Insecticide resistance is an increasingly serious problem for the control of medically and agriculturally important pests. At biochemical level two major mechanisms govern the development insecticide resistance in various insect species. These mechanisms include target site insensitivity and increased detoxification. For any proper control program, detection of these mechanisms is fundamental for success. Several methods have been used to detect and monitor resistance to insecticide. Traditional methods use bioassay tests to measure the resistance level in the field. Biochemical assays for some enzymes involved in the resistance mechanism to detect the early onset of resistance. More recently the advances in molecular biology and genetics have enable better diagnostic of biochemical mechanisms that lead to the development of resistance.
What is insecticide resistance?
Resistance to insecticides may be functionally defined as the ability of an insect population to survive exposure to dosages of a given compound that are lethal to the majority of individuals of a susceptible lineage of the same species (Beaty and Marquardt, 1996). It is a dynamic and evolutionary phenomenon that arose as a result of the combination of genetic makeup and selection pressure by insecticides. There are several mechanisms through which insects develop resistance to insecticides, and usually more than one of these mechanisms act simultaneously to provide strong protections against insecticides. Resistant insects may metabolize the insecticide at a faster rate than susceptible ones, thereby preventing it from reaching the target site where it is due to exert the lethal action. This mechanism is known as enhanced detoxification and is responsible for the rapid development of resistance. In some cases the molecular target at which the insecticide bind is altered to prevent or reduce the binding . Consequently the target site become insensitive to the insecticide, allowing the insect to evade its lethal action. Sometimes insects may just avoid the insecticide by changing their normal activity (behavioral resistance) or by absorbing the toxin more slowly through development of tougher cuticle that reduce the penetration of insecticides. This mechanism know as penetration resistance does not produce significant level of resistance and generally supplements other resistance mechanisms. Recently symbiotic bacteria have been found to confers insecticide resistance to their host(Kikuchi et al., 2012).
At molecular level, target site insensitivity and enhanced detoxification are the two principal mechanisms that lead to powerful development of resistance in various insect species. In this paper, we review these mechanisms of and the methods used to detect occurrence of insecticide resistance.