Visual Detection and Avoidance of Pathogenic Bacteria by Aphids

Aphids are diverse sap-sucking insects [1] that can be serious agricultural pests and vectors of plant disease [2]. Some species, including pea aphids (Acyrthosiphon pisum), are susceptible to infection by epiphytic bacteria that are commonly found on plant surfaces [3-5]. Pea aphids appear unable to recover from these infections, possibly because pea aphids are missing apparent orthologs of some immune response genes [6], and these aphids exhibit relatively low immune responses after pathogen exposure [7]. We therefore tested the ability of pea aphids to use avoidance as a non-immunological defense against Pseudomonas syringae, a widespread plant epiphyte and aphid pathogen [8, 9]. Pea aphids avoided highly virulent strains of P. syringae, but not all strains, and avoidance led to a significant reduction in infection among aphids. We found that aphids can use visual cues to detect the ultraviolet (UV)-based fluorescence of the bacterial siderophore pyoverdine [10] produced by virulent strains. Avoided epiphytic bacteria caused light leaving the surface of leaves to be richer in wavelengths that were tightly linked to both aphid visual sensitivities and the fluorescent emission spectra of pyoverdine, suggesting that pyoverdine fluorescence mediates avoidance and may be a visual cue used by aphids to detect epiphytic pathogens. Although pyoverdine production in Pseudomonas species may be a broadly reliable indicator of bacterial virulence within the phyllosphere, it was not directly responsible for virulence to aphids. Aphids may be under selection to avoid fluorescence on leaves, a phenomenon with potential use for the control of agricultural pest insects.