Insect Resistance Management when Planting Bt Corn Hybrids

Eileen Cullen, Extension Entomologist

Planning to plant Bt corn hybrids this spring? If so, growers and consultants are required by law to have an insect resistance management (IRM) plan in place. The IRM plan is implemented by planting refuge corn acres on each farm where a Bt corn hybrid is planted. Refuge corn acres are hybrids that do not contain the Bt insect trait used in the Bt planting. The refuge must be planted to 20-percent of the corn acreage on each farm, and there are specific configuration and distance requirements.

As a condition of registration of Bt crops, the US Environmental Protection Agency requires all farmers who use Bt crops to plant at least 20-percent of their corn acreage to a refuge. The aim of this strategy is to provide an ample supply of insects that remain susceptible to the Bt toxin. The non-Bt refuge will decrease the odds that a resistant insect can emerge from a Bt field and choose another resistant insect as a mate. By preventing the pairing of resistant genes, these refuges help ensure that susceptibility is passed on to offspring.

To implement EPA’s IRM refuge requirement, the Bt corn registrants (e.g., Monsanto, Dow AgroSciences, Syngenta, and Pioneer Hi-Bred) enter into a contractual agreement with every farmer who buys Bt corn that obligates the farmer to plant the appropriate refuge. EPA also requires the Bt corn registrants to establish a compliance assurance monitoring program (CAP) to identify and address noncompliant farmers. If a farm is found to be out of compliance, a grower will receive a warning from the seed company.  If the farm is out of compliance for a second year, the seed company will refuse to sell Bt corn to the grower. Field inspections may be performed by seed companies through the compliance assurance monitoring program. The purpose of the IRM refuge requirement is to maintain efficacy of Bt crops as an insect pest management tool by preventing or delaying development of insect resistance to these traits.

The remainder of this article serves as a primer for the 2007 season on Bt corn insect trait registrations and IRM refuge configurations. This article lists the IRM refuge requirements contained in the “grower agreement” (also called “stewardship agreement”) signed by each grower at the time of purchase of Bt corn seed.  Please consult the grower guide/product use guide from your seed company for full details.     

Lepidopteran Registrations - Registered Bt insect traits for field corn in 2007 include YieldGard® Corn Borer (Cry1Ab) and Herculex I® (Cry1F) for Lepidopteran control. Both traits protect corn against European corn borer, southwestern corn borer, sugarcane borer, and southern corn stalk borer; and provide corn earworm and fall armyworm suppression. Herculex I also includes western bean cutworm and black cutworm on its label.

• On each farm no more than 80-percent of the corn acres may be planted with a Bt Lepidopteran hybrid.
• 20-percent of total corn acres must be planted to a corn refuge that does not contain a Bt technology for control of any of these “caterpillar” pests.
• The refuge may be a separate field within ½-mile, but must be on the same farm and managed by the same grower. Within field refuge configuration options include a large block refuge, split planter to alternate four or more rows of non-Bt Lepidoptera corn, or plant the field perimeter four or more rows wide to meet the 20-percent refuge.
• The refuge corn can be treated with insecticide only when the level of pest pressure meets or exceeds economic threshold, and foliar Bt microbial sprays cannot be applied in the refuge.
• One type of Bt lepidopteran corn hybrid (e.g., YieldGard Corn Borer) cannot serve as a refuge for another group (e.g., Herculex I).

Corn Rootworm Registrations – Registered Bt insect traits for field corn in 2007 for northern, western, and Mexican corn rootworms include YieldGard® Rootworm and YieldGardVTÒ Rootworm (Cry3Bb1), Herculex® RW (Cry34Ab1/35Ab1) and Agrisure® RW (mCry3A).

• On each farm no more than 80-percent of the corn acres may be planted with a Bt corn rootworm hybrid.
• 20-percent of total corn acres must be planted to a corn refuge that does not contain a Bt technology for control of corn rootworms.
• The refuge must be planted within or directly next to each Bt corn rootworm field. The corn rootworm refuge cannot be separated by another field, it does not have the distance flexibility of the corn borer refuge.
• Within field refuge configuration options include a large block refuge, split planter to alternate four or more rows of non-Bt corn rootworm corn, or plant the field perimeter four or more rows wide to meet the 20-percent refuge.
• If the corn rootworm refuge is planted on rotated ground, then the Bt corn rootworm hybrid must also be planted on rotated ground.
• If the refuge is planted in continuous corn, then the Bt corn rootworm corn may be planted on either continuous or rotated ground.
• The corn rootworm refuge can be treated for control of corn rootworm larvae and other soil insect pests with soil-applied, seed-applied or foliar-applied insecticides.
• The corn rootworm refuge can be treated with a non-Bt foliar insecticide to control late season pests such as corn borer, however if adult corn rootworm (beetles) are present at this time, then the Bt corn rootworm hybrid field must be treated in a similar manner.

Stacked Lepidopteran + Corn Rootworm Registrations – Stacked Bt insect traits for field corn in 2007 include YieldGard® Plus and YieldGardVT® Plus (Cry1Ab + Cry3Bb1), Herculex® XTRA (Cry1F + Cry34Ab1/Cry35Ab1), and Agrisure® CB/RW (Cry1Ab + mCry3A).

Two different refuge planting options are allowed to meet the IRM requirements for stacked Bt insect trait corn hybrids. These options include a Common Refuge or Separate Refuge. For either refuge option below, if the refuge is planted on rotated ground, then the stacked Bt corn hybrid must also be planted on rotated ground. If the refuge is planted on continuous corn ground, then the stacked Bt corn hybrid may be planted on either continuous or rotated ground.

Common Refuge

• Plant one common refuge for both Lepidopteran insects and corn rootworms. The common refuge must be planted with corn hybrids that do not contain Bt technology for control of either rootworms or corn borers (non-Bt corn).  

• The refuge area must represent at least 20-percent of the grower’s corn acres (i.e, sum of stacked Bt corn and refuge acres).
• Refuge must be within or directly next to each stacked Bt corn hybrid field. It cannot be separated by another field, and does not have the distance flexibility of a corn borer only refuge.
• Refuge can be planted as a large block, split planter to alternate four or more rows of non-Bt corn, or plant the field perimeter four or more rows wide.
• The common refuge can be treated with a soil-applied, seed-applied, or foliar-applied insecticide to control rootworm larvae and other soil insect pests.
• The common refuge can also be treated with a non-Bt foliar insecticide for late season pests such as European corn borer, if pest pressure reaches economic threshold. However, if rootworm adults (beetles) are present at the time of foliar applications then the stacked Bt corn field must be treated in a similar manner.

Separate Refuge

• Plant a separate refuge for corn borers; and a separate refuge for corn rootworms.

• Corn Rootworm Refuge: 20-percent of total corn acres must be planted to a corn refuge that does not contain a Bt technology for control of corn rootworms; but can be planted with Bt corn borer hybrids.
• The corn rootworm refuge must be planted within or directly next to each stacked Bt corn hybrid field (e.g., large block, split planter or perimeter strips four or more rows wide).
• The corn rootworm refuge can be managed for corn rootworm larvae and other soil insect pests using a soil-applied, seed-applied, or foliar-applied insecticide.
• The corn rootworm refuge can be treated with a non-Bt foliar insecticide for control of late season pests such as corn borers if pest pressure reaches economic threshold. However, if rootworm adults are present at that time, then the stacked Bt hybrid field must also be treated.

• Corn Borer Refuge: 20-percent of total corn acres must be planted to a refuge that does not contain Bt-Lepidopteran protection, and must be planted within ½-mile of the stacked Bt corn field.
• The corn borer refuge can be treated with a soil-applied or seed-applied insecticide for corn rootworm larval control or other soil insect pests.
• The corn borer refuge can be treated with a non-Bt foliar insecticide if economic thresholds for late season pests such as corn borer are met; the stacked Bt corn hybrid would NOT have to be treated in a similar manner under this option.

References

USEPA. 2005. Bacillus thuringiensis Cry3Bb1 Protein and the Genetic Material Necessary for its Production (Vector ZMIR13L) in Event Mon 863 Corn & Bacillus thuringiensis Cry1Ab Delta-Endotoxin and the Genetic Material Necessary for its Production in Corn (006430, 006484) Fact Sheet.

http://www.epa.gov/pesticides/biopesticides/ingredients/factsheets/factsheet_006430-006484.htm

YieldGard  2007 IRM Guide.

http://www.monsanto.com/monsanto/us_ag/content/stewardship/irm/2007/yieldgard.pdf

Product Use Guides for Hybrids with Herculex® and YieldGard® Traits

http://www.pioneer.com/web/site/portal/menuitem.6cc64f075e4174bc81127b05d10093a0/