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Bt resistant rootworms starting to show up in surrounding states

Posted Wednesday, December 19, 2012, at 12:01 PM

Most of the corn planted in the United States is Bt corn, and the Cry3Bb1 toxin is the major one deployed in corn against corn rootworm.

Bt hybrids used for corn rootworm control are low- to moderate-dose events that leave survivors in every field. When enough heterozygotes survive and mate, a Bt-resistant population can increase rapidly.

Bt hybrids expressing protein Cry3Bb1 have been reported to show increasing damage by corn rootworms in Illinois, Iowa, Nebraska, and Minnesota.

Research from Iowa State University shows evolving resistance in progeny of adult western corn rootworms collected from northwestern Iowa fields in which Bt hybrids had been planted for several consecutive years.

The pest has also been seen in Illinois fields. As of now there is no evidence of a problem in Missouri. The fields that are most at risk are those which have been planted to corn for three or more continuous years, fields where Cry3Bb1 protein has been used, and those where there is a relatively high western corn rootworm pressure.

According to Wayne Bailey, University of Missouri Extension Entomologist, you need to be concerned but not real concerned, be vigilant.

The biology of corn rootworms is very closely tied to corn. Adults emerge in the summer, and can have a lengthy emergence period of several weeks. Females lay eggs in corn fields, and eggs overwinter in the soil.

Larvae hatch the following year in the spring. Corn rootworm larvae cannot survive long after hatching without corn roots. Simply rotating fields to soybeans will manage the majority of corn rootworms.

One of the telltale signs of corn rootworm damage is goose-necking or lodged corn.

In a year without windstorms, the lodging may not be noticeable. Also if you notice yield losses that cannot be attributed to weather, have unusually high numbers of adults emerging in your fields, and have been in continuous corn with the same Bt hybrid for three years or more, you may be at greater risk for corn rootworm resistance.

Repeated exposure of corn rootworms to the same Bt hybrids (as well as the same insecticide) will increase the rate of beetles developing resistance.

If you suspect corn rootworm resistance:

1. Verify the location of Bt corn and non-Bt corn refuge within the field to confirm the damaged corn is likely to be a Bt hybrid.

2. Look for goose-necking or lodging and dig and wash a few roots to confirm rootworm injury.

3. Contact your seed dealer and report unexpected corn rootworm damage. Companies that registered the Bt hybrids are obligated by their agreement with EPA to confirm expression of the Cry toxins in the plants and to collect beetles from these fields to test for resistance.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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