Bt Refuge is More Important than Ever!

With 10 years of experience to go on, Bt corn has proven to be an effective control measure that has practically eliminated the need for foliar insecticide use on field corn, changing how we control a primary pest in corn production. If resistance to Bt was to develop in our corn borer population, it would cause a major shift back to the more intensive crop management practices relied on in the past. So far no case of resistance by ECB to Bt corn has been reported, indicated that the insect resistance management strategies have been effective.

Now there are new Bt corn options, with hybrids that contain a different Bt that specifically controls corn rootworm larvae. And like buying a car, you can even get the "all options in one" package, the stacked hybrids containing both types of Bt to control both ECB and rootworm. But with progress comes risk. Unfortunately, corn rootworm has a history of successfully adapted to its control measures, developing resistance to certain soil insecticides, foliar insecticides and even crop rotation. Now that we are exposing it to transgenic Bt corn, resistance management strategies need to be followed and taken very seriously if we want to continue to use Bt corn in the future. But given all of the options now for Bt corn, the refuge requirements can get a bit confusing. Hopefully this article can help make it easier to understand.

20 Percent Rule for ALL Bt hybrids

One common rule for ECB Bt corn, rootworm Bt corn, or the stacked traits for controlling both (Herculex XTRA or YieldGard Plus) is that 20% of your total corn acreage needs to be planted as non Bt refuge. You can not rely on your neighbour's fields to be your refuge. It is your responsibility to ensure the proper use of Bt technology, which includes following resistance management strategies to ensure the longevity of the technology.

Refuge Location - Why ¼ mile away or within?

The whole point of a refuge is to increase the likelihood of mingling and mating amongst individuals that survived feeding in Bt fields with individuals from the non Bt refuge that have never been exposed to Bt so they can mate with each other and dilute the resistant traits within the gene pool. To complicate things, ECB and rootworm have very different mating behaviors, forcing us to follow different refuge requirements.

ECB females venture out of corn fields and find weedy areas along ditch banks and fields to find a mate. They fly up to ¼ mile away to find their mate. There, they mingle and mate and then fly into an ideal corn field to lay their eggs. This is why the non Bt refuge for ECB can be planted as far as (but not farther than) a ¼ mile from the Bt corn field.

Unfortunately things are different for rootworm. Male rootworm adults emerge out of the soil first. They walk along the soil surface and are able to detect where females are underground and about to emerge. They sit and wait for the females to come out and mate with them nearly on the spot or within that same area. The female rootworms do not leave the area until they have mated, decreasing the opportunity for them to mix and mingle with other individuals who have not fed on Bt yet.

Figure 1. Corn Rootworm Adults mating

Research has proven that some rootworm adults already do emerge alive from rootworm Bt corn. If the rootworms do not feed on the roots of the Bt hybrid until they are older/bigger larvae, they are less susceptible to the Bt and can survive. It is therefore crucial that the non Bt refuge is planted within or adjacent to the rootworm Bt cornfield, to help encourage mingling and mating among the individuals that come from the Bt and refuge areas. The refuge can be separated by a road or ditch from the Bt field but not by another field or larger obstruction.

With the stacked traits (Herculex XTRA or YieldGard Plus), the non Bt refuge must be planted within or adjacent to the Bt field, ensuring there is a opportunity for rootworm to mingle. ECB will be fine with this arrangement too since they again will fly out of the field and meet in "action sites" along field edges. As long as the refuge was planted within their flying zone, they are likely to mix and mingle. But rootworm adults need a little help from us to introduce them to individuals not exposed to Bt yet.

Refuge Configurations - How Can you Make it Work?

Within-Field Refuge Options

As long as there is a comparable non Bt hybrid available (ie. same maturity, good agronomics etc) to the Bt hybrid being planted, most growers prefer to plant the refuge within the same field as the Bt. This fits the bill for both rootworm and ECB requirements whether you plant ECB Bt, rootworm Bt or hybrids with the stacked Bt traits. Within field refuge options include Strips (at least 4 rows or wider), Blocks and Headland/Perimeter plantings (see Figure 2)

Figure 2. Within field refuge options include strips (at least 4 rows or wider), Blocks and Headland/Perimeter Plantings

Going Outside the Bt Field

When planting the refuge outside of the Bt field, distance is important. For ECB Bt corn, the refuge can be planted in any other field that you own as long as it is within ¼ mile from that field.

For rootworm Bt or the stacked Bt hybrids the refuge must be planted adjacent to the Bt field. This means that the field that borders or is directly across the road from the rootworm Bt field must be planted with a hybrid that does not contain the Bt gene to control rootworm. That field also needs to have the same crop history as the rootworm Bt field, making it just as likely to have rootworm currently present in the soil as the Bt field doe (see figure 3).

Figure 3. Diagram of Potential Refuge Option for Rootworm Bt or Stacked Bt Hybrids

If the Bt field (field on the left in diagram) is going to be rootworm Bt only (not stacked with both Bt traits) the adjacent field could be planted with ECB Bt corn since it won't have any effect on rootworm and can act as the refuge for the rootworm, but then another non Bt refuge field will need to be planted within ¼ mile of this field to act as the refuge for the ECB Bt field (see Figure 4)

Figure 4. BT Field: rootworm Bt only; ECB BT Corn, Non Bt Refuge Field

Regardless of which configuration you decide to use, it is your responsibility to ensure that it meets all refuge requirements to ensure that resistance does not occur so that Bt technology is here for the future.

For more information on Bt corn and Insect Resistance Management, visit the Canadian Corn Pest Coalition website at www.cornpest.ca

Related Links

• Canadian Corn Pest Coalition

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