Wolbachia are intracellular endosymbionts that may spread throughout populations by manipulating the reproduction of their hosts. The principal vector of the dengue virus, the mosquito Aedes aegypti, is not naturally infected with Wolbachia, but it has been transinfected experimentally. A recent discovery that Wolbachia suppresses dengue virus replication in the mosquito has driven investigation into its application as a biological dengue control agent. Successful implementation of this strategy may eliminate the need for mosquito eradication to control the spread of the disease.

In order for Wolbachia to invade target populations and supress dengue transmission, infected mosquitoes must be competitive with the native inhabitants. Wolbachia impose fitness costs on the mosquito which decrease the likelihood of the infection reaching fixation in a population. Consequences of Wolbachia infection are well understood in the adult host. However, during field releases, Wolbachia-infected mosquitoes are likely to encounter fierce competition with wild-type mosquitoes in larval habitats. The effects of crowding and competition on Wolbachia-infected larvae are therefore important to understand so that the potential for Wolbachia to invade field populations can be assessed.

We tested for effects of two Wolbachia infections, wMel and wMelPop, on the fitness of immature Ae. aegypti when developing under competitive conditions. Development of Wolbachia-infected larvae is delayed when competing in mixed cohorts with uninfected larvae. wMelPop-infected adults that develop slowly are smaller relative to uninfected, and have a shorter lifespan and a greater density of infection relative to faster developers. Adult females infected with either Wolbachia strain also have reduced fertility compared to uninfected females. These costs to fitness under larval competition may limit the potential for Wolbachia infections, particularly wMelPop, to invade areas where larval habitats are limiting. These results have implications for the use of Wolbachia as a biological control agent in areas where dengue is endemic.