Dynamics of the intestinal bacterial community in black soldier fly larval guts and its influence on insect growth and development

Artificial light source combined with functional microorganism improves reproductive performance of black soldier fly

Black soldier fly (BSF, Hermetia illucens) has been recognized as a promising insect species for sustainable conversion of organic waste into valuable biomass. Many studies have focused on the use of BSF larvae to improve the recycling of organic waste. However, few studies have been conducted on the reproductive efficiency of BSF adults. In particular, the major problems with artificial systems are directed oviposition and the poor oviposition rate due to inadequate sunlight. This study aimed to address the bottleneck by developing an effective artificial source and finding effective attractants. Results showed that our homemade artificial light significantly enhanced the number of BSF eggs and elevated the egg hatching rate compared to commercial artificial light. Simultaneously, the isolated strain of Trichosporon asahii BSFL-2 can induce gravid BSF females to oviposit and the combination of homemade artificial light with BSFL-2 resulted in a notable increase in the number of eggs collected in the BSF adults rearing system. Analysis of the gas chromatography-mass spectrometry results and experimental validation showed that 2,5-dimethylpyrazine produced by BSFL-2 strain was able to attract gravid females to aggregate and oviposit. This study demonstrates that the use of effective artificial light source and attractant is a crucial element in addressing the bottleneck of efficient indoor reproduction of BSF.

Bacterial volatiles from aphid honeydew mediate ladybird beetles oviposition site choice

BACKGROUND

The cotton-melon aphid Aphis gossypii Glover is a destructive pest worldwide that causes substantial damage to diverse crops. The ladybird beetle Propylea japonica Thunberg is the dominant predatory natural enemy of A. gossypii. To date, the chemical cues of P. japonica associated with the selection of oviposition sites remain unclear.

RESULTS

Our results revealed that crude honeydew, but not sterilized honeydew, was strongly attractive to mated P. japonica. A total of eight bacterial strains were isolated from crude honeydew, with two (Acinetobacter sp. and Pseudomonas sp.) showing significant attractiveness. Volatiles from these bacteria were identified, and three compounds-DL-lactic acid, 4, 6-dimethyl-2-heptanone, and didodecyl phthalate-were found to significantly attract mated P. japonica in olfactometer assays. Further cage experiments confirmed that P. japonica preferred oviposition sites near these volatile substances.

CONCLUSION

The oviposition site selection by the ladybird beetle P. japonica was found to be influenced by volatiles produced by bacteria associated with cotton-melon aphid honeydew. These findings contribute to biologically based, environmentally friendly pest management strategies in agriculture. © 2025 Society of Chemical Industry.

Effects of Feeding Sources and Different Temperature Changes on the Gut Microbiome Structure of Chrysomya megacephala (Diptera: Calliphoridae)

Chrysomya megacephala (Diptera: Calliphoridae), commonly referred to as the oriental latrine fly, is a synanthropic blowfly species frequently associated with decomposing organic matter. This study sought to investigate the influence of various feeding substrates and temperature conditions, specifically constant temperatures of 15, 25, 35 °C, and variable temperatures averaging 23.31 °C, on the gut microbiome of C. megacephala. The microbiome analysis was conducted using the Illumina HiSeq platform for 16S rRNA gene sequencing in Changsha, China. Across all experimental conditions, the gut microbiome of C. megacephala yielded 1257 operational taxonomic units (OTUs), which were categorized into 26 phyla, 72 classes, 165 orders, 270 families, 516 genera, and 794 species. The study showed significant differences in the gut microbiome of C. megacephala between different feeding sources and temperature conditions across the lifespan. Low temperature had the potential to reduce the proportion abundance of bacterial communities in the gut microbiome, while high and variable temperature increased them. Metabolism was the main predicted function with diverse phenotypic characters in the gut microbiota of C. megacephala. The presence of diverse bacterial phenotypes in the gut microbiome of C. megacephala highlights its significant interest for medicine and offers promising applications in industry and agriculture.

Efficacy of black soldier fly larvae in converting kitchen waste and the dynamic alterations of their gut microbiome

The escalating demand for food, driven by population growth and improved living standards, has prompted the development of efficient and eco-friendly kitchen waste (KW) treatment technologies. This study focused on the feasibility of utilizing KW through the application of black soldier fly larvae (BSFL), with a specific interest in the dynamic changes in the intestinal bacterial community during the treatment process. After a 10-day KW processing period, BSFL gained an average of 0.84 g/hundred worms/day, achieving a conversion efficiency of 18.52% for KW. This demonstrated their capacity to efficiently utilize KW nutrients for good growth performance. Additionally, the bioconversion of KW by BSFL could markedly decrease the presence of potentially pathogenic bacteria in the feed matrix within one day (P < 0.001), including Escherichia coli, Shigella spp., Salmonella spp., and Staphylococcus aureus. Notably, the diversity of the intestinal bacterial community in BSFL increased with age and sustained KW consumption (P < 0.05), accompanied by enhanced stability. In particular, the average relative abundance of potential probiotic genera associated with nutrient absorption and antimicrobial compounds synthesis, including Fusobacterium, Phascolarctobacterium, Enterococcus, and Actinomyces, increased. Conversely, the prevalence of pathogenic genera like Morganella and Escherichia-Shigella, decreased. Co-occurrence network analysis identified Lactobacillus, Brevibacterium, Erythrobacter, and Enterobacteriaceae as keystone species. Despite their low abundance in the BSFL intestine, these species were potentially crucial for KW bioconversion. Our findings underscore the potential of BSFL for sustainable KW conversion, providing strong support for effective waste management strategies.

Gut microbial communities and transcriptional profiles of black soldier fly (Hermitia illucens) larvae fed on fermented sericulture waste

Sericulture waste poses significant challenges to industrial and environmental safety. Black soldier fly larvae (BSFL) offer a promising solution for organic waste management by converting it into insect protein. This study aimed to develop a microbial fermented method for utilizing sericulture waste to feed BSFL and explore the underlying mechanisms. Our results showed that all fermented sericulture waste groups had positive effects on body weight, survival rate, substrate consumption rate, and substrate conversion rate. Metagenomic analysis revealed a notable increase in the abundances of commensal genera, including Sedimentibacter, Clostridium, Enterococcus, Bacteroides, and Bacillus, in the gut of BSFL fed on sericulture waste fermented with the most effective combination of microbial strains (B. subtilis, B. licheniformis, and E. faecalis). In contrast, BSFL reared on unfermented sericulture waste exhibited higher abundances of potentially pathogenic and harmful genera, including Providencia, Klebsiella, Escherichia, Brucella, and Enterobacter. Clusters of orthologous genes (COG) analysis indicated that altered microbial communities in the fermented group mainly participated in metabolic pathways, defense mechanism, and signal transduction mechanism. Transcriptome analysis further revealed that the upregulated genes were functionally associated with key metabolic pathways and immune mechanisms in the fermented group. These findings underscore the pivotal role of selected microbial fermentation in utilizing sericulture waste as BSFL feed, providing a sustainable solution for organic waste management.

Effects of swine manure mixed with circulating fluidized bed fly ash on black soldier fly (Diptera: Stratiomyidae) larvae and larval frass

Black soldier fly larvae (BSFL) were reared on mixtures of swine manure and circulating fluidized bed fly ash (CFA) in different ratios. The aim was to evaluate the impacts of insoluble inorganic matter on BSFL and larval frass. The growth performance and nutrient composition of the BSFL were measured under different treatments. The intestinal microbiota structure, morphological characteristics, and total proteolytic activity of the gut were analyzed. The larval frass was tested for nutrients and analyzed using energy-dispersive spectroscopy and scanning electron micrographs. In particular, the surface areas of microparticles from the larval frass (diameter < 0.0074 mm) were measured using Brunauer-Emmett-Teller method. It was found that CFA addition prolonged larval development and reduced the maximum larval weights. The mean larval length, crude protein, and highest larval weight showed negative regression with an increase in the CFA ratio (P < 0.05). Morphological images indicated that physical clogging might be the main influencing factor on larval growth. Moreover, the microbial diversity and complexity in the larval gut increased with CFA addition, but CFA addition had little effect on the composition of dominant phyla or genera (P > 0.05). Finally, the nutrient composition revealed that the frass met the organic fertilizer standard when the CFA addition ratio was less than 7.5%. The optimal addition ratio was 5%, at which the larvae had a more stable and healthier gut environment, but there was less of an effect on larval growth and nutrient composition. Moreover, particles from 5% CFA mixture had the highest surface area.

Evolution of the Black solider fly larvae gut antibiotic resistome during kitchen waste disposal

Kitchen waste (KW) is an important reservoir of antibiotic resistance genes (ARGs). Black solider fly larvae (BSFL) are extensively employed in KW disposal, closely linking to their robust gut microbes. However, antibiotic resistome in BSFL gut during the KW disposal processes and the mechanism remain unclear. In the present study, the antibiotic resistome in BSFL gut within the 12 days KW disposal processes were investigated. Results showed that, ARGs abundance initially increased and subsequently decreased, the five most prevalent core ARG classes were tetracycline, aminoglycoside, cephalosporin, lincosamide and multidrug. A total of 7 MGE types were observed and the horizontal gene transfer (HGT) of ARGs was predominantly mediated by plasmids. Host microbes were mainly categorized into Proteobacteria (98.12 %) and their assemblies were mainly classified into the deterministic processes. To elucidate the driving mechanisms, the mantel test and the structural equation model (SEM) were developed. Results indicated that microbial functions (0.912, p < 0.0001) and microbial community (1.014, p = 0.036), consistently showed very significant relationships with the patterns of ARGs, which presented higher direct effects than indirect effects. Overall, this study makes an initial contribution to a more deepgoing comprehension of the gut antibiotic resistome of BSFL during KW disposal.

Microbial changes and associated metabolic responses modify host plant adaptation in Stephanitis nashi

Symbiotic microorganisms are essential for the physiological processes of herbivorous pests, including the pear lace bug Stephanitis nashi, which is known for causing extensive damage to garden plants and fruit trees due to its exceptional adaptability to diverse host plants. However, the specific functional effects of the microbiome on the adaptation of S. nashi to its host plants remains unclear. Here, we identified significant microbial changes in S. nashi on 2 different host plants, crabapple and cherry blossom, characterized by the differences in fungal diversity as well as bacterial and fungal community structures, with abundant correlations between bacteria or fungi. Consistent with the microbiome changes, S. nashi that fed on cherry blossom demonstrated decreased metabolites and downregulated key metabolic pathways, such as the arginine and mitogen-activated protein kinase signaling pathway, which were crucial for host plant adaptation. Furthermore, correlation analysis unveiled numerous correlations between differential microorganisms and differential metabolites, which were influenced by the interactions between bacteria or fungi. These differential bacteria, fungi, and associated metabolites may modify the key metabolic pathways in S. nashi, aiding its adaptation to different host plants. These results provide valuable insights into the alteration in microbiome and function of S. nashi adapted to different host plants, contributing to a better understanding of pest invasion and dispersal from a microbial perspective.

Microbiome analysis of monarch butterflies reveals effects of development and diet

Diet profoundly influences the composition of an animal's microbiome, especially in holometabolous insects, offering a valuable model to explore the impact of diet on gut microbiome dynamics throughout metamorphosis. Here, we use monarch butterflies (Danaus plexippus), specialist herbivores that feed as larvae on many species of chemically well-defined milkweed plants (Asclepias sp.), to investigate the impacts of development and diet on the composition of the gut microbial community. While a few microbial taxa are conserved across life stages of monarchs, the microbiome appears to be highly dynamic throughout the life cycle. Microbial diversity gradually diminishes throughout the larval instars, ultimately reaching its lowest point during the pupal stage and then recovering again in the adult stage. The microbial composition then undergoes a substantial shift upon the transition from pupa to adult, with female adults having significantly different microbial communities than the eggs that they lay, indicating limited evidence for vertical transmission of gut microbiota. While diet did not significantly impact overall microbial composition, our results suggest that fourth instar larvae exhibit higher microbial diversity when consuming milkweed with high concentrations of toxic cardenolide phytochemicals. This study underscores how diet and developmental stage collectively shape the monarch's gut microbiota.

Turning trash into treasure: Hermetia illucens microbiome and biodegradation of industrial side streams

Black soldier fly larvae (BSFL) have attracted attention due to their ability to upcycle various biological side streams into valuable biomass, such as proteins, lipids, and chitin. In this study, we investigated the impact of high-fiber diets on larval growth performance and the shift of microbes in the gut. We tested empty fruit bunches (EFB), potato pulp (PP), and cottonseed press cake (CPC), with chicken feed (CF) used as a control diet. We found that larvae reared on the EFB, PP, and CPC were smaller than control larvae at the end of development due to the low nutritional value of the diets. However, survival rates of more than 90% were observed regardless of the diet. We used a cultivation-dependent approach to analyze the microbial community in the gut of BSFL, isolated, and identified a total of 329 bacterial strains. Bacillaceae were most frequently isolated from larvae reared on the high-fiber EFB diet. These isolates were predicted to degrade cellulose in silico and this was subsequently confirmed in vitro using the Congo Red assay. Whereas the members of Enterobacteriaceae and Morganellaceae were mostly found in guts of larvae reared on the high-protein diets CPC and CF. We conclude that the gut microbiome plays a crucial role in the digestion of fiber-rich plant organic material, thereby enabling the BSFL to successfully complete their life cycle also on substrates with low nutritional value. As a result, BSFL convert industrial side streams into valuable biomass, reducing waste and promoting sustainability.

IMPORTANCE

Organic side streams from various industries pose a challenge to the environment. They are often present in huge amounts and are mostly discarded, incinerated, used for biogas production, or as feed for ruminant animals. Many plant-based side streams contain difficult-to-digest fiber as well as anti-nutritional or even insecticidal compounds that could harm the animals. These challenges can be addressed using black soldier fly larvae, which are known to degrade various organic substrates and convert them into valuable biomass. This will help mitigate agro-industrial side streams via efficient waste management and will contribute to the more economical and sustainable farming of insects.

Black Soldier Fly (Hermetia illucens) Microbiome and Microbe Interactions: A Scoping Review

Black soldier fly (Hermetia illucens, BSF) is farmed worldwide to convert organic waste into usable biomaterials. Studies on the larval microbiome have been carried out to check for symbiotic or pathogenic microbes and their respective functions and fates. Some studies tested these microbes for industrial applications, while others tested the effects of exogenous microbes as probiotics or for substrate pre-processing to improve larval fitness, bioconversion rates, or nutritional qualities. This review examined all peer-reviewed literature on these topics to consolidate many disparate findings together. It followed the PRISMA guidelines for scoping reviews. The results found no evidence of globally conserved core microbes, as diet strongly correlated with gut microbiome, but some genera appeared most frequently in BSF larval guts worldwide regardless of diet. The gut microbes undoubtably assist in digestion, including pathogen suppression, and so microbial probiotics show promise for future investigations. However, the common gut microbes have not been explored as probiotics themselves, which would be a promising direction for future work. The impacts of BSF bioconversion on pathogens varied, so each rearing facility should investigate and manage their pathogen risks independently. The data summarized in this study provide useful reference points for future investigations into BSF-microbe interactions.

A new approach to biotransformation and value of kitchen waste oil driven by gut microorganisms in Hermetia illucens

Hermetia illucens larvae are known for their ability to recycle organic waste, but their capacity to recover waste oils and the role of gut microorganisms in this process are not fully understood. To gain further insights, the biological recovery of waste frying oil into valuable lipids and the influence of gut bacteria on this biotransformation were investigated. The larvae efficiently digested and absorbed waste frying oil, demonstrating their potential for converting various oils into insect fat. The presence of different fatty acids in their diet significantly altered gut bacterial communities, enriching certain genera such as Actinomyces, Enterococcus, and Providencia. Redundancy analysis revealed that the composition and structure of these bacterial communities were predictive of their function in the biotransformation of fatty acids and the lipid biosynthesis in the larvae. Specific bacteria, including Corynebacterium_1, Providencia, Actinomyces, Escherichia-Shigella, and others, were identified to play specialized roles in the digestion and absorption of fatty acids, contributing to lipid synthesis and storage. These findings highlight the potential of Hermetia illucens in the biological recovery of waste frying oil and underscore the crucial role of gut microbiota in this process, offering a sustainable approach to waste management and bioenergy production.

Study on the Structure and Function of Intestinal Microorganisms in Silkworm Maggot Exorista sorbillans

Insects have important symbiotic relationships with their intestinal microbiota. The intestinal microbiota is involved in or influences various processes in insects such as development, metabolism, immunity, and reproduction. Currently, research on the intestinal microbiota of parasitic insects is still in its early stages. The tachinid parasitoid Exorista sorbillans is a dipteran parasitic insect, with the silkworm (Bombyx mori) being its main host. Silkworms parasitized by E. sorbillans can suffer from severe silkworm maggot disease, which also poses a serious threat to sericulture. In this study, the intestinal microbiota of larval E. sorbillans at three instar stages was analyzed using 16S rRNA amplicon sequencing to explore the community composition of the intestinal microbiota. Additionally, using conventional culture methods, six cultivable strains were isolated and identified from the larval E. sorbillans on an antibiotic-free LB medium, and four cultivable strains were isolated and identified from the hemolymph of parasitized silkworms. This study investigated the E. sorbillans from the perspective of intestinal microbiota, elucidating the composition and structural characteristics of the intestinal microbiota of the tachinid parasitoid, and preliminarily discussing the functional roles of several major microorganisms, which helps to further clarify the potential mechanisms of interaction between the parasitoid and the silkworm.

Genetics, age, and diet influence gut bacterial communities and performance of black soldier fly larvae (Hermetia illucens)

BACKGROUND

The gut microbiota of black soldier fly larvae (BSFL, Hermetia illucens) play a crucial role in recycling various organic waste streams. This capability is linked to the presence of a potential common core microbiota in BSFL. However, subjective thresholds for defining core taxa and the difficulty of separating genetic and environmental influences have prevented a clear consensus in the literature. We analysed the gut bacterial communities of two genetically distinct BSF lines (wild type (WT) and lab-adapted line (LD)) raised on ten different diets based on common agricultural by-products and food waste in Southeast Asia.

RESULTS

High-throughput 16S rRNA gene sequencing revealed that gut bacterial communities were significantly influenced by genetics (p = 0.001), diet (plant/meat-dominated; p = 0.001), larval age (p = 0.001), and the interactions between all three (p = 0.002). This led us to investigate both common core taxa and lineage-specific core taxa. At a strict > 97% prevalence threshold, four core taxa were identified: Providencia_A_732258, an unclassified genus within the family Enterococcaceae, Morganella, and Enterococcus_H_360604. A relaxed threshold (> 80% prevalence) extended the core to include other potential common core taxa such as Klebsiella, Proteus, and Scrofimicrobium. Our data suggest that Proteus, Scrofimicrobium, Corynebacterium, Vagococcus_B, Lysinibacillus_304693 (all LD), and Paenibacillus_J_366884 (WT) are lineage-specific rather than members of a common core (> 90% prevalence in either LD or WT, with prevalence significantly different between lines (p ≤ 0.05)). Positive correlations were observed between several core genera and larval performance in LD, typical of a highly optimized lab-adapted line. Interestingly, only members of the genus Providencia appeared to play a crucial role in most aspects of larval performance in both genetic lineages.

CONCLUSION

Our study demonstrates that the gut microbiota of BSFL is influenced by genetic factors, diet composition, larval age, and their interactions. We identified a distinct lineage-specific core microbiota, emphasizing genetic background's role. Future studies should apply a standardized high prevalence threshold of at least > 90% unless there is a valid reason for relaxation or sample exclusion. The consistent association of Providencia spp. with larval performance across both genetic lines highlights their crucial role in the BSFL gut ecosystem.

Effects of black soldier fly larvae on biotransformation and residues of spent mushroom substrate and wet distiller's grains

Black soldier fly larvae (BSFL) could convert a variety of organic wastes, including spent mushroom substrate (SMS) and wet distiller's grains (WDG). Nevertheless, little is known about the conversion of these wastes by BSFL. Thus, this study investigates the conversion of SMS and WDG in five different proportions by BSFL. This study demonstrates that BSFL can convert SMS, WDG, and their mixtures. It can also encourage the humification of the substrate, increasing the amount of element in the residues. It is evident that there were differences in the carbon and nitrogen element fractionation mode as well as the microbial community present in the residue. The microbial community of the substrate and the physiochemical parameters are intimately related to this. Although the mixture treated with BSFL helps to generate a residue with more humus, it might not be stable.

Microplastics in the diet of Hermetia illucens: Implications for development and midgut bacterial and fungal microbiota

In a world with a population exceeding 8 billion people and continuing to grow, pollution from food and plastic waste is causing long-term issues in ecosystems. Potential solutions may be found by exploiting insect-based bioconversion. In this context, we investigated the impact of polyvinyl chloride microparticles (PVC-MPs) on the development of Hermetia illucens (black soldier fly; BSF) and its midgut bacterial and fungal microbiota. The impact of PVC-MPs was evaluated feeding BSF larvae with a PVC-MPs-supplemented diet. The larvae exposed to different PVC-MPs concentrations (2.5%, 5%, 10% and 20% w/w) developed into adults with no significant increase in pupal mortality. Faster development and smaller pupae were observed when 20% PVC-MPs was provided. The BSF larvae ingest PVC-MPs, resulting in a reduction in MPs size. Larvae exposed to PVC-MPs did not exhibit differences in gut morphology. Regarding the impact of PVC-MPs on the structure of both bacterial and fungal communities, the overall alpha- and beta-diversity did not exhibit significant changes. However, the presence of PVC-MPs significantly affected the relative abundances of Enterobacteriaceae and Paenibacillaceae among the bacteria and of Dipodascaceae and Plectospharellaceae among the fungi (including yeast and filamentous life forms), suggesting that PVC-MP contamination has a taxa-dependent impact. These results indicate that BSF larvae can tolerate PVC-MPs in their diet, supporting the potential use of these insects in organic waste management, even in the presence of high levels of PVC-MP contamination.

The gut bacterial composition across life stages of Sarcophaga peregrina (Diptera: Sarcophagidae) and the effects of amikacin on their development

Insects and microorganisms, ubiquitous organisms in the natural world, have developed intricate relationships throughout their evolutionary histories. However, most studies have concentrated on specific time points or life stages, but some limited studies have investigated the dynamics of microbial diversity within insects across life stages. Here, 16S rDNA sequencing technology was used to investigate the gut bacterial community across the life stages of Sarcophaga peregrina (Robineau-Desvoidy) (Diptera: Sarcophagidae). The results revealed that the gut bacterial diversity of S. peregrina varied with life stage and showed similarity in the nearby life stages. Proteobacteria, Actinobacteria, Firmicutes, and Bacteroidetes were the dominant phyla in S. peregrina. Genera such as Providencia, Ignatzschineria, and Myroides are implicated in potentially pivotal roles during the developmental processes of this flesh fly. Furthermore, the effects of amikacin on the growth and development of S. peregrina were not statistically significant. However, we did observe significant changes at the protein level, which suggests a close association between protein-level alterations and growth and development. Additionally, we speculate that S. peregrina regulates its nutritional status during nonfeeding stages to meet the demands of eclosion. This study represents the first comprehensive examination of the intestinal bacterial composition across various life stages of S. peregrina. Our findings deepen our understanding of the gut microbiota in this flesh fly and lay the groundwork for further exploration into the intricate interactions between microorganisms and insects.

The cluster digging behavior of larvae confers trophic benefits to fitness in insects

Collective behaviors efficiently impart benefits to a diversity of species ranging from bacteria to humans. Fly larvae tend to cluster and form coordinated digging groups under crowded conditions, yet understanding the rules governing this behavior is in its infancy. We primarily took advantage of the Drosophila model to investigate cooperative foraging behavior. Here, we report that Drosophila-related species and the black soldier fly have evolved a conserved strategy of cluster digging in food foraging. Subsequently, we investigated relative factors, including larval stage, population density, and food stiffness and quality, that affect the cluster digging behavior. Remarkably, oxygen supply through the posterior breathing spiracles is necessary for the organization of digging clusters. More importantly, we theoretically devise a mathematical model to accurately calculate how the cluster digging behavior expands food resources by diving depth, cross-section area, and food volume. We found that cluster digging behavior approximately increases 2.2 fold depth, 1.7-fold cross-section area, and 1.9 fold volume than control groups, respectively. Amplification of food sources significantly facilitates survival, larval development, and reproductive success of Drosophila challenged with competition for limited food resources, thereby conferring trophic benefits to fitness in insects. Overall, our findings highlight that the cluster digging behavior is a pivotal behavior for their adaptation to food scarcity, advancing a better understanding of how this cooperative behavior confers fitness benefits in the animal kingdom.

Evaluation of the Microbial Quality of Hermetia illucens Larvae for Animal Feed and Human Consumption: Study of Different Type of Rearing Substrates

In the context of climate change and depletion of natural resources, meeting the growing demand for animal feed and human food through sufficient, nutritious, safe, and affordable sources of protein is becoming a priority. The use of Hermetia illucens, the black soldier fly (BSF), has emerged as a strategy to enhance the circularity of the agri-food chain, but its microbiological safety remains a concern. The aim of the present study was to systematically review available data on the microbiological quality of BSF and to investigate the impact of using four different rearing substrates including classic options allowed by the EU regulation (cereals, fruits, vegetables) and options not allowed by EU regulations regarding vegetable agri-food (co-products, food at shelf life, and meat). A total of 13 studies were collected and synthesized, including 910 sample results, while 102 new sample results were collected from the present experiments in three farms. Both datasets combined revealed a high level of contamination of larvae, potentially transmitted through the substrate. The main pathogenic bacteria identified were Bacillus cereus, Clostridium perfringens, Cronobacter spp., Escherichia coli, Salmonella spp., and Staphylococcus aureus coagulase-positive, while Campylobacter spp. and Listeria monocytogenes were not detected. Any of these four substrates were excluded for their use in insect rearing; however, safety concerns were confirmed and must be managed by the operators of the sector using microbial inactivation treatment after the harvest of the larvae in order to propose safe products for the market. The results obtained will guide the definition of the control criteria and optimize the following manufacturing steps.

A gut commensal bacterium promotes black soldier fly larval growth and development partly via modulation of intestinal protein metabolism

IMPORTANCE

Black solider fly larvae and the gut microbiota can recycle nutrients from various organic wastes into valuable insect biomass. We found that Citrobacter amalonaticus, a gut commensal bacterium of the insect, exerts beneficial effects on larval growth and development and that the expression of many metabolic larval genes was significantly impacted by the symbiont. To identify the larval genes involved in the host-symbiont interaction, we engineered the symbiont to produce double-strand RNA and enabled the strain to silence host genes in the larval gut environment where the interaction takes place. With this approach, we confirmed that two intestinal protease families are involved in the interaction and provided further evidence that intestinal protein metabolism plays a role in the interaction. This work expands the genetic toolkits available to study the insect functional genomics and host-symbiont interaction and provide the prospective for the future application of gut microbiota on the large-scale bioconversion.

Distinct gut bacterial composition in Anoplophora glabripennis reared on two host plants

Anoplophora glabripennis (Coleoptera: Cerambycidae: Lamiinae) is an invasive wood borer pest that has caused considerable damage to forests. Gut bacteria are of great importance in the biology and ecology of herbivores, especially in growth and adaptation; however, change in the gut bacterial community of this pest feeding on different hosts is largely unknown. In this study, we investigated the gut bacterial communities of A. glabripennis larvae fed on different preferred hosts, Salix matsudana and Ulmus pumila, using 16S rDNA high-throughput sequencing technology. A total of 15 phyla, 25 classes, 65 orders, 114 families, 188 genera, and 170 species were annotated in the gut of A. glabripennis larvae fed on S. matsudana or U. pumila using a 97% similarity cutoff level. The dominant phyla were Firmicutes and Proteobacteria and the core dominant genera were Enterococcus, Gibbsiella, Citrobacter, Enterobacter, and Klebsiella. There was significantly higher alpha diversity in the U. pumila group than in the S. matsudana group, and principal co-ordinate analysis showed significant differences in gut bacterial communities between the two groups. The genera with significant abundance differences between the two groups were Gibbsiella, Enterobacter, Leuconostoc, Rhodobacter, TM7a, norank, Rhodobacter, and Aurantisolimonas, indicating that the abundance of larval gut bacteria was affected by feeding on different hosts. Further network diagrams showed that the complexity of the network structure and the modularity were higher in the U. pumila group than in the S. matsudana group, suggesting more diverse gut bacteria in the U. pumila group. The dominant role of most gut microbiota was related to fermentation and chemoheterotrophy, and specific OTUs positively correlated with different functions were reported. Our study provides an essential resource for the gut bacteria functional study of A. glabripennis associated with host diet.

Serratia marcescens in the intestine of housefly larvae inhibits host growth by interfering with gut microbiota

BACKGROUND

The structure of gut microbiota is highly complex. Insects have ubiquitous associations with intestinal symbiotic bacteria, which play essential roles. Thus, understanding how changes in the abundance of a single bacterium interfere with bacterial interactions in the insect's gut is important.

METHODS

Here, we analyzed the effects of Serratia marcescens on the growth and development of housefly larvae using phage technology. We used 16S rRNA gene sequencing technology to explore dynamic diversity and variation in gut bacterial communities and performed plate confrontation assays to study the interaction between S. marcescens and intestinal microorganisms. Furthermore, we performed phenoloxidase activity assay, crawling assay, and trypan blue staining to explore the negative effects of S. marcescens on housefly larvae's humoral immunity, motility, and intestinal organization.

RESULTS

The growth and development of housefly larvae were inhibited after feeding on S. marcescens, and their intestinal bacterial composition changed with increasing abundance of Providencia and decreasing abundance of Enterobacter and Klebsiella. Meanwhile, the depletion of S. marcescens by phages promoted the reproduction of beneficial bacteria.

CONCLUSIONS

In our study, using phage as a tool to regulate the abundance of S. marcescens, we highlighted the mechanism by which S. marcescens inhibits the growth and development of housefly larvae and illustrated the importance of intestinal flora for larval development. Furthermore, by studying the dynamic diversity and variation in gut bacterial communities, we improved our understanding of the possible relationship between the gut microbiome and housefly larvae when houseflies are invaded by exogenous pathogenic bacteria.

The Use of Probiotics during Rearing of Hermetia illucens: Potential, Caveats, and Knowledge Gaps

Given the novelty of the industrial production of the edible insects sector, research has primarily focused on the zootechnical performances of black soldier fly larvae (BSFL) in response to different substrates and rearing conditions as a basis to optimize yield and quality. However recently, research has started to focus more on the associated microbes in the larval digestive system and their substrates and the effect of manipulating the composition of these communities on insect performance as a form of microbiome engineering. Here we present an overview of the existing literature on the use of microorganisms during rearing of the BSFL to optimize the productivity of this insect. These studies have had variable outcomes and potential explanations for this variation are offered to inspire future research that might lead to a better success rate for microbiome engineering in BSFL.

Changes in Bacterial Community Structure Across the Different Life Stages of Black Soldier Fly (Hermetia illucens)

The digestive capacity of organic compounds by the black soldier fly (BSF, Hermetia illucens, Diptera: Stratiomyidae, Linnaeus, 1758) is known to rely on complex larva-microbiota interactions. Although insect development is known to be a driver of changes of bacterial communities, the fluctuations along BSF life cycle in terms of composition and diversity of bacterial communities are still unknown. In this work, we used a metabarcoding approach to explore the differences in bacterial diversity along all four BSF developmental stages: eggs, larvae, pupae, and adult. We detected not only significant differences in bacterial community composition and species richness along the development of BSF, but also nine prevalent amplicon single variants (ASVs) forming the core microbiota. Out of the 2010 ASVs identified, 160 were significantly more abundant in one of the life stages. Moreover, using PICRUSt2, we inferred 27 potential metabolic pathways differentially used among the BSF life cycle. This distribution of metabolic pathways was congruent with the bacterial taxonomic distribution among life stages, demonstrating that the functional requirements of each phase of development are drivers of bacterial composition and diversity. This study provides a better understanding of the different metabolic processes occurring during BSF development and their links to changes in bacterial taxa. This information has important implications for improving bio-waste processing in such an economically important insect species.