Responsible Deployment of Bt Corn Technology in Ontario

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Dr. Mark Sears, Professor of Entomology and Chair
Department of Environmental Biology, University of Guelph

Dr. Art Schaafsma, Assistant Professor of Field Crop Protection
Ridgetown College, University of Guelph.

Background

Drs. Sears and Schaafsma have been involved with crop protection and Bt technology for many years and published research on corn borers. They are supervising the work of Tracey Baute, a masters student, who has completed a thesis on the benefits of Bt corn to Ontario growers. They initiated the Bt Corn Coalition, a group representing the private and public sector consisting of growers, Provincial extension personnel and publically funded researchers from AAFC and UofG and seed industry representatives. The mandate of the group is to foster the responsible deployment of Bt technology in Ontario. They are also the only two Canadian representatives on the NC-205 committee. NC-205 is a United States Department of Agriculture (USDA) North Central Regional Research Committee that has conducted research on stalk-boring pests of corn continuously since 1954. It is comprised of representatives from 20 states, USDA-ARS, Mexico, and Canada. Its main focus recently has been the responsible deployment of Bt corn in North America. The primary concern for NC-205 has been the potential for development of resistance to Bt corn by the European corn borer. The initial recommendations of NC-205 regarding the management of resistance were published in North Central Regional Publication 602 during 1997. An electronic version is located at: http://www.extension.umn.edu/distribution/cropsystems/DC7055.html. Dr. Schaafsma, on behalf of the Ontario Bt coalition, attended a special NC-205 meeting in Kansas City which reviewed the initial recommendations and was charged with developing a position paper for optimal deployment and resistance management strategies for this technology. This article briefly summarizes research on Bt corn conducted by NC-205 members. The recommendations of NC-205 have been adopted by the Bt Corn Coalition and adapted for the Canadian context. This is summarized below.

Impact of Bt Corn Technology in Ontario

The availability of Bt corn hybrids marks the first opportunity to experimentally determine the impact of European corn borer on field corn by comparison of performance with the isolines of each Bt event. In this study over the past two years, yield protection provided by the Bt hybrids was generally 3 to 5 bushels per acre in areas of low infestation (0-2 cm tunnelling damage), 5 to 8 bushels in areas of moderate infestation (2-6 cm tunnelling) and 10 to 15 bushels in areas of high infestation (6-12 cm). A return to growers beyond the cost of this technology would be realized in areas sustaining moderate to heavy infestations in most years.

Generally, greatest infestations occurred in SW Ontario and greater yield losses due to corn borer were observed in fields in that area. This is likely due to the consistent two generations of corn borers that appear there each year compared with a single generation that is normally found in central and eastern Ontario. However, moderate to severe infestations were observed in some years at locations where only one generation is normally found and that light infestations frequently occur in SW Ontario.

Our data demonstrate that for every centimetre of stalk tunneling per plant by corn borer, yield is reduced by 0.007 to 0.01 bu/A, or 0.7% to 1% reduction. At high levels of infestation (10-15 cm tunnels/plant) this equals 8 to 15 bu/A loss in a typical field that averages 120-140 bu/A. At an average price of $2.50/bu this is equivalent to $20-$37.50 potential loss/A, which is greater than the premium of $9-14/A. Low infestations resulting in 5 cm or less of damage would reduce yield by 5 to 6 bu/A or $12.50-15.00/A. This value is at or just above the break even point for seed premium costs.

Some of the high performance standards chosen in the experiments within this project performed as well as the Bt hybrids, especially under low to moderate borer infestation. This does not necessarily detract from the impact of the Bt hybrids, but does indicate the effect of years of selection of standard hybrids for their tolerance of some corn borer infestation. The Bt gene technology will ultimately be transferred to all the best yielding corn hybrids and their agronomic traits plus near complete protection from corn borer attack will realize the optimum yield parameters.

Benefits of Bt corn to Ontario

Bt corn has been proven to be an excellent new technology for corn producers in Ontario. Bt corn provides effective and consistent control of European corn borer far better than insecticides, with less cost and fewer logistical, health, or environmental concerns. Bt corn has visibly demonstrated to us that the European corn borer causes significant yield loss in areas of Ontario where corn borers are abundant and present for much of the season. We have also confirmed that corn borer protection and protection against related secondary problems are the only benefits of Bt corn, and that this technology must be incorporated in top hybrids. Many top hybrids already have a small level of protection and perform as well or better than some Bt hybrids under low to moderate borer pressure. Bt corn has insurance value by reducing risk of yield loss from European corn borer in these areas, plus we have demonstrated that fusarium ear and stalk rots related to corn borer damage are also controlled. Buying Bt corn at a premium should be viewed as buying insurance. The two questions you should ask yourself when contemplating this kind of insurance are: As a grain producer, how often do I lose yield to corn borer? As a pork producer, is this added protection against borer-induced Fusarium rots valuable? There is no doubt that Bt corn is a valuable tool for Ontario corn producers, when used appropriately.

The greatest concern about Bt corn right now is how long will this technology last. All scientists across North America in public and private sectors agree that corn borer populations will develop resistance to the Bt toxin that is currently in Bt corn hybrids. It is simply a matter of time. The rest of this article will focus on how resistance develops in corn borer populations, why it could develop quickly, what strategy the industry proposes to overcome the risk, what responsibility the Ontario corn producer has in this strategy, refuge implementation issues and how the producer implements this strategy.

How resistance develops

The Bt toxin that corn plants produce through genetic engineering has a direct effect on corn borers when ingested. It binds with specific sites on the gut wall of the insect, allowing gut contents to leak into the body fluid eventually causing death. An insect is resistant if it does not respond in this way to the toxin. Resistance could occur because the toxin cannot bind to the specific site of the gut wall, or some factor in the gut breaks the toxin down or any other factor which changes how the insect might respond to the toxin. An individual insect cannot "decide" to become resistant. Resistance arises within populations and through natural selection by continued exposure to Bt toxins. If the entire population of corn borers with its vast genetic diversity is considered, most scientists agree that somewhere in that population there are genetically controlled mechanisms for resistance. Most often these mechanisms are already present somewhere in the population, and occasionally they will arise from random mutation. Very few individuals will express these resistance traits because normally they are not necessary for the survival of the species so their frequency is said to be very low. However when large segments of the population are exposed to Bt toxin, selection for individuals who survive will occur. The greater the proportion of corn borer populations exposed to the toxin and the greater duration of exposure, the faster the selection process occurs. If all corn acreage in North America were planted to Bt corn, the selection pressure would be extreme and resistance would be inevitable and develop quickly (some estimate within 3-5 years). There are numerous and repeated examples of poor deployment of insect control tools that have resulted in pest resistance and loss of that technology. We simply cannot afford make the same mistake with Bt technology.

The strategy to retard the development of resistance

The North American industry has adopted what is called the "high dose/refuge strategy" to manage corn borer resistance to Bt technology. The high-dose/refuge strategy involves exposing one portion of the pest population to Bt plants with an extremely high concentration of toxin, while maintaining another part of the population in a refuge where the pests do not encounter any Bt toxin. By maintaining the refuges in close proximity to the Bt corn, susceptible pests that survive in the refuge are expected to intermingle and mate with any toxin-resistant pests that survive on the Bt corn plants. Offspring from these matings will be unable to survive on Bt corn because they are more susceptible than their completely resistant parent. Population and genetic theory predict that this approach will substantially delay resistance, if it is appropriately implemented and its assumptions are met. The high-dose/refuge strategy has three essential assumptions.

Major resistance genes must be sufficiently rare so that nearly all such genes will be in heterozygous individuals. (A heterozygous individual has only one copy of the resistance gene and is referred to as a RS heterozygote). A gene frequency of less than one in 1,000 for major resistance genes is needed for the high-dose/refuge strategy to be successful.

Resistance genes must be nearly recessive. In other words, the RS heterozygotes should have very low survival on the Bt crop. RS survival rates that are less than 5% of the expected survival of homozygous RR resistant individuals on Bt corn are needed for the high-dose/refuge strategy to be successful.

Non-Bt refuges are needed to provide a source of susceptible pests to mate with the resistant ones so that their offspring will be RS heterozygotes. This requires random mating within the typical dispersal distances of the adults.

While these are the three critical assumptions, most of the theoretical models also have assumed that the pest population exhibits local random mating and no regional genetic isolation. In summary, the high-dose/refuge strategy can substantially delay resistance if (1) the frequency of major resistance genes and (2) RS heterozygote survival are low enough, and (3) there is random mating of adults within typical dispersal distances.

Responsibility of the Ontario corn producer

The Ontario Bt Corn Coalition and the NC-205 advocate the planting of at least 20% non-Bt corn refuge to allow survival and interbreeding of resistant and susceptible corn borers. The refuge should be planted no further than 1/4 mile from the farthest Bt corn. All corn producers need to take ownership of this strategy for it to be successful.

Implementation issues

There are a number of concerns regarding the high dose/refuge strategy. While the frequency of resistance genes has been confirmed to be very low, the survival of RS heterozygotes is still unknown because major resistance genes have not yet been found or characterized. Significant numbers of corn borer moths move only short distances. This will restrict the ability of resistant and susceptible moths to mix meaning that the refuge must be close to Bt corn. What is more alarming is that there is strong evidence that mating in corn borer populations is in fact non- random (violating one of the assumptions) and that populations are regionally genetically isolated resulting from substantial inbreeding. These two new facts will greatly accelerate the development of resistance. Ontario is one of the regions that has geographically isolated corn borer populations.

Economic analysis

How much refuge optimizes producer profits depends on the biological and genetic information discussed previously, along with the planning horizon (how long the technology is expected to last). Under normal growing conditions, and a 10-15 year planning horizon, economic models suggest that farmers capture most, if not all, of the benefits of Bt technology by planting 20-30% refuge. At lower levels of refuge, the economic models are more sensitive to the underlying biological and genetic uncertainties. Risk analysis shows that the cost to farmers of planting too much refuge is less than the cost of planting too little refuge. For example, under normal conditions and a 15-year planning horizon, increasing refuge from 10 to 20 percent is expected to decrease the value of Bt technology by less than 1%, while reducing the probability of resistance developing from 37% to less than 1%. On the other hand, reducing refuge from 10 to 5 percent is expected to cost farmers about 15% of the value of the technology, while increasing the probability of resistance developing from 37% to 74%. Therefore, producer economics and uncertainties about important model parameters suggest larger, rather than smaller refuges should be planted.

All scientific evidence points to planting more refuge rather than less, however, several members of the seed industry are advocating planting less. Twenty percent strikes a balance between biological, agricultural and economical considerations.

Bt Corn Protection Extends Into Adjacent Non-Bt Corn Refuges

Scientists associated with NC-205 found that damage from European corn borer was reduced in non-Bt corn adjacent to Bt corn. Under high borer pressure, up to 50% reduction in damage occurred in refuge corn within 5-10 metres of Bt corn. Damage increased gradually with distance from the Bt corn. Some reduction of damage continued out to 80 metres from the Bt corn, but was undetectable beyond 80 metres. Theoretical simulation models also predict this phenomenon. Refuge corn planted in narrow strips within a field of Bt-corn experiences less damage than blocks of refuge. Simulations show that refuge strips 6-12 rows wide are effective at delaying resistance and can provide similar economic return as a separate block of refuge corn adjacent to the Bt corn in a field.

Non-Field Corn Refuges

Many plants serve as aggregation areas and hosts for European corn borer and may provide refuges to conserve susceptibility in certain geographic areas. However, we do not know if these habitats will produce enough unselected individuals at the right time and whether their proximity to Bt corn allows for random mating. Until the contributions of these alternative hosts as refuges are known, refuge recommendations should be based solely on non-Bt corn.

Impacts On Natural Enemies and Other Non-Target Organisms

Recommendations regarding the size and distribution of non-Bt corn refuges have been made primarily to preserve susceptibility of the pest insects to Bt-toxins. Less attention has been paid to the potential effects of Bt corn on natural enemies in agricultural ecosystems and of the effects on other non-target organisms. Because of the extensive acreage that may be planted to Bt corn in the near future, NC-205 feels that this technology has potential to have widespread and lasting impacts on beneficial insects. These potential problems include direct effects and the indirect effect of substantial local/regional declines in the natural enemy prey base. These effects could ripple through other crops and habitats in unpredictable ways. While it is unclear if 20-30% refuge is sufficient to mitigate negative impacts on natural enemies in the long term, in the short- term, refuges of at least this size are prudent. A significant refuge should minimize negative impacts on beneficial insects that control other pests.

New Genes and Gene Combinations

New transgenic technologies, including gene stacks, introduction of other Bt toxins, and registration of novel toxins are under development and their resistance management implications need to be evaluated. When based on other Bt toxins, cross-resistance is an important issue. Cross-resistance among Bt toxins that share a common binding receptor is well documented, especially among those classed as Cry1 toxins. New non-Bt toxins may interact with Bt receptors, and each technology will need examination for resistance management implications. Gene stacks with other pest management traits (e.g., herbicide resistance) need to be examined for impacts on corn borer resistance management. The commercial availability and viability of these new technologies is unknown, which reinforces concerns about durability of existing strategies.

Changes in toxin concentration throughout the growing season

All current and future transgenic hybrids should be measured for toxin concentration throughout the season under a wide range of environmental conditions (e.g., soil, weather, irrigation). Toxin concentrations in some Bt corn hybrids can decline after pollen shed which jeopardizes the high-dose strategy, that is, the dose may not be high enough all season.

Minor Bt-Resistance Traits are Common in European Corn Borer

Laboratory selection for Bt resistance by some individuals in NC-205 have shown increases in Bt tolerance of 20- to 80-fold. These results demonstrate that minor resistance genes are common enough to be included in all of the original selection stocks, and there is substantial genetic variability for resistance in wild European corn borer populations. To date, however, survival of selected strains has not been documented on transgenic Bt corn hybrids. When major resistance genes are found, they are likely to occur in populations and genotypic combinations with minor traits that may increase their relative dominance threatening the high dose strategy.

Position Statement of the Bt Corn Coalition

The Bt Corn Coalition endorses the recommendations submitted by NC-205.

We recognize and accept that all individuals and institutions in Canada involved in Bt technology are responsible for its proper use and stewardship.

The seed industry accepts responsibility for maintaining and marketing Bt corn hybrids that deliver a high dose of Bt toxin throughout the season, and for continuing to produce high quality non-Bt corn hybrids that can serve as refuge areas for Bt-susceptible European corn borers.

Corn producers accept responsibility to apply appropriate measures for management of resistance by corn borer populations to Bt toxins to ensure extended use of this technology.

The research and extension community accept responsibility to provide scientifically based information pertinent to the use of this technology and to transmit this information in a timely and clear manner to the seed industry and to corn producers.

Our recommendations for implementing a resistance management strategy include:

All growers should plant a minimum of 20% non-Bt corn not sprayed with insecticides on their planted acreage each year.

Non-Bt corn should be planted within 1/4 mile of the farthest Bt corn in a field to provide a refuge where Bt-susceptible moths may exist.

Non-Bt corn hybrids for use as refuges in a field should be selected for growth, maturity and yield traits similar to the Bt hybrid used in the remainder of the field.

Refuge areas may be planted in blocks on the edges or headlands of fields or in strips across the entire field. When refuge corn is planted in strips across a field a minimum of 6 rows should be planted with non-Bt corn alternating with Bt hybrid across the entire field. Refuge created by mixing seed in the hopper is ineffective.

The Bt Corn Coalition recommends that individual corn producers using Bt technology be responsible to ensure that the minimum 20% refuge occurs on their farm.

The Bt Corn Coalition unanimously agrees that a single, clear and concise message concerning stewardship of Bt technology is essential for all groups and individuals concerned and for public awareness and acceptance.