A large and extremely diverse number of insects harbour maternally transmitted bacterial symbionts. Some symbionts manipulate host reproduction in order to benefit their own fitness, and the most common of these reproductive manipulations is cytoplasmic incompatibility (CI). In CI, uninfected females produce few or no viable progeny when mated to infected males. The bacterial endosymbiont Cardinium causes CI in its host, Encarsia pergandiella (Hymenoptera: Aphelinidae). I used population cages with varying initial infection frequencies to test a model of CI invasion. Cardinium was found to spread rapidly in all populations, even in cases where the initial infection frequency was well below the predicted invasion threshold frequency. Male age can also be an important factor in CI dynamics. I tested the effect of male age on the level of incompatibility induced by Cardinium in E. pergandiella. Male age was found to have a negligible effect on CI strength.