Practical Considerations for Bt Refuge Management
Recent Developments
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.
Practical Approaches
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.
2) Blocks
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!
3) Headlands
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.
4) Strips
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.