Nutritional effects on growth and development of sepsid flies

The plastic nutritional foundations of growth and reproduction are crucial in mediating animal (insect) life histories but remain largely unexplored. Here, we investigate how the nutritional quality of dung during larval feeding affects subsequent (adult) life history traits of a coexisting guild of four closely related European sepsid dung fly species: Sepsis cynipsea (L.), Sepsis fulgens (Meigen), Sepsis punctum (Fabricius), and Sepsis thoracica (Robineau-Desvoidy) (all Diptera: Sepsidae). Larvae were raised in standard cow dung, protein-rich (yeast-supplemented) dung, microbe-depleted (autoclaved) dung, and in a combined treatment of autoclaved dung with yeast enrichment. Responses of egg-to-adult survival, total development time, final adult body size, and female reproductive potential in terms of age at first reproduction and offspring number were assessed. Overall survival was lower in autoclaved dung, but also tended to be reduced rather than enhanced by potentially artificial microbial additives such as supplemental yeast, which apparently cannot be metabolized effortlessly by the fly larvae. The only trait positively affected was adult body size, for which we observed an increase in three of four species when larvae were raised in yeast-supplemented dung. Flies emerged smaller when reared in autoclaved dung, and the addition of yeast to autoclaved dung partly compensated the negative effect of autoclaving to result in a body size similar to control dung. Our results thus indicate that larval growth is likely reduced by autoclaving-induced killing of nutritional microorganisms in the dung, which however may recolonize. Whereas for some model insects nutrition is well-understood, more research is necessary to unravel the diverse effects of natural and artificial stressors on the nutrition of non-model insects, such as these dung-decomposing flies providing important ecosystem services in human-mediated grasslands.