Ant-aphid mutualisms and community composition influence the spread of aphid-vectored plant viruses

Plant viruses are ubiquitous, understudied and can have potentially large impacts on the population dynamics, community structure and evolutionary dynamics of natural (non-agriculture) plant communities.  Cucumber mosaic virus (CMV) infects over 1000 plants species in 100 families, and it is vectored by aphids in a non-persistent manner. For aphid-vectored plant viruses, vector density strongly influences virus incidence.  Many aphid species enter into mutualistic relationships with ants, and this mutualism can lead to increases in aphid densities.  Therefore, we predict that the presence of fire ants, an invasive ant species that forms strong facultative mutualisms with honeydew-producing aphids, will lead to increased CMV incidence in herbaceous plant populations.  For 2 years, plant species composition and abundance, fire ant density, aphid density and species composition, and CMV incidence were surveyed in herbaceous plant communities on tomato farms in north Alabama.  We found that fire ants can dramatically increase CMV incidence, but that plant species composition plays an important role in mediating this effect.  More specifically, aphid species identity and density on key host plant species was a major predictor of the effects of fire ants on the spread of CMV.  This research highlights the role that aphid-ant mutualisms can have in virus dynamics in herbaceous plant communities and the role that plant community composition plays in mediating these effects.