Changes in delta 13C stable isotopes in multiple tissues of insect predators fed isotopically distinct prey

Traditionally, researchers have used measurements of carbon stable isotopes to infer the composition of consumers' diets. However, since consumer's tissues may process carbon isotopes differently, particularly following a diet shift, it is possible to use measurements of carbon isotopes in multiple tissues to determine not only the composition of an individual's diet, but also the temporal dynamics thereof. This study examined how stable isotopes of carbon (13C/12C, expressed as delta 13C) changed in different adult tissues of two predacious beetles, Harmonia axyridis and Coccinella septempunctata (Coleoptera: Coccinellidae). In the laboratory, we switched ladybeetles from a C3-based diet (soybean aphids, Aphis glycines) to a C4-based one (corn leaf aphids, Rhopalosiphum maidis). The delta 13C of metabolically active tissues such as the body fat and reproductive organs changed rapidly (< or =5 days) following the diet shift. Tissues expected to be more metabolically inert, such as wings, changed more slowly over the same period. Although these general patterns were largely similar between males and females, females had more rapid changes in delta 13C in fat and reproductive tissues. However, females showed a significant depletion in delta 13C after 10 days, while males' delta 13C continued to increase. Given the results of this experiment, it is now possible to distinguish between ladybeetles eating a mixed diet (beetles with multiple tissues at similar, intermediate, equilibrial delta 13C signatures) from those that have shifted diets (beetles with different tissues at distinctly different delta 13C values). Thus, this approach can be used broadly to infer not only what constitutes the diet of a consumer, but also the temporal history of dietary intake.