The parasitic bacterium Wolbachia pipientis is estimated to infect as many as 75% of all terrestrial arthropods, making it potentially the most common parasite on Earth. Wolbachia resides within the cells of the host, and is transmitted almost exclusively from mother to offspring. The parasite exhibits a number of strategies that promote the production of infected females, usually at a cost to the host. Given its widespread occurrence and its intimate association with host reproduction, Wolbachia potentially has important demographic consequences for its host.

In the Mediterranean flour moth, Ephestia kuehniella, Wolbachia generates cytoplasmic incompatibility (CI), which causes embryonic death when sperm from an infected male fertilises eggs from an uninfected female. I examined the effects of Wolbachia on individual life history traits, sexual selection and population dynamics in this species. I show that infected males suffer a reproductive cost in terms of reduced sperm production. Coupled with the negative effects of CI on hatching success, this means that females should avoid mating with infected males. I therefore examine whether the parasite affects sexual selection in this host, and whether there are feedback effects at the population level.