Aedes aegypti control: the concomitant role of competition, space and transgenic technologies.
Over 2 billion people are currently at risk of infection with dengue fever. Use of the sterile insect technique (SIT) in controlling the vector, Aedes aegypti, has been recommended but remains a matter of some debate. Density-dependent, time-delay mathematical models are developed to establish the conditions under which a SIT programme can actually increase both isolated and connected mosquito populations. This potential problem is caused by an increased survival of wild-type juvenile stages as a result of the reduction in the density-regulated larval population. The non-spatial model demonstrates that releases that are insufficient to collapse the local population may result in an increase in the mosquito population. The range of release ratios over which this increase occurs is broadened if the sterile males have either reduced mating competitiveness or incomplete sterility. The release of insects carrying a dominant lethal gene (RIDL) that is tailored to act at the late larval stage is a recently proposed modification of the SIT. Simulations suggest that pest suppression is more effective with these 'RIDL males' than with conventional sterile males. As with the SIT, collapse of the simulated pest population is more restricted by the inclusion of residual fertility of the released males than it is by a reduced mating competitiveness. When migration between simulated populations can occur, the release of sterile males can have significantly detrimental effects to areas neighbouring the control zone. Increased connectivity between the target zone and surrounding populations can act to increase the non-target pest populations further. This undesirable effect was not evident in simulations based on the use of RIDL. Synthesis and applications. As the sterile insert technique, and related genetic advances, are increasingly considered as tools for pest management, it is important to consider their wider ecological implications, in particular with respect to areas neighbouring a targeted control. Although results presented here focus on the primary vector of dengue fever, they are clearly applicable to the management of any spatially distributed insect pest with over-compensatory population dynamics.