FIGURE 2-12 Insecticide resistance monitoring methods.

SOURCE: Adapted from information in Coleman and Hemingway (2007).

exposed to these chemicals. In West Africa, DDT and pyrethroid resistance due to kdr is widespread. This resistance was selected for by the evolutionary pressure of DDT used in growing cotton and other cash crops (Mouchet, 1988).

Surveillance of resistance within a population can be done by several methods. The simplest is to measure insecticide dose response, where insects are exposed to a range of concentrations of insecticide and the dose that kills either 50 or 95 percent (LD50 and LD95, respectively) of the population. Populations may be compared by calculating the resistance ratio. Alternatively, and more commonly in an NMCP, a single diagnostic dose of insecticide is used that kills susceptible insects. Alternatively, resistance may be measured at a mechanistic level using biochemical or molecular methods. Although technically more demanding, these methods provide more predictive and accurate information (see Figure 2-12).

Studies to determine the baseline of resistance should ideally be completed before an insecticide is selected (Casimiro et al., 2006a,b). Changes in this baseline can be monitored over time and insecticide treatments switched when required. This has been a component of the Lubombo Spatial Development Initiative (LSDI), a tri-national malaria control program between South Africa, Swaziland, and Mozambique to control malaria (Sharp et al., 2007a). Prior to

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