Through evolution plants and animals have developed defensive mechanisms, including chemical repellents and toxins, against attacking organisms. In turn the attacking organisms have developed mechanisms that enable them to detoxify or otherwise resist the defensive chemicals of their hosts. Thus, it appears that most pest species already contain genes that enable them to degrade enzymatically or otherwise circumvent the toxic effects of many types of chemicals that have been developed as modern pesticides. Pesticide resistance, therefore, is a genetically based phenomenon and occurs when a pesticide is used on a population containing some individuals genetically predisposed to be resistant to that pesticide. Repeated applications and higher treatment rates will kill increasing numbers of the pest but resistant survivors will pass the resistance genes to the next generation. Unless a different treatment regime is used the population will contain increasing numbers of resistant pests and where reproductive rates are high, e.g. in insects, the entire population will quickly become resistant.
Once a pest has developed resistance to a particular pesticide it is necessary to have other means of controlling it. One method is to use a different pesticide, especially one in a different chemical class that has a different mode of action against the pest. Care must be taken, however, that multiple resistance, i.e. resistance to several classes of pesticides, is not introduced into the population. The best strategy is to avoid building up a resistant population and various procedures have been developed for this. Various management strategies have been recommended to avoid the build up of resistant populations due to the use of pesticides. The US EPA and the Canadian PMRA have drawn up a voluntary labelling scheme, together with recommendations for avoiding resistance, giving mode of action and target site information to users.