|Keywords:||Disease/pest resistance; Biosafety/Foodsafety; Genetic engineering.|
|Correct citation:||Whalon, M.E. and Norris, D.L. (1996), "Resistance Management for Transgenic Bacillus Thuringiensis Plants." Biotechnology and Development Monitor, No. 29, p. 812.|
Bacillus thuringiensis (Bt) toxin genes have been genetically engineered into dozens of plant species. By expressing a Bt toxin within their tissues, the plants protect themselves from some insect pests without farmers resorting to pesticide sprays. In addition, plant tissues otherwise difficult to reach with conventional pesticides can now be protected. However, to conserve the effectiveness of Bt plants, management measures are essential to control the build-up of resistance by the target insects.
Insects have the potential to develop resistance to transgenic Bt plants
because the plants maintain a constant killing dose throughout the season.
Therefore, unlike Bt sprays which are inactivated over a short time, the
selection pressure of transgenic Bt plants on susceptible pest populations
will be much higher.
Resistance management is a way of sustaining the effectiveness of a pest control tool or tactic. It tries to delay or prevent adaptation in pest species by managing the factors that may contribute to resistance development. Its key goal is the preservation and management of genetic resources, i.e. the genes that are responsible for the susceptibility of a pest to the pest control tool or tactic (susceptible genes). This will require commitment and participation by farmers, pesticide or seed suppliers, and regulators to help prevent insect resistance through monitoring and proactive management.
Resistance management strategies
Resistance management programmes rely on four key strategies:
Diversification of mortality sources. The assumption behind diversification of mortality sources is that insects will not adapt as quickly if they are faced with more than one mortality mechanism. Several tactics can be used in both conventional (spray) and transgenic Bt programmes. Bt toxins can be rotated or alternated with other chemicals, toxins, or other control strategies. Similarly, two or more toxins could be mixed and introduced at once.
Nevertheless, the development of cross-resistance to Bt toxins is a possibility that could preclude the long-term success of this tactic. Many pests have demonstrated the ability to develop resistance to a wide variety of Bt toxins after initial selection by exposure to only one toxin. In addition, the effectiveness of the multi-toxin approach can vary depending on the initial level of resistance in the population and the manner in which resistance is inherited (dominant or recessive).
Reduction of selection pressure and use of refugia. Assuming that there is a fitness cost associated with resistance, reducing selection pressure may help the population revert back to a more susceptible state. Fitness costs, i.e. the associated 'cost' of resistance development to the insect such as reduced fertility, smaller body size/weight, also help maintain the existing susceptible population, thus preserving the important susceptible genes. Refuges may be an effective way to reduce selection pressure by providing an area for habitation and immigration of susceptible insects. In a transgenic deployment scheme, this can be achieved by providing a refuge of non-transgenic plants in one or more ways: (1) a seed mixture of transgenic and non-transgenic plants; (2) a spatial mixture, or field-to-field mosaic, that results in a patchwork of completely transgenic and completely non-transgenic plots; (3) a temporal mixture, or season-to-season sequence that alternates between transgenic and
However, some research suggests that refuges may actually speed up resistance development in cases where dispersal of resistant insects from the treated (transgenic) area into the refuge is high enough to effectively pass the resistance gene to the susceptible population. Therefore, resistance management strategies must take into account the movement dynamics of the target pest(s).
Prediction and monitoring of resistance. Waiting until resistance occurs before implementing a resistance management programme is ineffectual. An effective programme must include tactics for monitoring, predicting, and evaluating resistance progress. Sampling of insect populations at regular intervals is a good approach for monitoring resistance progress. However, this requires either a simple, low-cost diagnostic tool for use in the field (especially in developing countries) or more advanced facilities where insect specimens can be sent for diagnosis. In either case resistance monitoring requires a high level of commitment from farmers and extension personnel since the collection and testing of samples is time- and labour-intensive. Biotechnology companies can also encourage monitoring by developing and implementing resistance monitoring protocols for transgenic crops that they introduce commercially. In the USA, the development of a resistance management and monitoring plan has sometimes been a requirement for registration of the transgenic crop. Computer-based models could be a useful tool for predicting trends in pest populations.
Policy implementation. An appropriate policy for transgenic plant deployment will be tailored to each region's specific needs and available resources, but it should take into account the resistance management concerns as described above. The following sections examine in greater detail a possible assessment process for developing an appropriate transgenic deployment policy.
Deployment approaches: Biotechnological considerations
The mixing of Bt toxins with other toxins to manage resistance can make use of transgenic technology. Seeds of different genetic lines, each engineered to express a different toxin, could be mixed, or one plant variety could be engineered to express multiple toxins. The US private biotechnology company Mycogen has developed MATTCH Bioinsecticide, a Bt product which is an assembly of two different lepidopteran active Bt toxins encapsulated in Pseudomonas fluorescens. Multi-gene, multi-toxin plants are likely to appear on the market in the future as well.
Another consideration that will affect the choice of deployment strategy is the level of toxin expressed in the plant. Many researchers agree that a high dose approach in combination with other conditions such as presence of refugia (to insure that mating between resistant and susceptible individuals 'swamps out' the resistance gene), low initial frequency of resistance genes in the pest population, and recessive inheritance of resistance genes are promising conditions for delaying resistance development. However, most researchers also fear that a high dose approach, if not carefully managed, could still lead to rapid selection for resistance. A high dose could effectively destroy the susceptible (homozygous) and partially-resistant (heterozygous) individual insect while allowing survival of the more rare homozygous resistant individuals. These strongly resistant (homozygous) insects could mate with each other and resistance could develop rapidly within the population. On the other hand, low and moderate dose expression may not effectively control the pest population, resulting in unacceptable crop damage. In these cases, growers would be forced to resort to chemical and other control methods to avoid significant economic loss to the crop.
Induced toxin expression within the plant only when the target is present could be an alternative to lower selection pressure. Transgenic plants can be engineered with a gene promoter that initiates expression of the toxin in plant tissues via chemical induction (e.g. application of a regulatory chemical by the farmer) or by physical induction (e.g. response to insect feeding injury). Induced expression lowers the selection pressure because it is limited to the period of time when the plant is actively expressing the toxin. Another Bt expression pattern that diminishes selection pressure is tissue-specific expression, wherein toxins are expressed only in certain tissues (e.g. plant tissues of economic importance like bolls, buds, and fruit).
Other concerns: Gene escape
Transgenic cultivars can be problematic in those countries where wild native plants may acquire Bt genes from cross pollination. For example, transgenic rice has great implications for many nations because of the potential to outcross to wild rice varieties, thus allowing those weeds to escape insect herbivory and become even more destructive. A 1996 study in Denmark by Mikkelsen et al. showed that genes inserted into a crop plant could move rapidly into their wild, weedy relatives. The Denmark study has heightened concerns that transgene escape is indeed a possibility that must be taken into account in any transgenic deployment plan. The transfer of Bt genes to wild related species could have a direct impact on resistance development in pests that also feed on these wild species. Essentially, Bt-enhanced weeds could function as an additional selective pressure on the insect pests and increase the rate of resistance development.
Decision-making for transgenic plant deployment
Researchers emphasize that decisions to deploy Bt transgenic crops should be made on a case-by-case basis within each crop production system. To determine if transgenic Bt is suitable for deployment, a case-by-case (country-by-country) assessment process requires identification and understanding of the local ecological, environmental and agricultural conditions as well as the biology and host plant interactions of the target species.
The process begins by collecting data to assess three main factors: (1) the crop itself for features that could impact on selection for resistance; (2) the target pest, its host range, and its propensity to develop resistance to Bt; and (3) assessment of the crop and pest data in relation to available transgenic technology. This includes an estimate of the suitability of the available transgenic technology to the crop-pest complex and, if positive, of which deployment strategies are appropriate to maintain susceptibility in the pest population. This factor will gain importance in the future as transgenic technology progresses and more deployment options become available.
The information gathered can be used to assess several criteria or conditions under which the introduction of Bt transgenic plants could lead to rapid resistance development in pest species. Examples of criteria include: the ecological risk of Bt gene transfer to other related species, the presence or absence of refugia to counteract resistance development, the relative economic importance of the target pest, and the atmosphere of cooperation (strong or weak) among growers, industry and government.
|Regulatory options for Bt transgenic plant deployment
(*) + = easiest, straightforward to implement and enforce;
Regulatory policy options
The success of the policy will depend on the number of entities or people regulated as well as the particular crop and production system involved. Obviously, the fewer the number of people that are targeted for regulation, the easier it will be to enforce. The current regulatory apparatus, the particular transgenic crop under consideration, and the existing seed handling system are important factors in determining effective regulatory options in each country.
Due to loose or non-existent regulation of other pesticide chemicals, however, many countries lack a historical precedent on which to build new transgenic related policy. These countries may find the process of developing transgenic deployment policy extremely challenging. The table on page 11 summarizes several possible regulatory options, targets, and enforcement methods. Many of the policy choices are mutually compatible, especially simple options such as labelling, that can be used in conjunction with any of the more complex regulatory options. We will now examine some of these options in more detail.
Labelling. Package labelling is an obvious starting point for countries considering deployment of transgenic products because it is straightforward, relatively easy to enforce, and it can serve as a foundation for formulating more extensive regulation. Since biotech companies already provide 'proper use' labels on most products, industry cooperation and compliance with this type of regulation can be high. Label requirements should include: (1) identification of transgenic status; (2) recommended planting ratio (percentage of transgenic versus non-transgenic seed) or recommended refugia for resistance management; (3) a warning against misuse or overplanting of the transgenic seed; (4) an agency or industry contact in the event that resistance to the transgenic crop becomes evident in the pest population.
Licensing. Licensing of any type requires an efficient and stable bureaucratic apparatus for its implementation and enforcement. While licensing is common in the developed world, countries with less evolved institutions may find it more difficult to implement. The private US company Monsanto has used licensing to achieve compliance on utilization, production and sales of transgenic Bt crops like potato, cotton, and corn.
Licensing targets could include seed companies, seed distributors, sales representatives, or end-users. The enforcement apparatus would then vary based on the particular target. For example, a national government agency could enforce licensing agreements at all target levels, although government licensing of seed companies would be easier to implement than government licensing of individual farmers. Likewise, seed companies could enforce licensing at the seed distribution and sales levels, or a local grower organization may act as licenser and enforcer at the grower level. In countries that lack strong institutions/agencies for enforcement, local seed distributors or grower organizations may provide the most effective means for regulation. In addition, the establishment of a penalty (fine, forfeiture of seed, etc.) for license infringement or, conversely, an incentive for compliance will be necessary in most cases to insure effective enforcement.
In all cases, the establishment of such a policy will require the input of many resources: capital, personnel, facilities, and educational campaigns to train individuals in the license application process and proper use of the seed. Extension workers would be well-suited to train the end-users, but the extensionists themselves will require initial training by national agency or seed company personnel. The burden of these costs will most likely fall on the national government, but in some cases countries may be able to establish partnerships with the seed companies that require a commitment of resources, at least in education and training, as a condition for importing/selling the seed or transgenic product.
Central control of seed. Governments with strong national agencies could directly regulate imports, exports and distribution via the seed companies. Implementation and enforcement would require a new agency branch and administrative staff, but some of the education costs associated with licensing could possibly be eliminated. However, the complexity of the programme could increase with governmental attempts to regulate national distribution of seed. In addition, the government may need to implement strong penalties or incentives for compliance at two levels (the seed company/distributor level and the grower level) to prevent the misuse of seed once it has been distributed. Alternately, the government could make the seed companies shoulder some of the costs of educating the end-users and then hold the seed companies accountable for subsequent misuse of the seed. However, this too would increase the complexity and cost of enforcement.
The assessment of success for any transgenic regulatory policy will
be its ability to delay the development of resistance in the target pest(s).
Some regions may choose to implement an integrated 'package' of policies,
e.g. labelling and seed licensing, designed to give maximum benefit. The
likelihood of success of the policy will also depend, in part, on its level
of complexity. Programmes increase in complexity as the regulatory apparatus
becomes more removed from the target, and increasing complexity can make
successful policy implementation more difficult. For example, the licensing
of growers by a local growers association is relatively simple and presumably
easier to implement compared to the complexity of grower licensing by a
national agricultural agency.
Ultimately, the effectiveness of any transgenic regulatory policy will require continual monitoring, assessment, and feedback to insure proper use of seed, regulatory compliance, and control of resistance development. Should incidences of resistance in target pests occur, the current policy should be modified immediately and deployment of transgenic crops should be halted until the pest population has been destroyed by other means.
Mark E. Whalon/Deborah L. Norris
Department of Entomology & Pesticide Research Centre, Michigan
State University, East Lansing, MI 48823, USA. Fax (+1) 517 353 5598;
This article is based on a paper earlier presented at the IBS-CamBioTec Regional Seminar on Planning, Priorities and Policies for Agricultural Biotechnology, October 6-10, 1996, Lima, Peru.
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