Performance Tradeoffs for Two Hosts within and between Populations of the Oligophagous Grasshopper Hesperotettix viridis (Acrididae)

Abstract

Diet selection by insect herbivores often varies among sites in response to combined ecological and evolutionary factors. The oligophagous grasshopper Hesperotettix viridis feeds almost exclusively on composites (Asteraceae) in the tribe Astereae. While host lists compiled from throughout the species' geographic range are extensive, restricted diets within populations suggest local adaptation by H. viridis to specific but different host plants. We evaluated population differentiation in performance (developmental rate, individual weight gain, and survivorship) by H. viridis to two host plant species (Solidago mollis and Gutierrezia sarothrae) using reciprocal transplant experiments in the field. Even though out experiments included a conditioning treatment, individuals from all populations generally performed better on host plant species from which they were collected than on alternate hosts. Physiological tradeoffs in performance on alternate host plants best explain this result, a result expected if population differentiation in host plant use exists. Grasshoppers originally collected from G. sarothrae developed faster, survived longer, and gained more weight when fed G. sarothrae. Similarly, individuals originally collected from S. mollis developed faster and gained more weight on S. mollis compared with G. sarothrae although survivorship did not differ. Consumption of host species in paired choice tests revealed overwhelming preference for host species from which the grasshopper was collected, regardless of the host conditioning. Differentiation in performance on the two host species including negative tradeoffs also occurred at a single site, coupled to sensory biases in host selection reinforce the evolution of diet specialization in H. viridis.