Introduction

Larvae of the black soldier fly (Hermetia Illucens) can be used for the production of protein and fat for poultry farming and aquaculture. Larval biomass can be obtained by utilizing local side streams from agricultural or industrial production as insect feed. The residue from black soldier fly larvae (BSFL) farming, so-called frass, consists of insect excrements and feed residues. Frass is produced as a side stream itself and can be used as a valuable organic fertilizer.

Goals

The aim of our study was to evaluate different side streams as feed for BSFL and how the frass differs chemically and microbially depending on the infeed substrate. We also investigated the effect of thermal hygienization at 70°C to reduce microbes, as required by the EU directive, with regard to the effects on the microbiome that promotes plant growth. The findings should contribute to the development of frass as fertilizer for agriculture.

Methods

Our larvae were fed with potato peelings, potato pulp, apple pomace and rapeseed press cake to study their growth behaviour, while substrate, larval and feed samples were determined by elemental analysis. The frass was hygienized and the count of colony forming units (CFU) on selective and non-selective media was compared with that of an untreated sample. Subsequently, microorganisms were isolated and identified by Sanger sequencing. Amplicon sequencing of the frass samples was carried out for comparison.

Results

In the feeding trials, substrate-dependent differences in BSFL growth and chemical composition of the frass were found, with chicken feed showing the best results, also with regard to its use as NPK fertilizer. Hygienization was shown to have the desired effect, namely a reduction in CFU. The amplicon data so far showed recognizable variations in the microbiome, depending on the feed used.

Summary

Overall, we were able to show that not every industrial side stream is suitable for optimal larval growth characteristics. We were able to demonstrate the CFU-reducing hygienization effect in frass. We also found chemical differences and a clear variance in the microbiome depending on the feed used for BSFL rearing.