Insect Resistance Management Requirements for BT Corn
Tracey Baute, Field Crop Entomologist, OMAFRA-Ridgetown and
Albert Tenuta, Field Crop Pathologist, OMAFRA-Ridgetown

Whole/Separate field - Planting whole fields to non-Bt hybrids may be your simplest option as it reduces the need for dumping seed boxes and switching hybrids in the field during planting. Keep in mind though, these whole fields must represent 20 per cent of your total corn acreage and be positioned so that they are less than one quarter-mile from any of your Bt corn.

Transgenic Bt corn hybrids have been available commercially in Canada since 1997 and have clearly proven their effectiveness in controlling European corn borer (ECB). Public and private sector scientists across North America agree, however, that there is potential for corn borer populations to develop resistance to the Bt. With this in mind, the Canadian Food Inspection Agency requires that all producers who grow transgenic crops also adopt insect resistance management (IRM) strategies to ensure their long-term performance. Following a good IRM plan will be key to the sustainability of this technology. If you are planning to plant Bt corn this year, you will need to implement an IRM plan.

Key Steps to implementing an IRM plan for European corn borer:

1. Grow Bt corn hybrids only in fields that are at risk from European corn borer.

2. Plant non-Bt corn refuge areas . If you are planning to use Bt corn this year, you will also need to plant non-Bt corn refuge areas. These areas

ensure the presence of European corn borer moths that have not fed on Bt corn, and that can mate with resistant moths coming from the Bt field. This increases the chance of genetic mixing of susceptible and resistant borers, and decreases the potential for development of fully resistant populations.

- This planting style involves continually alternating strips of Bt and non-Bt hybrids within the same field. The strips must be at least six corn rows wide to be considered an effective refuge area. Strips of less than 6 rows wide are not recommended because they increase the chance for the corn borer larvae to come into contact with both Bt and non-Bt plants. When this happens, the larvae may ingest some Bt toxin, but not enough to kill them. This will help contribute to the development of a population of larvae that are resistant to the Bt toxin.

General requirements for refuge:

. 20% of total corn acreage must be planted to non-Bt hybrids. Refuge may be planted as complete fields, blocks, headlands or alternate strips that are at least 6 rows wide.

. Bt corn fields must be planted within 400 meters (1/4 mile) of the refuge.

. Do not mix Bt and non-Bt seed as this practice actually increases the chance of resistance.

With the understanding that not all fields are alike, there are various refuge planting schemes you can follow. Whichever one you choose, it is important to ensure that your refuge area is planted at the same time as your Bt fields, and that you select hybrids for the refuge area that are similar in maturity, resistance, lodging, etc. as your Bt hybrids. This will work to your advantage at harvest time, a
s well as ensure that the moths are attracted to both your non-Bt and Bt areas. The use of ECB insecticides is not permitted in the refuge.3. Keep careful and accurate records as to where Bt and non-Bt corn has been planted for both insect monitoring and hybrid performance evaluation purposes.

- This style involves some interruption of planting to switch hybrids mid-field, but also creates an effective corn borer refuge area. One potential problem in this arrangement is the possibility of having a non-Bt hybrid "land-locked" by the main Bt hybrid in the field.

4. Scout for insects and diseases in both your Bt and non-Bt fields. Bt corn hybrids are effective at controlling European corn borer, but may not be resistant or tolerant to diseases and other corn insect pests such as wireworms, white grubs, seedcorn maggots, aphids or mites.

5. Monitor for resistance of European corn borer to Bt corn . If feeding damage is detected, investigate the cause. Verify from your field records that a Bt hybrid was planted where the surviving larva was found. If needed, get help in accurately identifying the feeding caterpillar. Notify seed company representatives immediately if you suspect a problem. The Canadian Corn Pest Coalition and the Canadian Food Inspection Agency have developed procedures to determine if resistant ECB have developed in your field.

- Depending on the dimensions of the field, headlands of 24 to 30 rows in width can generally meet the 20 per cent requirement on fields up to approximately 60 acres (again, depending on shape). For example: a field 1,500 feet by 1,000 feet has a total of 34 acres and 24 rows (30-inch row width) of headland, which would represent 6.5 acres or slightly over 19 per cent refuge area. This approach can offer some advantages: a) the non-Bt hybrid is accessible for early harvest if needed; b) the susceptible moths emerging from the headlands are in close proximity to the fencerows and grass areas where corn borer mating occurs, insuring a high probability of mixing with resistant moths, and c) if you farm in an area where outer edges of fields are less productive regardless of corn borer pressure (i.e., shade, animal damage, compaction) you effectively plant less expensive seed on those areas without sacrificing yield.

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