Figure 3.

Larval insecticides. (A) The epidemiological consequence of the evolution of resistance, if resistance is evolutionarily costly. The cost is reflected as a factor γ by which the biting rate of resistant mosquitoes is divided and the adult mortality rate of resistant mosquitoes is multiplied. (Note that a cost with respect to fecundity is not considered, as fecundity has no epidemiological relevance.) The effect of the insecticide in sensitive mosquitoes is given as the proportion of juvenile mosquitoes that are killed by the insecticide. The line shows the combination of the mortality of sensitive larvae and the cost of resistance where R₀, calculated from the Ross–Macdonald equation, is equal for sensitive (with increased juvenile mortality but no cost) and resistant mosquitoes (with no juvenile mortality but reduced biting rate and increased mortality rate), and the shaded area shows parameters where evolution of resistance increases the effectiveness of control. (B) Evolution of resistance for four levels of insecticide-induced juvenile mortality (shown by the four lines) as a function of the cost of resistance for fecundity and mortality rate. The equations used to calculate the evolutionary response are identical to the ones in Fig. 1, except that (i) juvenile survival of sensitive mosquitoes is decreased by the use of the insecticide, (ii) the mortality rate and fecundity of adult mosquitoes are independent of the insecticide, but in resistant mosquitoes mortality is increased and fecundity decreased by a factor identical to the cost of resistance. (Note that the effect of resistance on biting rate is ignored, for it has no direct effect on evolution.)