Source: CropWatch, University of Nebraska, Lincoln

The U.S. Environmental Protection Agency has registered two new Bt corn hybrids with the Bt proteins, Cry34Ab1 and Cry35Ab1, for use against corn rootworm larvae. They will be sold under the trade name Herculex RW by Pioneer Hi-Bred International, Inc. and Mycogen Seeds/DowAgroSciences LLC, which jointly developed the genetic material.

These are different Bt proteins than those used in YieldGard Rootworm hybrids or any other Bt corn to date. Company and university research indicates that they effectively protect roots from rootworm feeding injury.

Similar to previous Bt corn hybrids, EPA has required registrants to implement a resistance management plan as a condition of registration. The resistance management plan for Herculex RW hybrids is similar to that for YieldGard Rootworm hybrids:

1. No more than 80% of the acreage on a farm can be planted to Herculex RW hybrids,

2. A minimum of 20% of the acreage on a farm must be planted to a hybrid not containing a Bt protein active against rootworms, and

3. The refuge must be within or adjacent to the Herculex RW field.

Additional refuge details are outlined below.

Adding another source of Bt corn with a different type of Bt protein for rootworm management will provide growers another option to increase the diversity of controls available for this important insect.

Refuge requirements

Grower agreements (also known as stewardship agreements) will specify that growers must adhere to the refuge requirements as described in the grower guide/product use guide and/or in supplements to the grower guide/product use guide.

1. Refuge size. The use of Cry34/35 corn from event DAS 59122-7 requires an accompanying 20% refuge.

2. Refuge location. The rootworm refuge is required to be planted within or adjacent (e.g. across the road) to the Cry34/35 corn field.

3. Refuge management options. The rootworm refuge may be managed in such a way that there is little or no yield loss to rootworms, but must be managed in a way that it is sufficiently productive of susceptible rootworm adults. The in-field refuge options may be planted as a single block or as a series of strips measuring at least four crop rows wide.

• Seed mixtures of Cry34/35 and refuge corn are not permitted.

• If the refuge is planted on rotated ground, then Cry34/35 corn also must be planted on rotated ground.

• If the refuge is planted in continuous corn, the Cry34/35 field may be planted on either continuous or rotated land (option encouraged where western corn rootworm rotation-resistant biotype [soybean variant] may be present).

• Application of soil insecticide is permitted in the refuge.

• Seed treatment is permitted in the refuge, either for rootworm protection or for controlling secondary soil pests.

• If aerial insecticides are applied to the refuge for control of CRW adults, the same treatment also must be applied in the same time-frame to Cry34/35 corn.

• Pests other than adult corn rootworms can only be treated with CRW-labeled insecticide on the refuge acres without treating the Cry34/35 acres if treatment occurs when adult corn rootworms are not present. Pests on the Cry34/35 acres can be treated as needed without having to treat the refuge.

• The rootworm refuge can be planted to any corn hybrid that does not express Bt proteins for rootworm control (e.g. lepidopteran-protected Bt corn, herbicide-tolerant corn, or conventional corn).

• The refuge and Cry34/35 corn should be sown on the same date, or with the shortest window possible between planting dates, to ensure that corn root development is similar among varieties.

• Growers are encouraged to plant the rootworm refuge in the same location each year, as it allows the rootworm population to remain high and the durability of the trait is extended. This option may be preferable to growers who wish to only think of their refuge design once and for those who grow continuous corn. However, for those growers who need to employ crop rotation, a fixed refuge would be impractical.

For more information

Bacillus thuringiensis Cry34Ab1 and Cry35Ab1 proteins and the genetic material necessary for their production (plasmid insert PHP 17662) in Event DAS-59122-7 corn, Fact Sheet, published by the U.S. Environmental Protection Agency at www.epa.gov/pesticides/biopesticides/ingredients/factsheets/factsheet_006490.htm

Murt McLeod, Tom Doerge and Steve Butzen. 2005. Pioneer® Brand Hybrids with the Herculex® RW Trait, Crop Insights Vol. 15, No. 13. www.pioneer.com/usa/agronomy/insects/hx/hxrw.htm

Mycogen Seeds Announces New Herculex® RW Grain Corn Hybrids:, www.dowagro.com/mycogen/resource/grain/releases/20051006.htm

Bob Wright
Extension Entomologist