Abstract
Larvae of the black soldier fly (BSF) Hermetia illucens are polyphagous feeders and show tremendous bioconversion capabilities of organic matter into high-quality insect biomass. However, the digestion of lignocellulose-rich palm oil side streams such as palm kernel meal (PKM) is a particular challenge, as these compounds are exceptionally stable and are mainly degraded by microbes. This study aimed to investigate the suitability of BSF larvae as bioconversion agents of PKM. Since the intestinal microbiota is considered to play a key role in dietary breakdown and in increasing digestibility, the bacterial and fungal communities of BSF larvae were characterized in a culture-dependent approach and screened for their putative entomopathogenicity. The lethality of six putative candidates was investigated using intracoelomal injection. In total, 93 isolates were obtained with a bacterial share of 74% that were assigned to the four phyla Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. Members of the genera Klebsiella, Enterococcus, and Sphingobacterium are part of the core microbiome, as they were frequently described in the gut of Hermetia larvae regardless of diet, nutritional composition, or rearing conditions. With 75%, a majority of the fungal isolates belonged to the phylum Ascomycota. We identified several taxa already published to be able to degrade lignocelluloses, including Enterococcus, Cellulomonas, Pichia yeasts, or filamentous Fusarium species. The injection assays revealed pronounced differences in pathogenicity against the larvae. While Alcaligenes faecalis caused no, Diutina rugosa weak (23.3%), Microbacterium thalassium moderate (53.3%), and Pseudomonas aeruginosa and Klebsiella pneumoniae high (≥80%) lethality, Fusarium solani injection resulted in 100% lethality.
Highlights
Introduction iationsHermetia illucens (Diptera: Stratiomyidae; BSF), commonly known as the black soldier fly, is native to the tropical and subtropical regions of America
The cultivable microbial gut community of BSF larvae reared on palm kernel meal (PKM) was predominantly composed of bacteria
The course of infection and the pathogenicity of Klebsiella and Pseudomonas species were comparably high (16.7% and 20.0% survival), albeit only P. aeruginosa caused the significantly greater killing of the larvae than the moderate M. thalassium (p = 0.02)
Summary
Hermetia illucens (Diptera: Stratiomyidae; BSF), commonly known as the black soldier fly, is native to the tropical and subtropical regions of America. In addition to a wide tolerance range for environmental factors, their short life cycle of 40–45 d makes the fly an interesting candidate for industrial applications [2]. Based on the polyphagous diet of the larvae, they have an extremely wide substrate spectrum. Various studies report their bioconversion capabilities of different organic matter and waste such as kitchen scraps, pineapple peelings, soybean residue [3], brewer’s spent grains [4], fish offal [5], as well as chicken and swine manure [6,7] into high-quality insect biomass. The high protein and fat content of the larvae are promising for novel uses in the feed sector, as an alternative to fish and soy meal formulations in aquaculture or livestock, respectively [8]
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