Insect food preferences analysed using 13C/12C ratios.

Stable carbon isotope ratio analysis is a powerful technique in tracing ecosystem carbon flows, especially those between primary and secondary producers. The distinctive 13C/12C ratios of plant species tend to pass along the food chain with little further fractionation, hence the stable carbon isotope composition of an animal is an important clue to what it has eaten. We compared the stable carbon isotope composition of plants and insects in an old field in Georgia. Of the dominant plants in the old field, 6 were C4 species and had δ13C1 values of-10.9‰ to 12.9‰, and 7 were C3 species with values of-27.3‰ to-29.1‰. Insects known to be feeding on only one plant species had δ13C values within 1‰ of the isotopic composition of the plant. Wasp larvae parasitizing two insect species had δ13C values 1.3 and 1.7‰ higher than that of the food plant. A variety of insects of unknown food habits collected on monospecific and mixed species plant stands in the old field had δ13C values ranging from-10.1‰ to-30.0‰. Two species of leafhopper and a grasshopper had isotopic compositions within the range of C4 plant values; a tortoise beetle and a lace bug had isotopic compositions within C3 plant values. Other insects had intermediate δ13C values, suggesting a mixed diet composed of both C3 and C4 plants. The carbon isotopic ratios of field collected insects appears to be a useful qualitative indicator of their feeding preference.