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Refuge in corn fields still a concern

Acceptance is better than before.

March 10, 2008  By Ralph Pearce

When Bt corn became commercially available in the mid 1990s, the yield advantages were clear, even if the requirements for aspects like refuge, minimum distance and record keeping were not. Ten years later, with some significant advancements in breeding and technology, grower aptitude in planting these hybrids has advanced as well. It is to the point where there is more than a passing familiarity with pest management practices concerning the use of Bt and now, corn rootworm (CRW) Bt corn hybrids.

Shotholes, one of the many signs of European corn borer damage, indicate continued vigilance is needed, in spite of relatively high rates of compliance with refuge crops.

Growers today are well acquainted with the primary requirements for growing genetically modified corn as a tool to manage existing populations of European corn borer (ECB), and are becoming aware of the increased risk of corn rootworm resulting from an increase in corn on corn with the biofuels trend.

Far from becoming complacent however, the industry as a whole is working to further acceptance of the requirements, in advance of newer hybrids with stacked genetic traits. Predictions of a day in which refuge crops will be unnecessary or where a glyphosate tolerant hybrid could be used in place of a conventional non-Bt hybrid are cause for concern by some.


The concern is rooted more in the ultimate impact of newer genetically modified (GM) events. Despite their insistence, science and industry can provide solutions that appear to be sound and lasting. But nature’s ultimate impact can be far more random and severe.

Acceptance and reluctance
There are several types of Bt, each controlling a specific group of insects. Some control beetles while others control larvae of the moths and butterfly group, Lepidoptera. The advent of ECB Bt technology has been reliant on a few single Cry proteins, which cause a disruption in the epithelial layer of the gut in some members of the Lepidoptera group, namely European corn borer and corn earworm larvae.

The theory behind establishing refuge and the associated guidelines was to slow development of resistance against those single Cry proteins. “The original refuge was set conservatively to
consider all the possible options, and it was based on science, too,” insists Tracey Baute, field crops entomologist for the Ontario Ministry of Agriculture, Food and Rural Affairs at Ridgetown. “We realized that we can’t take a risk, and it’s a good idea to establish resistance management strategies so that the insect does not become resistant to one mechanism of control. We all agreed that 20 percent refuge fit the bill, and it was easy enough and flexible enough for the grower to work it into his plan.”

Since its establishment, there has been some reluctance to planting refuge: initially growers relied on their neighbour’s conventional, non-Bt fields for their requirements. However, the Canadian Food Inspection Agency (CFIA) quickly joined the international scientific community, recognizing the need for six primary guidelines for Bt corn and pressed seed companies to carry that message to growers.

In the US, some researchers, including Dr. Kevin Steffey of the University of Illinois at Urbana-Champagne, have voiced their frustration at growers who have admitted they plan on or continue to plant 100 percent Bt hybrids. Dr. Marlin Rice of Iowa State University also has warned against abandoning corn refuge, echoing Steffey’s comments about preserving the technology responsibly. His commentary appeared as a guest column in the University of Guelph-Ridgetown Campus weekly newsletter in April 2007. Others in the US and Canada have suggested the 20 percent rule, and the other requirements, are based only on theory, not empirical data, thus questioning their value.

However, with the introduction of CRW Bt hybrids, and with stacked trait hybrids set to become a new standard, the theory behind refuge may become more pressing. Future hybrids are expected to have two Cry proteins that control ECB and two other Crys that control rootworm, all in one hybrid. Despite the principle of using two Cry proteins against the same pest at one time to reduce the risk of resistance, Baute maintains much is unknown about how these pests will adjust.

Corn rootworm has been in Ontario since the 1970s but has been controlled largely through rotation, which is why it is such a concern with talk of corn on corn or continuous corn crops.

“The seed trade is going to be coming out with these multiple stacked products that are going to be used even in fields that only have one of these pests as a problem. But both insects are being exposed to Bt,” she explains. In theory, with two different modes of action, the insect is less likely to develop resistance. “But it can happen and a lot of the work isn’t done yet. We need to know whether that resistance can occur. Until we have both answers, we’re just guessing at the theory because you never know with nature. It tends to adapt to survive rather than go extinct.”

Project now underway
It is of vital importance to the seed trade that any and all questions are answered, says Gary Bauman, manager of technology and agronomic services for Syngenta Seeds Canada. On the one hand, he lauds the efforts of growers to implement the requirements for growing refuge crops, and he believes that most growers who are not complying are likely doing so because of miscalculations or time constraints coupled with shifts in acreages at planting.

Based on the most recent industry survey from 2007, most were found to be in compliance with refuge requirements. Where the numbers did drop were in areas of southwestern Ontario where growers were new to planting corn; compliance there dipped from roughly 85 percent to 65 percent. In light of word from the CFIA that 2008 will be a year they are monitoring fields in Ontario, Quebec and Manitoba, Bauman says there will be a renewed effort in communications and education to recognize the need for greater diligence with refuge crops.

However, Bauman also notes there is a considerable learning curve with the newer CRW and stacked traits hybrids, an example of which is the minimum distance requirement. “If the grower is just planting corn borer technology, then the guidelines on refuge percentage, the distance and the record keeping, they’re all pretty clear,” says Bauman. “Now that they have the rootworm trait, the 20 percent refuge still stands, but now when we start talking about proximity, the distance, the 1000 feet, is no longer sufficient.”

In fact, for growers with CRW Bt hybrids, the simplest solution will be to plant their refuge crops in the same field. The shift in that particular guideline is due to the fact that rootworm adults tend to stay in the same field where they emerge to mate. “The next best thing which is acceptable is that the refuge is planted in an adjacent field,” states Bauman. “You do not want any part of the field that has CRW Bt corn in it where the adults emerging will have more than a quarter of a mile before they hit a refuge area. It’s pretty clear that’s something the industry and growers have to work better at and not just adopt the ECB guideline.”

Another difference with CRW Bt hybrids is that growers can use Poncho 1250, Cruiser 250 or a granular application of Force on the refuge crop, as an insurance against surviving emerging adults. The current guidelines for ECB Bt hybrids make no such allowances.

Low, medium and high dose
As part of the learning curve where CRW technology is concerned, the ceiling has yet to be reached in terms of potency. According to Bauman, the VT YieldGard hybrids from Monsanto, Pioneer’s Herculex and Syngenta’s Agrisure RW hybrids are all at medium strength. “They control the first instar very well, with some effect on second instar feeding, but generally the later instars will survive feeding, so you can have as many of the numbers that they have in the grower’s guide,” he says. “Five percent of the larvae that were hatched and feeding in there could emerge as adults. The fact you can find emerging adults of corn rootworm in a Bt field is a reality and therefore, it’s very important to have that refuge principle in effect, so that you get the mixing of the populations on an ongoing basis.”

Feeding on main roots and smaller root mass are something to be avoided, hence the heightened awareness surrounding use of CRW Bt corn hybrids.

Bauman adds that current corn borer technology is all high dose, with very few survivors, where later feeding might occur because the protein has been deactivated as a result of drought. The other instance might be a later instar from a refuge crop, surviving and moving into the field to feed.

Management a multi-faceted matter
The idea of managing for CRW is dependent on other factors, as well. Alan McCallum, an independent crop consultant from Iona Station, Ontario, believes the advancing technology of stacked traits and enhanced hybrids will usher in the need for much more detailed planning on the part of the grower. Faced with higher prices, parity in currencies and speculation of tightening supplies in wheat and corn, McCallum sees the increasing demand for better management and planning.

“There’s going to be a lot of thinking around corn on corn going in to 2008, and what are the considerations we have to make, particularly with insect tolerance and herbicide tolerance, and whether we’re worried about taking volunteer corn out of a corn crop,” says McCallum. “Certainly we have a lot of tools available, and the fact that we have both genetic resistance and seed treatment control for rootworm helps, but it takes more planning to make it work.”
One aspect that could help further the acceptance of refuge would be to ensure availability of the same base genetics and resulting yield potential to refuge hybrids. “From a grower’s standpoint, they would love to see the refuge-type hybrids being available in the premium genetics,” suggests McCallum. “Seed supply is such that we’re seeing a larger yield penalty because the only refuge hybrids that are available in some maturities are the older genetics, so we end up paying for that lack of technology and a lack of the premium genetics, and compliance becomes more difficult.”

Be certain the pests are present
Of course, increased scouting and diligence as to the presence of the pests targeted by the technology would also help. According to Baute, a sense of complacency may be setting in, given the technology that is now available to growers. “The hybrids are getting so good now and with the agronomics, some growers don’t even feel the need to go out and make sure it fits their needs,” she says. The first order of business for any grower should be to confirm that corn borer and rootworm are both pests on the farm. Otherwise, she adds, focus on the one pest that is present and find the best agronomics for the hybrid that applies to the conditions on that farm.

Without taking such basics into consideration, growers can be mixing up which traits and guidelines to use, leaving themselves ill-equipped to properly manage corn borer or rootworm. “That’s what scares me, because if it gets really inconvenient, some growers may stop planting refuge and that’s not the goal for anyone here,” concedes Baute. “And CFIA is watching and there are flags for some potential problems there.” Still, Baute is encouraged by the acceptance of refuge requirements and the successful use of Bt technology for more than a decade. The unknown factor in 2008 is the development of rootworm and its potential for resistance.

From a private sector perspective, however, Bauman points out research work being done by the Corn Pest Coalition. With the help from Dr. Mark Sears of the University of Guelph, along with OMAFRA staff, Syngenta, Monsanto, Pioneer and Dow, this co-ordinated effort is trying to develop an effective protocol for corn rootworm larval feeding and assessing injury levels. The project would then determine the economic injury levels of that feeding and whether resistance is developing. “That’s a big project underway right now within the industry,” says Bauman. -end-

Requirements for management of Bt and CRW corn hybrids

The Canadian Corn Pest Coalition (consisting of an expert panel of Canadian researchers), in concert with the Canadian Food Inspection Agency, the Ontario Ministry of Agriculture, Food and Rural Affairs, the Ontario Corn Producers’ Association and seed industry representatives, have agreed on these requirements for planting Bt corn.

Requirements for management of ECB Bt corn hybrids:
1. Plant a minimum of 20 percent of total corn acreage to non-Bt refuge.
2. Refuge must be within 400 metres (roughly 1000 feet or 1⁄4 mile) of Bt corn.
3. Keep Bt and non-Bt seed separate at planting to decrease chance of resistance.
4. The use of ECB insecticides is not permitted in refuge.
5. Refuge hybrids should be of the same maturity as Bt hybrids.
6. Growers must keep careful and accurate planting records, including location of Bt and non-Bt hybrids, for monitoring of insects and hybrid performance.

It is also strongly advised that growers scout Bt corn for the presence of European corn borer feeding. Where there is greater than five percent ECB feeding, contact the seed company agronomist (procedures exist to assess possible resistant populations).

Requirements for management of CRW Bt hybrids:
1. Plant a minimum of 20 percent of total corn acreage to non-Bt refuge.
2. Refuge must be adjacent to or within CRW Bt field.
3. Refuge may be treated with CRW larval control with soil applied or seed insecticides (provided economic thresholds warrant their use). Insecticide use for adult control is not permitted.
4. CRW Bt corn and refuge planting must be owned or managed by the same grower.
5. If refuge is treated for other late season insect pests such as corn borer, then the CRW Bt field must be similarly treated.
6. Where adjacent field option is used, CRW Bt corn and refuge fields should have the same
rotation histories and maturities.
7. Mixing CRW Bt and non-Bt corn seed at planting is not permitted.

More information is available from the second edition of the publication, A Grower’s Handbook, on-line at: -end-

*Article used with the permission of the Canadian Corn Pest Coalition.


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