Effect of different ratios of wheat to corn flour in the diet on the development and isotopic composition (δ¹³C, δ¹⁵N) of the red flour beetle Tribolium castaneum

Abstract

The back-calculation of the diet is a common application of stable isotopes in animal ecology. The method is based on a predictable relation between the isotopic signature of the diet and the animal’s tissues. Frequently, the assumption of a constant difference in isotopic signatures (trophic shift) is made. Carbon isotopic ratios of C₃ and C₄ plants differ by approximately 10 ‰, making wheat (C₃-plant) and corn (C₄-plant) ideal materials for isotopic studies in nutritional ecology and especially for testing the back-calculation method. In this experiment, red flour beetles, Tribolium castaneum, were reared on wheat flour, corn flour and three different mixtures thereof, either in pure flour or with the addition of yeast inoculum or yeast grains. Development of T. castaneum on these experimental diets was monitored, and isotopic signatures of carbon and nitrogen in emerging adults were analysed. The values of trophic shift of C and N isotopes for wheat and corn flour were different, and the values for the mixtures did not correspond to those expected from a linear mixing model. The latter can be taken as an indication that the tiny larvae of T. castaneum may be capable of differentiating between particles of wheat and corn flour, making this animal model unsuitable for testing the back-calculation method.

Keywords:

• Beetles
• C₃-plants
• C₄-plants
• Carbon-13
• Diet
• Nitrogen-15
• Trophic shift

Acknowledgements

Thanks are due to Dr. Klingler, Tribolium Network/Ludwig-Maximilian-University of Munich, Germany, for providing the initial stock of T. castaneum and advice on the culture, to H. Baumgaertner for assisting in the diet preparation and animal rearing, to B. Fischer for chemical analyses, to R. Langel, KOSI, Georg-August-University Göttingen, for IRMS analyses and to Ms. S. Nugent for checking the English content as well as to the reviewers whose comments contributed to the improvement of the manuscript.

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Ulfert Focken

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