Bottom-up Mediation of an Ant-Membracid Mutualism: Effects from Different Host Plants

The relationship between the membracid, Publilia modesta, and the tending ant, Formica obscuripes, was either a mutualism or commensalism depending on the host plant species. Experimental manipulation of the presence of ants in two different years indicated that the presence of ants had a positive effect on nymph numbers on both host plants, Chrysothamnus viscidiflorus and Wyethia spp. However, Wyethia spp. senesced before membracid nymphs reached adulthood, causing extensive mortality of membracids. In contrast, C. viscidiflorus plants senesced after nymphs had developed into adults. The increased number of nymphs in the presence of ants translated into more new adults on C. viscidiflorus, but not on Wyethia spp. Poor host plant choices may render the presence of ants irrelevant for such insects on some host plants. Being a host plant generalist may lead to significant variability in the outcomes of mutualistic interactions.