The effect of ants on the frequency of an aphid-bacteria protective mutualism

Hamiltonella defensa is a bacterial symbiont of aphids known to confer protection against parasitoid wasps. The frequency of infection and advantage provided by the bacteria vary widely. Because ant-aphid associations are commonplace and also important to aphid fitness, we hypothesized ants play an important role in determining the net benefit of H. defensa for aphids and explaining the frequency of infection in natural populations. We tested the effect of ants on H. defensa frequency in aphids by excluding F. obscuripes ants from O. frigidae aphid aggregations and comparing infection frequencies between manipulated and undisturbed aphid groups. We measured embryo number in individual aphids, number of aphids, aphid mummies, and ants tending per aphid group. When parasitoids were absent, we found no direct effect of ants on H. defensa frequency in O. frigidae. Due to few parasitoid attacks, we were unable to test correlations of infection frequency and ants with parasitoids. The effect of ants on parasitoid attack rate and thus H. defensa frequency is not clear. However, we found parasitoid DNA was absent in ant-tended aphids without H. defensa. We hypothesize parasitized aphids without H. defensa are removed by ants before observation. Finally, H. defensa significantly increased fecundity of O. frigidae, but we found a trend suggesting infection was associated with smaller groups of O. frigidae. In a natural setting, H. defensa may have an unknown cost to O. frigidae that counteracts the benefits of high fecundity and explains the lack of fixation of H defensa in O. frigidae.