Manipulating the frequency of intra-plant parts influences the foraging behaviour of a facultatively florivorous grasshopper

Abstract

Studying density- (total available resources) and frequency-dependent (relative abundance of resources) foraging by herbivores can shed light on our understanding of insect–plant interaction. However, these concepts are rarely investigated on different resources within an individual plant, such as flowers and leaves, even though this can be informative on how each plant individual allocates different resources and defends against florivory and folivory. We performed manipulative experiments in an insectary on the facultatively florivorous grasshoppers (Xenocatantops humilis) and flowering twigs of the tropical dogwood (Mussaenda erythrophylla). The tropical dogwood was used because each plant has both typical leaves and modified calyx lobes of the flowers which function as attractive flags. By manipulating the densities and frequencies, in terms of biomass, of leaves and flag calyx lobes, we aimed to investigate the following questions: (1) Do the grasshoppers exhibit density- and/or frequency-dependent foraging of the intra-plant structures? (2) Do the grasshoppers show a dietary preference for the intra-plant structures? (3) Do relative and/or total biomasses of the intra-plant structures affect the intensity of corolla feeding? Our results provide support for frequency-, but not density-dependent foraging. We found that relative consumption and total herbivory increase with the relative biomass of the preferred flag calyx lobes, whereas a larger corolla diameter increases the intensity of corolla feeding. Our findings suggest that the colourful flag calyx lobes attract unwanted herbivores but could potentially be a decoy against folivory.