Power of the crowd: substrate-dependent impact of black soldier fly larvae on bacterial community composition in substrate and larval gut

Background: Black Soldier Fly (BSF) larvae are used as a sustainable component of animal feed, because they can convert a wide range of organic waste types into insect biomass. In the decomposing substrate, they interact with a rich microbial community of bacteria and fungi, which strongly depends on the type of substrate. These microorganisms may impact larval performance, whereas the larvae themselves can alter substrate properties and bacterial communities – an impact that is enhanced as larvae aggregate. However, the relative importance of substrate type and larval density on bacterial community dynamics is unknown. We investigated four larval densities (0 (control), 50, 100, or 200 larvae per container (520 ml, Ø 75 mm)) and three feed substrates (chicken feed, chicken manure, and camelina oilseed press cake) and sampled bacterial communities of substrates and larvae at three time points over 15 days.Results: We found that BSF larvae altered bacterial community composition over time, in all three feed substrates and that substrate type was the strongest driver of bacterial successioncommunity composition. The impact of the larvae depended on substrate and larval density, which was possibly related to substrate nutritional value, foraging behaviour, and larval performance. Larval and substrate microbiota differed for chicken manure and camelina, whereas they overlapped in chicken feed.Conclusion: These findings demonstrate the flexibility of the association between bacteria and BSF larvae and support the substrate-dependent impact of BSF larvae on bacteria both within the larvae and in the substrate. This study indicates that bacterial community composition and possibly insect microbiological safety can be manipulated through substrate composition and larval density.