1. Bacterial symbionts have the potential to alter insect fitness, which could influence insect competitive ability. To investigate this possibility, within-host competitions were staged between individuals of the parasitoid, Encarsia inaron Walker, that were differentially infected with the bacterial symbionts Cardinium and Wolbachia.
2. When parasitoids of different infection status parasitised the same whitefly host, there was no evidence that symbiont infection influenced the outcome of competition.
3. Using symbionts as markers, a significant advantage for the second (superparasitising) wasp was detected when eggs were deposited within 4 h of one another, probably as a result of ovicidal behaviour by the superparasitoid, but not when eggs were deposited 1 day apart. Additionally, the emerged sex ratio of superparasitoid offspring was male biased when eggs were deposited 4 h after the initial eggs, but had an even sex ratio when deposited 1 day later.
4. The present study demonstrates the potential utility of symbiont infection as a marking system for investigating within-host competition among parasitoids.