Measurement of predatory behaviour in cow dung-colonising insect larvae, using compound-specific (13)C-tracing of dietary fatty acids.

Studies of the trophic interactions between organisms in opaque environments where direct observation can be difficult, such as soil or leaf litter, often require the use of indirect inferential approaches. Here, the use of compound-specific (13)C-tracing of dietary biomarker fatty acids is evaluated as a method for studying predation by larvae of the dung-breeding fly, Mesembrina meridiana (Diptera: Muscidae); the technique was used to differentiate dung from high-enrichment (13)C-labelled prey in their gut contents. Potential prey, (13)C-labelled larvae of the dung-breeding fly, Neomyia cornicina (Diptera: Muscidae), were placed into unlabelled dung microcosms in the laboratory. A single 7-day-old M. meridiana larva was allowed to feed in each microcosm for 8 h. The magnitude of increases in the δ(13)C values of fatty acids (i14:0, 14:0, i15:0, a15:0, 15:0, 16:0 and 18:0) in the gut contents, relative to those of M. meridiana deprived of prey, demonstrated the predation of N. cornicina larvae which were estimated to have constituted at least 35% of the average dietary wet mass of these M. meridiana larvae. The tracing of specific labelled compounds increased confidence in dietary assessment and helped to avoid systematic errors associated with compound-dependent efficiency of assimilation in the gut. The results demonstrate the potential value of this method in helping to elucidate trophic interactions in predator-prey systems within opaque environments. The precision of the quantitative dietary estimation that arose from these isotopic data was superior to that generated using fatty acid distributional data, a widely used and evidentially independent line of evidence.