Context dependent benefits of aphids for bacteria in the phyllosphere.

Insect herbivores, like aphids, are common on plants yet how they interact with plant microbiomes remains largely unknown. For instance, for the widespread bacterial epiphyte and potential aphid pathogen Pseudomonas syringae, aphids could impact bacterial populations by serving as secondary hosts or by altering the epiphytic habitat through feeding and/or waste secretion. Here we examined if the pea aphid, Acyrthosiphon pisum, could influence epiphytic populations of P. syringae. First, we quantified epiphytic growth ability without aphids and virulence to aphids across 21 diverse P. syringae strains. For eight strains that varied in these traits we then assessed the influence of aphid presence on epiphytic bacterial growth. In some cases, P. syringae benefited significantly from the presence of aphids, with up to 3.8 times more cell doublings. This benefit was not correlated with strain traits, but rather with initial population densities; smaller bacterial populations received relatively more benefit from aphids, and larger populations received less benefit. Honeydew, the sugary waste product of aphids, in the absence of aphids was sufficient to increase P. syringae density on leaves. We conclude that aphid honeydew can sometimes increase P. syringae epiphytic growth, but that the bacteria may not benefit from using aphids as hosts.