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Larvae of the black soldier fly (BSF) <i>Hermetia illucens</i> 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 <i>Actinobacteria</i>, <i>Bacteroidetes</i>, <i>Firmicutes</i>, and <i>Proteobacteria</i>. Members of the genera <i>Klebsiella</i>, <i>Enterococcus,</i> and <i>Sphingobacterium</i> are part of the core microbiome, as they were frequently described in the gut of <i>Hermetia</i> larvae regardless of diet, nutritional composition, or rearing conditions. With 75%, a majority of the fungal isolates belonged to the phylum <i>Ascomycota</i>. We identified several taxa already published to be able to degrade lignocelluloses, including <i>Enterococcus</i>, <i>Cellulomonas</i>, <i>Pichia</i> yeasts, or filamentous <i>Fusarium</i> species. The injection assays revealed pronounced differences in pathogenicity against the larvae. While <i>Alcaligenes faecalis</i> caused no, <i>Diutina rugosa</i> weak (23.3%), <i>Microbacterium thalassium</i> moderate (53.3%), and <i>Pseudomonas aeruginosa</i> and <i>Klebsiella pneumoniae</i> high (≥80%) lethality, <i>Fusarium solani</i> injection resulted in 100% lethality.