In this article we will discuss about the representation of High-Dose/Refuge Strategy.
Over the past one decade—researchers, government regulators, environmentalist and even private sectors have extensive debate over the occurrence of resistance to Bt crops and delay in the development of resistance process.
In this context one of the promosing strategies is the high dose/refuge resistance management. This strategy has been enforced for Bt cotton, maize and potato in the USA, Bt maize in Canada and Bt cotton in Australia.
Before highlighting how high dose/refuge strategy works, it is indispensable to examine the genetic basis of resistance. In many instances, resistance to an insecticide is caused by a mutation in one gene of an insect. If there are two possible forms of the gene (alleles) i.e. R is the mutant allele, conferring resistance and S is the normal allele, conferring susceptibility. Each insect has two copies of the gene.
Then there are three possible genetic types (genotypes) of insects. They are SS, RS, and RR. Once insecticides contact insects, the response of RS genotype insects to the insecticide is intermediate between that of the SS and RR insects.
But it is more similar to that of the SS insects indicating that the R allele is partially recessive. To enforce the high-dose/refuge strategy, it is indispensable to have a level of toxin in the Bt cultivar that is high enough to kill almost all of the RS insects.
Refuges are non-Bt crops planted in the field to maintain Bt susceptible insects in the population. In refuges strategy, fields are planted with non-Bt plants or growing non-Bt plants within fields of Bt plants. This would slow down development of resistance particularly if resistance is not stable in insect population.
The large number of insects with the SS genotype that survive on the refuge plants are then available to mate with the small number of RR insects that surive on the Bt plants. After mating, SS × RR offspring will be RS and they will not survive when they feed on high-dose Bt plants. Refuges practice has found to be successful in reducing fixation of resistance (Fig. 20.8 (a)). 