The development of insect resistant crop varieties has been the most successful application of agricultural biotechnology research so far. The bt transgenic crops derive their resistance from the insecticidal gene of the bacterium Bacillus thuringiensis. Cotton, corn, and potato engineered with such genes were grown commercially for the first time in 1996. Transgenic bt cotton was grown on 1.8 million acres last year, accounting for about 12% of US cotton acreage.

A major worry lurking behind this success is the potential vulnerability of bt crops to eventual adaptation by insect pests. Large scale deployment of bt transgenics will certainly impose a selection pressure for pre-existing bt- resistant insects to increase their numbers. As a result, the effectiveness of this environmentally sound method of pest control would be reduced. Although several resistance management strategies have been proposed to slow the evolution of insect adaptation to bt genes, they are not based on empirical data, such as the initial frequency of resistance alleles in the population, but rely instead on theoretical estimates that may prove inadequate.

According to Bruce Tabashnik, University of Arizona, excitement over the success of bt plants "must be tempered with an admission of ignorance" on how to effectively manage pest resistance to ensure long term durability of the approach. Thus a study conducted by a team led by Fred Gould of North Carolina State University may be a turning point in bt research because it provides the first direct estimate of the field frequency of bt-resistant insects (1). They report that in tobacco budworms (Heliothis virescens), a major cotton pest, 1 in 350 individuals carried an allele for resistance to the bt toxin. This estimate is considerably higher than those assumed in earlier theoretical models, and thus forebodes a swift evolution of resistant insect populations. Tabashnik calls this study "a timely finding" which "provides inspiration to plunge ahead" into larger field tests of resistance management tactics (2).

The study was a mammoth effort that began with collecting 2,000 male insects from four cotton-growing states in 1993, before transgenic bt crops were grown commercially. As the resistance trait is recessive, it is difficult to detect heterozygous insects but estimates of the number of such heterozygotes carrying recessive alleles are critical as those individuals are predominant in any population. The collected males were then individually crossed with females of a strain selected for its extreme high resistance to CryIA(c), the bt gene used in cotton against tobacco budworm.

The resulting first and second generation progeny from 1025 successful crosses were tested for resistance to bt toxin using artificial diets in the laboratory. Three males from the sample of 1025 were confirmed to be carrying an allele for resistance to bt toxin, leading Gould and co-workers to conclude that field frequency of bt resistance alleles was about 3 in 2,000. William Moar of Auburn University comments, "Gould's research definitely illustrates that resistance management procedures such as refuges, intense field monitoring of transgenic plants for potential escapes, and alternate control strategies are essential to maintain the viability of this valuable resource."

To slow the adaptation of insects to bt cotton, the EPA has mandated that cotton growers should plant at least 4% of their crop with non-transgenic cotton and this refuge cannot be treated with any insecticides. The idea is that such 'refuges from toxin' will harbor susceptible insects and thus retard the evolution of insect resistance against the bt gene. Gould et al. predict that with 4% refuge, the bt cotton could remain efficacious to tobacco budworm for 10 years. This is not bad considering that insects have developed resistance to many pesticides and conventional varieties in less time than that. However, the current bt cotton has less resistance to other pests such as cotton bollworm and European corn borer, and thus the authors predict a boom cycle of only 3-4 years for this variety. Again Tabashnik puts it elegantly - "Nothing will be gained and much can be lost if we pretend to know more about resistance management than we really do".

References
1. Gould, F. et al. 1997. Proc. Natl. Acad. Sci., USA 94:3519-3523

2. Tabashnik, B.E. 1997. Proc.Natl. Acad. Sci., USA 94:3488-3490.

C. S. Prakash
Center for Plant Biotechnology Research
Tuskegee University

8/28/97