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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 C4 photosynthetic pathway and consume plants that possess only the C3 pathway. The relative abundances of stable carbon isotopes in insect tissues, which indicate consumption of C3 or C4 plants, were used to determine if insects were consuming C3 and C4 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 C4 species. Insect consumption of C4 plants was °50% of that expected if insects were consuming C3 and C4 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 C4 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.

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