Several species of parasitoid wasps have been used to control aphids in the field in order to improve the environmental sustainability of the current agricultural practices. Behavioural and physiological defences have been reported in aphids and the role of symbiotic bacteria, such as Hamiltonella defensa, Serratia symbiotica and Regiella insecticola, has been molecularly dissected to understand their protection against hymenopteran parasitoids. Interestingly, the relationship between the presence of defensive symbionts and the type of aphid-ant mutualism (obligate, facultative or absent) is still understudied, whereas ants could strongly influence the ecological costs of defensive symbionts. In the present paper, we performed a screening of 16 aphid species collected in Italy and compared the presence and abundance of defensive symbionts to the degree of myrmecophily of the sampled aphid populations revealing that Hamiltonella defensa is the most common defensive symbiont. Our data suggest a direct effect of ant-tending on the abundance of H. defensa such that if aphids are maintained in insectaries in the absence of ants, aphids increase the amount of this defensive symbiont making the composition of their microbiome context-dependent.
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