Resistance is a natural capability of a biotype of insect species that can withstand certain dose of pesticides which normally sufficient to kill a different biotype of that insect, and that can be pass-on to offspring as well. Among many types of insect resistance, metabolic resistance is the one that deals with metabolic process of insecticide or absorption of toxin in the body of insects, and modification (usually genetic or may be acquired) in the metabolic process of insects that can deviate the death of that insect. If we keep using low dose of insecticide at sub-lethal dose, and insect, in a very non-technical terms, get used to it and that dose does not harm them. Although it has been proved that sub-lethal doses if not kill the insect, but can alter physiological process like delays in developmental stages i.e. larval or pupal stage etc, reduced number of eggs, or lesser hatching percentage etc. Development of resistance by low concentration of insecticide is a slower process and greatly depends on the site of action of insect.
Development of resistance in insects against pesticides and in bacteria against antibiotic is a very common phenomenon to come across with. Scientists developed various strategies to overcome or manage this issue, like changing pesticide every time we use, using appropriate doses, preferring selective insecticides etc. One might have come across his or her physician saying, “complete your antibiotic course even if you feel better”. That strategy is also to manage the development of resistance in bacteria considering the bacteriums life cycle and residue period of the medicine etc.
In nature insect with resistance does exist but since resistant insects cross with other susceptible populations the resultant insects become susceptible (very simple dominant and recessive characters rules). Development of resistant in insects is more likely to develop in confined place like green houses where influx of susceptible population is limited. While using insecticide it is more likely that insecticide may not hit on entire insect population and/ or some of the insects remain un-hurt for available for mating and results in susceptible insects for subsequent crops or season. It would be havoc if entire susceptible population of insects vanishes and the resistant population keeps multiplying.
Same development of resistance phenomenon apply to Bt or GM crops as well, but the difference is insect resistant GM crops are toxic (to caterpillars) and entire insect population is exposed to toxic leaves. By cultivating Bt cotton or insect resistant GM crops and if we cover the entire area under insect resistant crops , it is most likely that all insects feeding on these plants will ultimately die as they feed on insect resistant plant and entire insect population will wipe off, except the one which has natural resistance. In such scenario, resistant population of insects will mate with resistant insects (as there is no susceptible insect population anymore) will result in super bugs which will impossible to control and considering the rate of increase of insects, their population will cover the earth very soon. This is the worst scenario one can think of. To address this worst possible scenario, it was proposed that GM cultivation must be accompanied by 5-10 per cent non-GM crop so that susceptible insect population can survive on these non-GM crops and mate with resistant insect survived on GM crops. This is idea of cultivation of refugia with GM crop. In case of cotton, 8 per cent area under cotton was planted with Bt cotton, hence we still have a natural non-GM cotton growing, and it is hoped our farmer will continue growing some area with non-Bt cotton in future as well. The public sector research institutes are still developing non-Bt cotton varieties with good yields and fiber traits, so there will be a choice of for farmers to choose between Bt and non-Bt cotton. Secondly, in Pakistan mix cropping is practiced and insects get a sufficient chance to feed and multiply on non-GM crops, in the presence of GM cotton in the fields, contrary to USA or Australia where mono cropping is practiced and large acreage comes under single crop and insects get very little chance to feed on non-GM crops. This clearly shows refugia planting is more important in developed countries than in Pakistan.