Bt Corn for Rootworm Brings Changes in Refuge Requirements

Growers planning to plant the new Bt corn hybrids for corn rootworm control this spring will see a few changes in refuge requirements from those in place for Bt corn hybrids for the control of European corn borers.

In 1997, the Canadian Corn Pest Coalition developed an Insect Resistance Management strategy to minimize the potential development of resistance to Bt corn hybrids by European corn borer. Part of the strategy requires growers who grow Bt corn to plant refuge areas of non-Bt corn to provide a safe haven for Bt-susceptible corn borers. This ensures that some European corn borer moths that have not fed on Bt corn are within close proximity of potentially resistant moths from the Bt field for mating. The genetic mixing of susceptible and resistant borers decreases the potential for fully resistant populations to develop.

The refuge strategy for Bt corn hybrids for the control of corn rootworm is very similar to the strategy for Bt corn hybrids for the control of European corn borers. The table below compares the refuge requirement specific to European corn borer Bt Corn and corn rootworm Bt Corn. The main difference between the two strategies is the proximity of the refuge (non Bt hybrids) to the Bt hybrids and option to use insecticide treatments. These differences are as a result of the differences in behaviour of these two insects.

Refuge Strategies

Specific for European Corn Borer Bt Corn

• Plant at least 20% of the total corn acreage to non-Bt hybrids;
• Refuge of non-Bt hybrids must be planted within 400 metres (1/4 mile) of the Bt hybrids, implemented as complete fields, blocks, headland or alternate strips that are at least six rows wide;
• The use of ECB insecticides is NOT permitted in the refuge planting;
• Do not mix Bt and non-Bt seed at planting because of the it increased risk of resistance development;
• Refuge and Bt corn hybrids must be of similar maturity.

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Specific for Corn Rootworm Bt Corn

• Plant at least 20% of the total corn acreage to non-Bt corn rootworm hybrids;
• Refuge must be planted within or adjacent to the Bt corn rootworm hybrids; planting options include non-Bt corn rootworm hybrids planted either as in-field strips, at least six rows wide or as external blocks or fields immediately adjacent to Bt rootworm hybrids;
• If adjacent field option is used, Bt corn hybrids and refuge fields should have the same crop rotation histories and maturities;
• Bt corn rootworm hybrids and refuge must be owned or managed by same grower;
• Soil-applied insecticide treatments for control of corn rootworm larvae may be applied to the refuge area. Insecticide use for adult control is not permitted;
• Do not mix Bt and non-Bt seed.

Growers planting Bt rootworm hybrids can plant a common refuge for both corn borers and corn rootworms, following the Bt Rootworm refuge requirements or they can plant separate refuge areas for corn borers and for corn rootworms, following the appropriate refuge requirements for each as listed above.

Keep careful and accurate records as to where Bt and non-Bt corn has been planted for effective insect monitoring and hybrid performance evaluation purposes.

More information regarding the Insect Resistance Management strategies for Bt technologies is available at the Canadian Corn Pest Coalition web site.

Related Links

• Canadian Corn Pest Coalition website

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