Tri-trophic interactions mediate the spread of a vector-borne plant pathogen.

Many insect herbivores are vectors that transmit plant pathogens as they forage. Within food webs, vectors interact with a range of host plants, other herbivores, and predators. Yet, few studies have examined how tri-trophic interactions involving vectors affect the spread of pathogens. Here we assessed effects of food web structure on aphid vectors and the prevalence of an aphid-borne persistent pathogen (Pea enation mosaic virus, PEMV) in pea plants. We experimentally manipulated ladybird predators, alternative host plants, and non-vector herbivores and assessed responses of pea aphids and PEMV using structural equation models. We show that variation in bottom-up, top-down, and horizontal interactions mediated PEMV prevalence. Predators reduced PEMV prevalence by consuming aphids, but an alternative host plant (vetch) had the opposite effect by promoting aphid movement and abundance. Non-vector herbivores (pea leaf weevil) increased PEMV susceptibility in peas. In turn, weevils offset the positive effects of predators on PEMV, but increased the negative effects of vetch. Our results show that tri-trophic interactions within insect and plant food webs can mediate vector biology with synergistic and opposing effects on pathogens. Continuing to assess how community-wide interactions affect vectors will aid in our understanding of vector-borne pathosystems.