It is common for pesticide to work well for several years, but then loose their effectiveness, because insects evolve resistance to these poisons. Professor Wilson and his colleagues conducted research on a global populations of fruit flies.Wilson and his team investigated the insect resistance that has arisen to DDT. Insect's resistance to pesticides revolves around the DDT-R gene.DDT-R produces a metabolic enzyme known P450, which is the agent responsible for breaking down DDT and other poisons. Normally these metabolic enzymes, found in all living organisms from bacteria to humans are present in low amounts. However Wilson and his colleagues found that when insects become resistant to pesticides, there is a dramatic increase in the amount of one of these enzymes due to the overexpression of the DDT-R gene.
     This occurs because the DDT-R gene becomes mutated by insertion of another piece of the fly's DNA known as a jumping gene, into the controlling sequence of the gene.This insertion messes up the normal expression of DDT-R, leading to over production of its product, the cytochrome P450 metabolic enzyme. The result is that as soon as an pesticide enters the body of the insect, it is broken down so efficiently that the poisons never reach their  target tissue to cause death.
            Let me give you an example of how a resistance genetic mutation can spread so quickly. If a farmer has a field with 17 billion pest insects and uses a new pesticide, 16.9 billion are immediately susceptible and are killed. The resistant ones survive and pass on the resistance gene to the next generation. Now the resistant group makes up a larger percentage of the whole. The  farmer sprays later in the growing season and again kills pests, but primarily susceptible ones. After a few more times using the pesticide, there is an increasingly larger percentage of the population that is resistant.
          One has to understand that you cannot win with using pesticide or insecticide for your bees
           george