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Emerging Corn Rootworm Resistance to BT: Strategies to preserve the technology

Elson Shields
Department of Entomology, Cornell University

In 2016, corn rootworm control failures have been once again reported in central NY.  As reported in 2013, the control failure was in fields planted to corn containing a single rootworm BT toxin, Cry 3Bb1 (Figure 1).  Severe rootworm injury is usually easy to spot during the growing season by the goose-necked corn plants.  The plant tilts over due to the loss of roots and then the stem straighten up, leaving a curved plant stem.  Many times during fall harvest, lodged plants are blamed on corn rootworm damage.  However, during the fall, many of the lodged plants have straight stems and have fallen over due to their poor rooting, top heavy condition and wet soils in the fall.  The key diagnostic characteristic for rootworm damage in the fall is the curved stem.

Root damage and plant lodging from corn rootworm larval feeding. The curve stem of the plant is characteristic of this type of feeding, which occurs in July. The plant tips over from loss of roots and then turn back up. These plants contain CRW-BT Cry 3Bb1 protein and the larvae feeding on the roots were not killed by the toxin.

Root damage and plant lodging from corn rootworm larval feeding. The curve stem of the plant is characteristic of this type of feeding, which occurs in July. The plant tips over from loss of roots and then turn back up. These plants contain CRW-BT Cry 3Bb1 protein and the larvae feeding on the roots were not killed by the toxin.

Keeping the various BT traits straight and which corn varieties they occur in is an ever changing and difficult task.  One very helpful tool is the “Handy BT Trait Table” published by Dr. Chris Difonzo at Michigan State University.  The 2016 version of the table can be located at www.msuent.com/assets/pdf/28BtTraitTable2016.pdf.  The 2017 trait table is not available until March 2017 and can be located by conducting a Google search using “Handy BT Trait Table” as the search words.

For corn rootworm control with plant incorporated BT, we have only three groups of BT proteins in our toolbox.  The first protein Cry 3Bb1 was first incorporated into corn by Monsanto and was the first BT-CRW protein marketed.  The second protein is Cry 34/35, a two protein/toxin mixture, incorporated into corn by Dow and the third group is the Syngenta proteins mCry3A and eCry3.1Ab.  Some corn varieties have two groups of CRW proteins incorporated called a pyramids.  For example, a common pyramid is Cry 3Bb1 and Cry 34/35.

As predicted, corn rootworm is developing resistance to these widely deployed corn incorporated proteins across the corn belt with resistance to Cry 3Bb1 the most wide spread.  Multiple field failures have been reported to mCry 3A in the Midwest and fields with poor control have been reported planted to corn with Cry 34/35.  In the Northeast, reports of resistance to Cry 3Bb1 occurred in NY in 2013 and 2016 with reports of resistance to mCry 3A in Pennsylvania in 2014.  With new technologies on the distant horizon, producers should make a focused effort on preserving the current BT-CRW technology as long as possible.

Strategies to preserve the technology:
If a farm is experiencing a loss of control or performance from the BT-CRW toxin, there are several effective strategies to use which has both the benefit of reducing losses in future years and helping to preserve the technology from increased insect resistance.

  • Rotate the field experiencing damage to a non-corn crop:

Since corn rootworm eggs overwinter in corn fields and the newly hatched larvae must find a corn root to survive, crop rotation remains an effective CRW management tool.  Simply rotate the field to another crop for a season and then the field can be rotated back to corn the following year without the need for any CRW management measures.

  • If the field cannot be rotated, use a soil insecticide with conventional corn variety:

If growers still have insecticide boxes on their planter, excellent control can still be achieved using an application of soil insecticide in a 7” band in front of the press wheel.  In furrow applications have a reduced efficacy compared to the 7” band.  The use of a liquid insecticide such as Capture either in furrow or in the fertilizer band is widely considered as ineffective against CRW larvae.

  • If the field cannot be rotated, use a corn variety with a different BT-CRW toxin:

If the field has experienced loss of performance with a corn variety containing Cry 3Bb1, then plant a corn variety using Cry 34/35.  Substituting a corn variety with mCry 3A or eCry3.1Ab CRW toxin is not the best choice because of potential cross resistance (observed in the laboratory) with Cry 3Bb1.  If the loss of performance is due to a corn variety containing Cry 34/35, then any of the Cry 3 choices would be appropriate.

  • If the field cannot be rotated, it is a poor choice to plant a corn variety containing two CRW-BT toxins, with one of the toxins identical to the toxin failing in the field:

For example, if Cry 3Bb1 is giving poor control in the field, planting a variety which contains both a Cry 3 toxin (Cry 3Bb1, mCry3A or eCry3.1Ab) and Cry 34/35 with only a 5% refuge in the bag puts a lot of selection pressure on the CRW population to develop resistance against Cry 34/35.  The dual CRW-BT toxin containing corn varieties were allowed to be marketed with only a 5% “refuge in the bag” under the conditions both toxins where effective on the insect population.

  • Resist the temptation to layer a soil insecticide over a failing CRW-BT event:

When a soil insecticide is used in addition to a CRW-BT trait with a history of poor CRW control, this combination increases the development of CRW resistance to the event.  A soil insecticide applied either in furrow or banded in a 7” band protects the area surrounding the base of the plant from larval feeding.  However, this insecticide application has no impact on the CRW larvae feeding on corn roots outside the insecticide treated area surround the plant base.  CRW larvae feeding outside the insecticide protected zone are still being exposed to the failing BT toxin and larvae continuing to survive feeding on the toxin will continue to interbreed with other survivors, resulting in an increased level of resistance in the next generation of rootworms.

 

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