After an underlying use of the insecticide, not the greater part of the insects present may have been murdered. Perhaps it drizzled, and the chemicals washed off, or the individual who connected the chemicals didn't utilize enough, or the creepy crawlies may have been more tolerant of that compound. The creepy crawlies that survived the main presentation to the synthetic at that point reared and passed on that imperviousness to their posterity, and through proceeded with bug spray applications that would execute the weaker bugs, the ones that survived were the ones who could oppose the chemicals best.