RELATIVE FITNESS OF ORGANOPHOSPHATE-RESISTANT AND SUSCEPTIBLE STRAINS OF CULEX-QUINQUEFASCIATUS SAY (DIPTERA, CULICIDAE)

The effects of chlorpyrifos resistance on fitness in C. quinquefasciatus Say was evaluated in terms of life-span, fecundity, fertility and rate of development, an these parameters were integrated by calculating the intrinsic rate of natural increase. A chlorpyrifos-resistant strain from Tanzania had very significantly reduced biotic fitness compared to a standard susceptible strain, CfCA, from Sri Lanka. However, after 5 generations of backcrossing the resistant gene into the genetic background of the susceptible strain, fitness was closer to that of the susceptible strain. The reduced fitness of the original Tanzania strain therefore appears to be mainly due to background genetic factors rather than to the major resistance gene itself.