Insect resistance is not a mutational reaction to an applied pesticide, rather, it is pre-adaptive.  Resistant individuals emerge in insect populations due to normally occurring mutations in DNA replication.  These mutations would most likely go undetected in the insect population.  With the application of a chemical insecticide, however, one is subjecting any and all resistant individuals to positive selection for resistance to that particular chemical.  So while the non-resistant insects have been disposed of, a few resistant individuals live on.  And since resistance is a heritable trait, those original resistant insects will establish a new population consisting entirely of resistant individuals.  Whether we increase the concentration of that particular chemical or employ a completely novel chemical, the story of resistance remains the same.

        Resistance in insects has been categorized into four major types:
            --Increased metabolism
            --Target site insensitivity
            --Reduced cuticular penetration
            --Behavioral resistance

        With an increased metabolism mechanism, the insect mobilizes its normal metabolic enzymes to attack toxic molecules and push the toxins out of its system as quickly as possible.  Some members of the genus Lepidoptera also possess detoxification enzymes in their guts that can neutralize some toxins as soon as they are ingested.

        Target site insensitivity involves a conformational change at the receptor site for a particular toxin that inhibits or excludes binding.  Target site insensitivity may also be conferred by a reduction in the total number of receptors for a particular chemical.  This type of resistance has been documented in the housefly for the chemical insecticide DDT.

        Reduced cuticular penetration simply means that the insect possesses a thicker exoskeleton than normal, and therefore is absorbed into the insect’s system much more slowly.  Although fairly ineffectual on its own, this mechanism works particularly well in conjunction with other types of resistance.  For example, if an insect had a slight metabolic resistance, the reduced penetration would amplify the insects ability to degrade and get rid of a dose of insecticide over time.

        Behavioral resistance is the most difficult mechanism to account for, from a control perspective.  Simply put, an insect exhibits behavioral resistance by avoiding plants that possess non-host chemicals.  This mechanism may be more of an increased sensitivity to a particular chemical than a physical resistance, but the characteristic is still heritable.

        A note on behavioral resistance:  Since behavioral resistance is actually an avoidance of a particular chemical no selection is involved, other types of resistance do not arise.  Therefore, it seems logical that the phenomenon of behavioral resistance may help to maintain susceptibility to a particular chemical within a population.  It had been proposed that by using insecticides with repellent properties, we can reduce the rate at which resistance develops in a particular population.  However, it is unclear whether this type of approach would only serve to indicate susceptible individuals; non-susceptibles would continue to arise due to random mutation.

        In summary, whenever a chemical insecticide is used on a population of insects, whether it be synthetic (DDT) or a natural product (Bt toxin) all we are doing is selecting those individuals who, by chance, are resistant to that chemical.  In the short run, we have severely crippled that insect population for the season.  In the long run, however, we will only have to spend more money to develop a new and "better" chemical to deal with the population of resistant insects that will establish itself within a few generations.