Direct and indirect effects of herbivory: Feeding by spittlebugs affects pollinator visitation rates and seedset of Rudbeckia hirta

Abstract

In this study, I examined the effect of the spittlebug Philaenus spumarius on the reproductive output of the monocarpic forb Rudbeckia hirta. Herbivores may affect reproductive output either by reducing resources available for flower and seed production, or by affecting floral and vegetative plant characters, which in turn affect visitation rates by pollinators. For this purpose, I examined the effects of four densities of spittlebugs on plant stature, floral characters, pollination rates, and seed production of R. hirta. I then used path analysis to compare the relative importance of the resource and pollinator limitation pathways. The results indicate that Philaenus reduces both pollinator visitation rates and the number of flowers per flower head, which in turn reduces the number of viable seeds. However, the main route whereby Philaenus affects seed production is by reducing the number of flowers, whereas there seem to be only limited consequences from reducing pollinator visitation rates. The reduced pollinator visitation rates may still affect total plant fitness by reducing male reproductive success.