Changes to Bt Corn Refuge Requirements

Tracey Baute, Field Crop Entomologist, OMAFRA-Ridgetown

Anyone planting Bt corn this spring will need to plant a non Bt corn refuge too. Though surveys and field inspections done by the Canadian Food Inspection Agency (CFIA) are showing that approximately 80% of Canadian corn growers are following through on the Insect Resistance Management (IRM) requirements for Bt corn, there is always room for improvement. Hopefully with the changes made to refuge planting strategies by the Canadian Corn Pest Coalition, approved by CFlA, it will make following through on these IRM
requirements a little easier.


REFUGE SIZE

Whether planting Bt corn for European corn borer (ECB) control, corn rootworm (CRW) control or both using the newer stacked products, 20% of the total corn acreage must be planted to non Bt to reduce the risk of these insects developing resistance to Bt. Nothing has changed about that rule and the longer we intend to use Bt technology, the more important it is to follow through on planting a sufficient size of Bt refuge.

REFUGE PLACEMENT

There are now several types of Bt corn available to you, including those that control ECB alone, CRW alone, or both ECB and CRW Where the refuge can be planted depends on the insect that is being targeted. Corn borer and corn rootworm have very different mating behaviours and other factors that require us to set different distances for the refuge requirements for Bt corn for ECB control compared with Bt corn for CRW control. Corn borer can fly up to a 1/4 mile or 400 metres outside the corn field they originated from to find a mate. So there is a good opportunity for the moths coming from Bt and non Bt plantings to mix and mingle and dilute the resistant gene pool so to speak, as long as there is a refuge planted within 400 metres of the Bt field. Corn rootworm has a very different "dating" scene. Instead, the adult male beetles emerge from the soil first and walk along the soil searching for female adults in the immediate area, usually without leaving the field they originated in. This requires us to bring mates that would not have been exposed to Bt corn closer to them to help dilute the resistant gene pool. For CRW Bt corn, the refuge needs to be planted adjacent to or within the Bt corn planting. This includes the stacked products that control both ECB and CRW because we need to ensure that we are getting the rootworms close enough together to mate.


Refuge Planting Options include Whole Field (top left), Headlands (top right), Blocks (bottom left) and Strips (bottom right). If planting refuge in
strips, each strip must be a minimum of 4 rows or wider.

REFUGE PLANTING SCHEME

Which planting scheme you use really depends on convenience and which option will ensure that you are following through on the distance requirement necessary for the type of Bt corn you are planting. Entire fields can be planted as refuge, as long as the field is your own and not your neighbour's, and as long as it is within the correct distance requirement. Some growers like to plant their headlands as the refuge for ease of combining, as long as they are following through ensuring the headlands are making up 20% of their acreage. Others like to plant in blocks within the same field. Both blocks and headland schemes work really well as the refuge for CRW Bt or stacked CRW and ECB Bt corn, as they ensure that the refuge is within the same field as the Bt planting.

Where we hope to help make it a little easier is if you prefer to plant the refuge as alternating strips of Bt and non Bt hybrids within the same field. Before now, the width of the refuge strips were required to be at least 6 rows or wider. With a few more years of research under our belts on corn borer and rootworm and their potential to develop resistance, the Canadian Corn Pest Coalition has agreed on relaxing a little about the width of the refuge strips required. The strips can now be four corn rows wide or wider to be considered an effective refuge. This should help those growers who had smaller planters or field widths to be able to try this type of planting scheme. However, strips of less than 4 rows wide are not recommended because they increase the chance of resistance.

Of course, no matter which field scheme used, you must ensure that you are following through on all of the IRM requirements for that type of Bt corn. The current IRM requirements for each Bt corn type are as follows:


Insect Resistant Management Requirements for ECB Bt Corn

· Minimum of 20% of total corn acreage planted to corn not having the Bt trait for ECB control;
· Refuge must be planted within 400 metres (1/4 mile) of the ECB Bt planting(s) ;
· Do not mix ECB Bt and non-Bt seed at planting because it increases the chance of resistance;
· The use of ECB insecticides is not permitted in .the refuge planting;
· Refuge and ECB Bt corn hybrid must be of similar maturity (within 100150 CHU) to be equally attractive to the adult moths;
· Keep accurate records of corn field planting(s) .

Insect Resistant Management Requirements for CRW Bt Corn (YieldGard or Herculex RW)

· Minimum of 20% of total corn acreage planted to non Bt refuge
· Refuge must be adjacent to or within the CRW Bt corn field;
· Refuge may be treated for CRW larval control with soil-applied insecticides if economic thresholds prescribe it. Insecticide use for adult
x control is not permitted;
· CRW Bt corn plantings and refuge must be owned or managed by same grower;
· If refuge is treated for other lateseason insect pests such as corn borer, then CRW Bt corn field must also be similarly treated;
· If the adjacent field option is used, CRW Bt corn and refuge fields should have the same crop rotation histories and maturities;
· Mixing CRW Bt and non-Bt corn seed at planting is not permitted;
· Keep accurate records of corn field planting(s) .


Insect Resistant Management Requirements for CRW Bt Corn (YieldGard Plus or Herculex Xtra)
· For YieldGard Plus or Herculex Xtra Bt corn (containing both ECB and CRW controlling proteins), follow the same insect resistant
x management requirements as CRW Bt corn (directly above), with the exception that the refuge can not be treated for corn borer at any
x time.
· Refuge must be adjacent to or within the YieldGard Plus or Herculex Xtra corn field.

If you are looking for more information on Bt corn and resistance management, including an online copy of the newest edition of the publication "A Grower's Handbook; Controlling Corn Insect Pests with Bt Technology," visit the Canadian Corn Pest Coalition Website at www.cornpest.com.