective trait.  Trait rotation may require you to step outside of your comfort zone, try new products, and spread seed purchases across a broader range of commercial hybrids.

Scouting for CRW root damage.  Check your Bt fields mid-season for unexpected root damage.  Dig plants and assess damage using the root node injury scoring system.  Scores of 0.5 or greater on pyramid stacks should be reported to your seed manufacturer’s representative.

Adult Suppression.  Apply foliar insecticides as warranted to control adult CRW beetles, reduce silk clipping, and minimize future larval generations.  The scouting data will also provide insights into how to best plan for the following year.  Soybeans should also be scouted for variants.

Should I use a soil-applied insecticide (SAI) in combination with Bt traits for ‘insurance’ protection?  This is a question with no easy answers.  Using a soil-applied insecticide is becoming a more common practice in combination with single-traited CRW-Bt hybrids, to add a second mode of action to the program.  The insecticide may very well give a yield response versus not using one, but it’s probably a false sense of security.  When a Bt trait is faltering, the insecticide simply masks the developing Bt resistance issue and does nothing to remove the selective pressure for resistance, which only hastens the onset of resistance.  For these reasons, University researchers are advising against using soil-applied insecticides with Bt traits and the proposed EPA rules would actually prohibit it by label.  The recommended practice is to rotate to a conventional non-Bt hybrid with a soil-applied insecticide.  However, conventional hybrids have generally not kept pace with yield potential of their transgenic counterparts and this combination would perform well only under low pressure situations.  In high pressure scenarios, using a pyramid stack without an insecticide may still prove to be most effective and economically viable option.  If using insecticides in the program, try to rotate mode of action and always follow label considerations for potential crop injury from interactions with your herbicide programs.

The objections to practice change can be overcome with sound agronomic advice and influence.  Your FS Crop Specialists can help you maintain the proper balance of remaining profitable while applying IPM principles that protect Bt technologies for the long-term. 

References:

http://www.pnas.org/content/111/14/5141.full.pdf

http://www.extension.iastate.edu/CropNews/2014/0820hodgsongassman

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2014-0805-0002

http://bulletin.ipm.illinois.edu/?p=1913

http://ipcm.wisc.edu/blog/2015/02/managing-corm-rootworm-in-2015

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