Mutualists that provide redundant services to the same organism have been shown both to coexist and compete for access to the partner. Aphids, for example, are known to receive protection against natural enemies from both heritable bacterial symbionts as well as ants, which tend aphids in exchange for sugary honeydew excretions. Little is known, however, about how ant attendance might influence interactions between aphids and their protective bacteria. Assuming costs to infection, we hypothesized that protective symbiont prevalence would decrease in the presence of ants, because of duplication of services. We surveyed natural aphid populations and found Hamiltonella defensa, a bacterium that provides resistance to aphids against parasitoid wasps in laboratory settings, among several aphid species, including Obtusicauda frigidae (Oestlund 1886). We excluded ants from protecting O. frigidae aggregations and measured the effect of ant absence on the frequency of parasitoid attack and H. defensa infection in aphid groups. Ant presence did not reduce H. defensa frequency in aphids, likely because ants and H. defensa did not significantly lower parasitoid wasp attack rates. Metabolic costs likely control H. defensa prevalence more than the effect of ant-tending.
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