Insecticide Resistance

See also Controlling Negative Effects of Bt Plants

Insecticide resistance is an important topic in the debate over Bt Plants. Selective pressure is an integral part in how insecticide resistance develops. Bt plants create a high pressure for resistant insects against Bt toxins (DeMaagd, R. A., and et al. , 1999). Insects can develop full or partial resistance to the Bt toxins expressed in the Bt plants (Andow, D. A. and et al., 2000).

How Bt plant resistance develops
Insects are known to be able to develop resistance to insecticides. In a population there may be a few insects that carry the genes for resistance. Rare mutations can originate resistance genes in a population. In the natural environment the mutant insect is usually out competed and produces less offspring. Upon exposure to insecticides, insects that do not carry the resistance genes die, thus allowing the individuals with the resistance genes to survive and reproduce, creating more resistant insects (Bacillus thuringiensis ). As the number of resistant insects increase, the number of resistant insects mating together increases, which increases resistant progeny (Yong-Biao, L., and et al ., 2001; Tabashnik, B. E, and et al ., 2002). In a relatively short amount of time the whole population is resistant.

Bt Resistant Diamond Back Moth Larave permission pending from Capinera, J. L., 2000

What are the mechanisms behind Bt toxin resistance?
The Bt resistance trait is thought to be autosomal recessive (Tabashnik, B. E., and et al. , 2002; Yong-Biao, L., and et al ., 2001). There is probably no more than a few genes that codes for resistance, because cross resistance has emerged in insects and the backcrosses of resistant progeny suggest one or a few loci.

Mutations at the loci for Bt resistance seem to be very rare, because there are only a few places on the chromosome that confer resistance when mutated (Yong-Biao, L., and et al ., 2001). Insects that have induced resistance from mutations have the same three mutations (Yong-Biao, L., and et al ., 2001).

Another molecular mode for resistance is to interfere with the toxin being converted into its active state. Some insects have found means to inhibit the full conversion of the protein to toxin. This inhibition seems to be a minor mode of resistance, since the intermediates still have low toxicity (Yong-Biao, L., and et al. , 2000).

How to stop Bt toxin resistance
There are several strategies thought to stop resistance to Bt toxins by insects. Crop rotation, refuges, and various expression patterns of Bt proteins are all proposed mechanisms for stopping Bt toxin resistance.

Crop Rotation
One form of rotation is where Bt plants and non-transgenic plants are grown in alternating seasons (Wu, K., and Guo, Y., 2003). The other rotation method is to change which Bt toxin is expressed in the plant (Bacillus thuringiensis ). This practice is supposed to keep resistance genes unstable in the population (Wu, K., and Guo, Y., 2003).

Bt Toxin Expression
Three different expression patterns have been purposed to control resistance. The use of multiple toxins being expressed in one plant at once is designed to decrease cross resistance. This idea is very similar to the idea behind the Bt sprays. Therefore, it has the same drawbacks with specificity (Wu, K., and Guo, Y., 2003). Overexpression of one Bt toxin is another idea being experimented with (Zwahlen, C., and et al. , 2000). The chloroplasts inside the plant are transformed in order to get overexpression of the Bt toxins (Zwahlen, C., and et al. , 2000). The last expression method being tested is expression of toxins only in certain tissues to reduce selection pressure for resistance insects (DeMaagd, R. A., 1999; Wu, K., and Guo, Y., 2003).

Refuges are discussed in Controlling Negative Effects of Bt Plants.

permission pending from Bacillus thuringiensis

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