Black soldier fly larvae rearing on side streams: impact on bacterial biota in different gut regions and larval growth

Black soldier fly (Hermetia illucens) larvae (BSFL) are capable of digesting a broad range of substrates and to convert them into protein-rich biomass. Differences in the nutritional composition of feeding substrates may result in differences in gut microbial profiles in the larvae, during the rearing cycle as well as at the end of the whole cycle. Moreover, depending on the insect developmental stage and the region of the gut, differences in microbial profiles may occur. General insight in these aspects may help to optimise the growth of larvae. When BSFL are reared to serve as ingredient in animal feeds, the highest added value can be generated by using low-quality (industrial) side streams as substrates, such as dried distillers grains with solubles (DDGS) and organic kitchen waste (swill). Therefore, the aim of this study was to rear BSFL on DDGS and swill, and monitor their microbiota in function of larval development and gut regions. Moreover, the growth rate of the larvae was compared with their performance on commercial chicken feed as control. During and after rearing, samples of the substrate, larvae, and residue were taken to monitor their bacterial biota. Four different gut regions were isolated from last instar larvae: the anterior, middle and posterior midgut, and the hindgut. The bacterial composition in the different gut districts for the three substrates are now being determined to study the community in each region and their similarity with the whole larvae. Also the bacteria in the substrates and in the residues will be determined to identify correlations. Illumina MiSeq sequencing targeting the 16S rRNA genes within the total DNA from the sample is currently being performed. This technology only provides relative abundances of bacterial genera present in the samples. Therefore, in addition quantitative real-time PCR (qPCR) is being performed, as this allows to transform relative into absolute abundances, giving a more realistic view on the bacterial community. As to growth performance, the weight gain of the BSFL was very comparable when reared on DDGS or chicken feed. After 14 days of rearing, the larvae reached a maximum weight of approximately 300 mg. Larvae reared on swill clearly grew slower and their weight did not exceed 200 mg, even after 21 days of rearing.