Practical Considerations for Bt Refuge Management
Over the past few months nearly all stakeholders in Bt technology have come together to agree on the principal criteria for an effective Bt refuge. This has included an across-the-board acceptance in both Canada and the U.S. that the level of non-Bt corn refuge acreage should be set at 20 per cent. Recent studies in New York and Pennsylvania have shown that plant species other than non-Bt corn do not provide sufficient Bt refuge. We are in a similar situation in Ontario; the only effective host to ensure adequate refuge against the build-up of Bt resistance in the corn borer population is non-Bt corn.
The other issue and perhaps much more problematic from a practical sense is the location of this 20 per cent refuge area relative to Bt corn. Movement of corn borer adults between Bt and non-Bt corn is important for genetic mixing of resistant and susceptible types. This genetic mixing reduces the chances of inbreeding among resistant moths and thus significantly delays resistance development. The concern is that these moths do not fly great distances. In the case of second generation corn borer, research suggests the adult females stay within 1,500 feet from where they emerge. The implication is that all Bt corn needs be within one quarter-mile (0.4 km) of a non-Bt refuge for the refuge to be effective.
Besides size and proximity of the refuge, the other factor in slowing resistance is that Bt corn plants need to offer a high dosage of the Bt toxin to the feeding corn borers. Not all Bt events are the same in satisfying this high dose strategy. When hybrids of the major Bt events were exposed to resistant borers, the damage to corn plants in events 176 and DBt 418 was significantly greater than in the other events. These borers, developed at Kansas, were from a population that became 70 times more resistant than wild borers after being exposed to Dipel (a Bt spray) for 18 generations. There were also some larvae surviving in these two events. There are two conclusions that we draw from this evidence. The first is that the risk of developing resistance to Bt is high and the second is that a high dose is clearly necessary to avoid survival of partially resistant borers.
Since both the value of Bt technology and the threat of resistance are real, producers need to give some consideration on how to best deploy Bt refugia on their own farms. Here are some points to consider:
1) Whole Field
In terms of simplicity, planting whole fields to non-Bt hybrids represents the best option. Emptying seed boxes and switching hybrids is minimized with this approach. However, as pointed out earlier 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.
Many field sizes and configurations will make the whole field option impossible and require that a specific block or portion of a field be planted as non-Bt refuge. This means some interruption of planting to switch hybrids mid-field, but would represent an effective corn borer refuge. One potential problem in this arrangement is the possibility of having a non-Bt hybrid land locked by the main Bt hybrid of the field. In some seasons the idea of having to harvest a wetter, non-lodging Bt hybrid in order to get at a non-Bt hybrid that is drier, suffering from corn borer damage and at risk of lodging may not be a very attractive option!
Planting non-Bt hybrids on the headland areas of a field may represent an effective refuge strategy. Some quick calculations indicate that 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 would represent 6.5 acres or slightly over 19 per cent refuge. Other considerations regarding headland refugia are: 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 fence rows 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.
Planting continually alternating strips of Bt and non-Bt hybrids can be an effective refuge. This approach will eliminate the need to dump planter boxes and switch hybrids over the course of a field or farm but will be restricted to planters with individual row hoppers. The key factor in these strips is that they need to be at least six corn rows wide to offer a suitable refuge. This option therefore becomes impossible for those producers with four-row planters. Producers with six-row planters could in fact dedicate three rows to Bt and three rows to non-Bt. However, if you are in an area where corn borer pressure is traditionally high and your goal is to plant as much Bt corn as possible, you are now restricted to the 50 per cent level. The obvious candidates for these alternating strips are those producers with eight- or 12-row corn planters. Dedicating three end row units to non-Bt refuge hybrids will effectively give you 38 per cent (eight-row planter) and 25 per cent (12-row planter) of your corn acreage as refuge. For reasons mentioned earlier this more intimate pairing of hybrids should coincide with additional care in selecting refuge hybrids that will be very similar to your Bt hybrid in terms of maturity, resistance to lodging, etc.
Corn producers are obviously the key to the successful implementation of any of these Bt refuge strategies. Hopefully the recent consensus among government, industry and producer associations on Bt refuge issues is a sign that all players will continue to contribute to the successful stewardship of Bt technology.