Insect Herbivory and Photosynthetic Pathways in Old‐Field Ecosystems

Three old—field plant communities of varying composition near Aiken, South Carolina, were used to test the hypothesis that phytophagous insects avoid consuming plants possessing the C 4 photosynthetic pathway and consume plants that possess only the C 3 pathway. The relative abundances of stable carbon isotopes in insect tissues, which indicate consumption of C 3 or C 4 plants, were used to determine if insects were consuming C 3 and C 4 plants in proportion to their abundance in the plant community. In one community, the carbon isotope ratio for insects was significantly less than that expected for proportional consumption and indicated avoidance of C 4 species. Insect consumption of C 4 plants was °50% of that expected if insects were consuming C 3 and C 4 plants in proportion to their abundance. In the other two communities, the differences between observed and expected isotopic ratios were not significant. Levels of insect consumption of C 4 plants in these two communities were, respectively, °82% and °126% of those expected for proportional consumption. The results suggest that the degree of avoidance varies among plant communities.