Conditional reduction of predation risk associated with a facultative symbiont in an insect

Symbionts are widespread among eukaryotes and their impacts on the ecology and evolution of their hosts are meaningful. Most insects harbour obligate and facultative symbiotic bacteria that can influence their phenotype. In the pea aphid Acyrthosiphon pisum, an astounding symbiotic-mediated phenotype has been recently observed: when infected with the symbiotic bacteria Rickettsiella viridis, young red aphid larvae become greener at adulthood and even darker green when co-infected with Rickettsiella viridis and Hamiltonella defensa. As body colour affects the susceptibility towards natural enemies in aphids, the influence of the colour change due to these facultative symbionts on the host survival in presence of predators was tested. Our results suggested that the Rickettsiella viridis infection may impact positively host survival by reducing predation risk. Due to results from uninfected aphids (i.e., more green ones attacked), the main assumption is that this symbiotic infection would deter the predatory ladybird feeding by reducing the profitability of their hosts rather than decreasing host detection through body colour change. Aphids coinfected with Rickettsiella viridis and Hamiltonella defensa were, however, more exposed to predation suggesting an ecological cost associated with multiple infections. The underlying mechanisms and ecological consequences of these symbiotic effects are discussed.