Testing host-associated differentiation in a quasi-endophage and a parthenogen on native trees

Host-associated differentiation (HAD) is the formation of genetically divergent host-associated sub-populations. Evidence of HAD has been reported for multiple insect herbivores to date, but published studies testing more than one herbivore for any given host-plant species pair is limited to herbivores on goldenrods. This limits the number of pair-wise comparisons that can be made about insect life-history traits that might facilitate or inhibit host-race development in general. Two traits previously proposed to facilitate HAD include endophagy and parthenogenesis. We tested for HAD in two herbivores, a quasi-endophagous caterpillar and a parthenogenetic aphid, feeding on two closely related species of hickories. We found that the quasi-endophage is panmictic, whereas the parthenogen exhibits HAD on their sympatric host plants, pecan and water hickory, at a geographic mesoscale. This is an important first step in the characterization of HAD in multiple insect herbivores using North American hickories, a host-plant system with many shared parthenogens.