Effect of spatial variability in treatment levels on the spread of resistance. The model underlying this graph assumes an area of untreated individuals (f = 0) and an area where all hosts are treated (f = 1), with migration of mosquitoes between both areas. The proportions of resistant parasites at equilibrium in (a) the untreated area and (b) the treated area are given as a function of the cost of resistance (defined by equation 12) and the proportion of mosquitoes migrating from one area to the other. The parameters are as in Fig. 4.