Last time we discussed two strategies that have been used to develop transgenic plants with resistance to insects.
Note that there are some important differences between these two strategies. Bt toxin is normally lethal to the insect that ingests this protein. In contrast, the protease inhibitors and alpha-amylase inhibitors are not lethal but will merely slow down the growth, maturation and life cycle of the insect. From studies done in other systems, the selection pressure that is applied by the Bt toxin is more likely to result in the evolution of insects with resistance to this toxin. In contrast, resistance is less likely to develop to a control method that merely slows down development.
Both of these methods have shown promise and commercial products have been developed that incorporate the Bt toxin in potatoes, cotton and corn. What are the economics of using these Bt crops? I don't know the details of this for corn, but the cost of hybrid corn seed with a Bt toxin gene is a little higher than regular corn hybrids. Growers must therefore weigh this against the cost of other measures to control European corn borer (insecticides), or the yield loss that might result from not using any insecticide, with a non-Bt hybrid.
For cotton, the economics are a little more complicated. Bt cotton seed costs about the same as regular cotton seed. However, the grower must pay Monsanto a licensing fee of $32 per acre to plant Bt cotton. In 1996, approximately 1.8 million acres of cotton were planted to Bt cotton, generating revenues of about $60 million dollars for Monsanto! The normal cost for insect control in cotton might be between $100 and $200 per acre, to the best of my knowledge. Remember that cotton accounts for the largest fraction of insecticide use in the U.S. If the Bt cotton yields the same as conventional cotton, then there is a clear advantage to the Bt cotton purely on the basis of production costs. Even if there is a need for one or two applications of insecticides at about $10 per acre per spray, the Bt cotton is likely to come out ahead.
This is a fairly simple analysis, but it gives an indication as to the economic advantages of using a genetic/biotechnological solution to controlling insect pests. In addition to these cost considerations, there are a number of other advantages to using this method (expressing proteins in plants to control insects) that are discussed below, followed by a discussion of the major problem, development of insects with resistance to these control measures.
What additional benefits are likely to come from these biotechnology methods to control insect pests?
What are the potential problems with these biotechnology methods to control insect pests?
The most important concern is that insects will develop resistance to these Bt toxins. It has been argued by some that this is unlikely to occur, because Bt spores have been used over the past 20 years and there have been essentially no reports of insect resistance developing against these controls. However, there are a number of important differences between using Bt spores as insecticides and expressing a Bt toxin in transgenic plants.
Currently, Bt crops typically express only a single Bt toxin protein, at relatively high levels, throughout the growing season of that plant. Under these conditions, it is much more likely that insects will develop resistance to Bt crops than they will to Bt spores. First, there is high selection pressure when a population of any organism is exposed continuously to a toxin. This is compared to the transient exposure that insect populations face with Bt spores. Second, insects need develop resistance only to a single toxin expressed in the plant, compared to the suite of toxins in spores. Simultaneous development of several different resistance mechanisms is highly unlikely.
Therefore, in spite of the fact that few examples of resistance to Bt spores have been reported, there is good reason to expect that insects will quickly develop resistance to Bt toxin in plants. Already there are some results pointing in this direction.
These and other results clearly demonstrate that the genetic potential to develop resistance to Bt toxins exists in insect populations. I think it is widely accepted that insects will develop resistance to this control method, just like they have developed resistance to most other insecticides.
How can the potential problem of insects developing resistance to Bt toxins be countered? Many of the companies that are involved in this area of research and development are very concerned about this problem. A couple of strategies have been proposed to delay or prevent the appearance of resistance.
The first is to plant a mixture of Bt and non-Bt varieties, not interspersed but in adjacent blocks. The non-Bt blocks are known as refuges. My understanding is that the current recommendation to growers is to plant 20% non-Bt corn which can be treated with another insecticide, or 5% non-Bt corn which cannot be treated with any insect control. These provide refuges for insects that are not resistant to the Bt toxin.
To discuss how refuges can delay the development of insect resistance, first we will discuss the likely genetics of this trait. Resistance is recessive, i.e. individuals that are r/r will be resistant, while sensitive individuals can be either S/S or S/r. (S and r are different alleles at the same locus.) If resistance to a single Bt toxin is a dominant trait, there is little that can be done to prevent this spreading rapidly through an insect population. The refuge model also assumes that Bt-resistant individuals, and likely the heterozygotes, are at a selective disadvantage under normal (non-Bt) conditions compared to the sensitive insects. The refuge model works like this, at least in theory:
This strategy is based on population genetics and is sound in theory. However, it does rely on grower compliance. Will a farmer willingly plant some of his/her crops with a variety that is susceptible to insects? Maybe he/she will rely on the neighbor to be the one to plant the non-Bt varieties. It should be emphasized that this strategy is not going to prevent the development of insect resistance to Bt toxins, it is designed merely to slow down the inevitable spread of resistance.
Are there alternatives that are being considered to enhance the long term usefulness of Bt toxins in plants? This is an important question because Bt toxins are a valuable biological resource, one of the few examples of highly active insecticidal proteins. It is in no one's interest to see this resource lost quickly by using it injudiciously in the first generation of transgenic crops. What alternatives are being considered?
In the future it may be possible to design genes that can encode novel proteins to perform specific functions, such as kill or inhibit the growth of insects. But our current understanding of protein structure and function do not yet make this sort of rational design of proteins possible.
Two final points to consider with regard to transgenic plants with resistance to insects: