Interactions between insect vectors and plant pathogens span the parasitism-mutualism continuum

Plants infected with vector-borne pathogens can suffer severe negative consequences, but the extent to which phytopathogens affect the fitness of their vector hosts remains unclear. Evolutionary theory predicts that selection on vector-borne pathogens should favour low virulence or mutualistic phenotypes in the vector, traits facilitating effective transmission between plant hosts. Here, we use a multivariate meta-analytic approach on 115 effect sizes across 34 unique plant-vector-pathogen systems to quantify the overall effect of phytopathogens on vector host fitness. In support of theoretical models, we report that phytopathogens overall have a neutral fitness effect on vector hosts. However, the range of possible fitness outcomes are diverse and span the parasitism-mutualism continuum. Contrary to previous predictions we found no evidence that transmission strategy, or the direct effects and indirect (plant-mediated) effects, of phytopathogens have divergent fitness outcomes for the vector. We discuss these findings in the context of plant - pathogen - vector ecology.