In this article we will discuss about pest resistance management and Bt-resistance management of crops.
Pest Resistance Management:
Crop breeders have long known that some conventionally bred cultivars with resistance to insect and microbial pests may perform wonderfully for the first few years after they are deployed commercially, but then fail miserably in controlling the targeted pests in later years because the pest has evolved a way to cope with the resistance mechanism in the cultivar.
Sometimes there is a slow decline in effectiveness of the cultivar, but in other cases the onset of control failure is rapid and unpredictable. If you are a subsistence farmer, the failed performance of such cultivars can mean hardship, especially if the cultivar had previously performed well and long enough for you to gain confidence in it.
Indeed some of the criticisms of the Green Revolution of the 1960s and 1970s centred on rice cultivars that were rapidly adapted to by insect and microbial pests. For example, brown plant-hopper populations adapted to the single-gene resistance in the first Green Revolution rice’s within 2-3 years of their widespread cultivation, and single-gene resistance to the rice blast fungus has been notoriously unstable.
The longevity of cultivars with single blast resistance genes in Japan has been less than 3 years. In industrial countries, breeders and seed producers sometimes try to deal with pest adaptation to widely used crops that have one resistance mechanism by maintaining, in reserve, replacement cultivars with different resistance mechanisms, for example wheat rust.
These systems are sometimes able to replace cultivars in a single season as was the case with the southern corn blight epidemic in the USA. In developing countries, instituting such a system for subsistence crops is difficult or impossible because of limited infrastructure and resources.
Not all pest-resistant cultivars are rapidly adapted to by their target pests. Entomologists and plant pathologists have worked hard to predict whether a specific resistant cultivar is likely to work well for a long time under field conditions.
This characteristic called “durable resistance” has proven to be partially predictable, but many plant pathologists are only willing to judge the durability of a specific type of pest resistance in retrospect.
The general problem of pests adapting to any approach used to control them has been the bane of agriculturalists for centuries. Weeds, pathogens, and insects have all overcome various cultural, chemical, and biological approaches used for their control.
Over 500 insect species are known to have adapted to at least one insecticide, and it often takes less than three years for this adaptation to evolve. In many developing countries this can severely disrupt food production because replacement insecticides are often not available, and the beneficial insect populations have been decimated by insecticide use.
In 1997 and 1998, there was a tragic series of over 400 suicides among cotton farmers in Andhra Pradesh, India in response to crop failures that were in part the result of pest adaptation to insecticides. The farmers were heavily in debt because of several seasons of crop failures, caused by irregular rainfall and heavy infestations of the insect pests Spodoptera litura and Helicoverpa armigera.
Application of large doses of highly toxic insecticides such as monocrotophos and methomyl were not effective because of pest resistance to these compounds, and their toxicity to predatory and parasitic arthropods which otherwise could have provided some level of natural biological control.
In the 1970s entomologists, plant pathologists, and weed scientists began a concerted effort to use knowledge of evolutionary biology and population genetics to develop strategies for slowing the rate at which pest populations evolved adaptations to control tactics such as pesticides and pest-resistant crops.
This approach called “pest resistance management” now seems highly appropriate for crops developed using genetic engineering, because there is good reason to predict that some approaches to the development and deployment of engineered pest-resistant crops will last much longer than others.
When new genetically improved crops that expressed insecticidal proteins from Bacillus thuringiensis (Bt) were first developed, there was much concern in the United States about insects adapting to these toxins.
Unlike conventionally bred resistant crops, where a resistance mechanism can only be moved within a single crop species, the Bt toxins were being moved into multiple crops, so insects that fed on more than one crop would get multiple exposures.
Unlike insecticides that are sprayed only during some time periods in the season when pest pressure is high, the newly developed crops produce the toxin all season long, so all insects in a population can be exposed to the toxin. Everything known about pest adaptation indicated that overuse of such crops could give great control for a limited number of years followed by failure.
In the United States there was one other pertinent fact about Bt crops. B. thuringiensis, the bacterium that was the source of the toxin genes in the crops, has long been sprayed on crops by organic farmers and others as an alternative to chemical insecticides.
Organic farmer groups and their supporters protested that the overuse of Bt toxins in genetically engineered crops, and the subsequent development of adapted pests, would leave them without an effective pest control tool.
This highlighted two issues: one was the plight of the organic farmer and the other was the unique, environmentally benign nature of Bt toxins compared to conventional pesticides. A set of Bt toxins, sometimes referred to as Bt endotoxins, were known from previous uses to be effective at killing either some caterpillars or some beetle species, but they had no effect on almost all other species.
From an environmental perspective these are wonderful toxins, and unless other toxins with this high target specificity can be quickly found, the overuse and loss of Bt toxin efficacy in transgenic crops could send cotton and potato farmers back to spraying environmentally disruptive chemicals.
Prior to the commercialization of any genetically improved crops, the US-EPA held meetings of Scientific Advisory Panels to get advice from experts outside EPA regarding risks of genetically improved crops. One of the recommendations of these panels was to institute resistance management programs.
When EPA granted conditional registrations for the first Bt corn cultivars in 1996, one of the conditions was the development of resistance management plans by the year 2000. The EPA felt that such plans were not immediately needed because they expected adoption of these Bt cultivars to proceed more slowly than it actually did.
The conditional registration for Bt cotton included a resistance management plan, but this plan is now being re-examined because it lacks rigor. More recent conditional registrations of newer corn cultivars have included more stringent resistance management plans. The imposition of resistance management plans is something new for the US-EPA and the agency has been gaining sophistication in this area over time.
In 1998 the EPA convened a Scientific Advisory Panel to reassess the issue of Bt resistance management. The report of this panel laid out some clear recommendations to the EPA. After considering a number of potential resistance management strategies, the panel recommended that “a refuge/high dose strategy must be employed for target pests within the current understanding of the technology.”
They added that “regulatory strategies should serve to provide growers with a sustainable approach that encourages compliance.” These were important recommendations worthy of careful examination. The high dose portion of this strategy is most easily understood by analogy to the use of antibiotics.
When doctors prescribe antibiotics they often give their patients the admonition that even if they feel completely cured after three days, they should continue to take the antibiotic for the full time prescribed.
The reason for this is to produce a high dose of antibiotic for an extended time period that will kill even those rare bacterial cells that have a mutant gene conferring partial tolerance of the antibiotic.
After three days you may have killed 99 per cent of the targeted bacteria, but if the 1 per cent that survive have a gene that confers partial tolerance and are transmitted to another individual, his or her infection will be more difficult to treat.
More importantly, when that next individual takes the antibiotic, the partially tolerant bacteria may evolve even higher tolerance if among the millions of bacteria involved in the infection there are a few bacteria with other mutations that add to the tolerance conferred by the initial mutation.
When a patient takes an antibiotic for the full period prescribed, the expectation is that even the partially tolerant bacteria will be killed. As long as it takes more than one evolutionary step to result in complete tolerance of the antibiotic, the prolonged use of the antibiotic should derail the adaptive process by inhibiting the first step.
The use of a high dose of Bt toxin in crops serves a similar purpose. In all cases studied to date it takes more than one gene, or at least more than a single copy of a gene to confer high tolerance of Bt toxins.
When Bt crops are first commercialized it is estimated that about 1 in 1000 individuals may carry one copy of a gene for tolerance of Bt, and only 1 in 1 million would carry the two copies needed to achieve a high level of tolerance. The high dose approach is set up to ensure that each plant that produces Bt toxin produces enough to kill most of the partially tolerant individuals.
But if the high dose is used by itself, some insects out of the billions that can infest a local area may have two copies of the gene. If they survive and mate, the Bt crops could rapidly lose effectiveness. This is where the refuge part of the “refuge/ high dose” approach comes in. All of the current target insects for Bt crops are obligate sexual.
That means that they must mate to reproduce, and that their offspring obtain half of their genes from each parent. If a small portion of a farm is planted to a cultivar that does not produce Bt toxin this area serves as a refuge for Bt-susceptible insects. Because the highly tolerant insects are expected to be so rare, they are likely to mate with susceptible insects produced in the refuge.
The offspring of these mattings will have only one gene for tolerance, and so will be killed if they feed on a Bt-producing plant. By combining the refuge and the high dose, this strategy derails the evolutionary process as long as more than one gene copy is required to survive the high dose.
Pests can eventually adapt to such a strategy but the time period required can be 10 to 100 times longer than expected if this strategy is not implemented. The 1998 EPA Scientific Advisory Panel was clear about what constitutes a high dose and what constitutes an effective refuge. They defined a high dose as 25 times the amount of toxin needed to kill susceptible target insects.
They concluded that an effective refuge existed when for every insect with a resistance gene produced in the Bt crop there would be 500 susceptible insects produced that could mate with the resistant insects. These are stringent requirements and they work in concert.
If a crop does not quite produce a high dose, the expected number of insects with at least one resistance gene increases. This results in the need for a larger refuge to produce the 1:500 ratio. How do these recommendations line up with Bt crops that are now on the market in the United States? With Bt potato the data indicate that there is a high dose for the target pest, Colorado potato beetle.
However, it is not confirmed that farmers are planting effective refuges. With corn, most cultivars produce a high dose for the European corn borer, but not for the corn earworm.
Refuges currently appear large enough for the European corn borer, in part because of lack of full adoption of Bt corn, but the refuges may sometimes be too far from the Bt corn to allow insects from the refuge to cross-mate with insects from the Bt crops. In cotton there is a high dose for the tobacco budworm but not for the corn earworm .
Proposed refuges of about 10 per cent are expected to be sufficient for the tobacco budworm, but may not be sufficient for the cotton bollworm. There appears to be a high dose in cotton for the pink bollworm, but this has not been completely confirmed. There is certainly room for improvements when the producers of Bt crops present their new resistance management plans to the US-EPA in 2000.
At a recent USDA/ EPA workshop, there was debate as to whether similar resistance management strategies would be required in China. The expert statement was conditioned on the assumption that no Bt corn was grown in China. If that assumption held then the targeted Chinese pest on Bt cotton, Helicoverpa armigera, could utilise non- Bt corn to produce the Bt-susceptible insects.
It appears that there now will be Bt corn grown in China, and the Bt cotton that is being planted in China does not have a high dose for the target insect. This is not the kind of scenario that is likely to retard the evolution of adapted pests.
If Chinese farmers and administrators come to rely on Bt corn and cotton in the next few years using the current technologies and risk management, there is a clear risk that yields will show variation over time.
There is economic pressure in many developing countries to adopt Bt crops that were developed to control U.S. pests. If these crops are sold as “second hand” cultivars to these countries, it is hard to imagine that they will usually be effective at thwarting pest adaptation.
Unless there are careful contingency plans to deal with the eventual failure of these cultivars, their adoption by developing countries could lead to higher than necessary yield variation. For example, it has been argued that situations such as that in Andhra Pradesh demonstrate the urgent need for the release of Bt cotton in India.
Two kinds of Bt cotton have been field-tested in India: one from the Monsanto Company and one produced by an Indian research institute. Many farmers will be eager to adopt Bt cotton, which can help to control S. litura and H. armigera. The availability of Bt cotton may also attract many new farmers to invest in growing cotton.
However, if a carefully designed resistance management plan is not implemented, the farmers may suffer another severe setback in a few years should the pests adapt to the Bt cotton. In desperation, these farmers may again turn to highly toxic insecticides, repeating a tragic cycle.
By the time insect pests adapt to Bt crops, biotechnology will develop replacements. If it was easy to develop replacements, competitive market pressures in the U.S. cotton and corn seed trade would have already resulted in alternative toxins being produced.
The only even marginally novel toxin in corn is the Cry9C Bt toxin that AgrEvo has marketed, but because of health concerns, the US-EPA has approved its use only in corn grown for livestock feed.
It has been said that the people with the most to lose from pest adaptation to Bt toxins are those who own the companies that own the genetically engineered seeds, but the aggressive stances of some companies against implementation of resistance management plans recommended by US crop scientists does not, however, indicate sincere concern.
An explanation for this discrepancy could be that as with the pesticide market, most of the important profits from Bt crops are expected to come in the first years of widespread use, and resistance management could interfere with these early profits. Public and commercial interests are therefore expected to clash.