One third of the food produced for human consumption is currently lost or wasted. Insects have a high potential for converting organic waste- and by-products into food and feed for a growing human population due to symbiosis with microorganisms. These symbioses provide an untapped reservoir of functional microbiomes that can be used to improve industrial insect production but are poorly studied in most insect species. Here we review the most current understanding and challenges of valorizing organic waste- and by-products through insects and their microbiomes for food and feed, and emerging novel food technologies that can be used to investigate and manipulate host(insects)-microbiome interactions. We further construct a holistic framework, by integration of novel food technologies including holo-omics, genome editing, breeding, phage therapy, and administration of prebiotics and probiotics to investigate and manipulate host(insects)-microbiome interactions, and solutions for achieving stakeholder acceptance of novel food technologies for a sustainable food production.

Large scale production of insect larvae is considered a sustainable way to upcycle various organic waste- and by-products into more valuable food and feed products. The sustainability of insect larvae production depends on the substrates and species being used, but comparative studies that include both growth and efficiency are lacking. Here we compare larval fitness, including survival, development time, weight, substrate conversion efficiency, substrate reduction, and metabolic parameters across different combinations of densities and waste- and by-product-based substrates on the two fly species, the house fly (Musca domestica) and the black soldier fly (Hermetia illucens). The waste- and by-product-based substrates were a brewer's spent grain-based substrate, a digested sludge-based substrate, and a wheat bran/deproteinized grass-based substrate all highly abundant and of low value. Substrate and density significantly impacted on most larval growth and metabolic performance traits, but dependent on species. The brewer's spent grain-based substrate generally gave the highest performance in terms of larval weight, larval yield, and substrate conversion efficiency for both species, while a high density gave a higher larval yield and substrate conversion efficiency, but lower larval weight. Generally, black soldier fly larvae showed lower metabolic costs and higher net growth efficiency than house fly larvae. Altogether, our results demonstrate that both larval species, substrate, and larval densities affect larval growth and metabolic performance, and subsequently the scope for valorizing waste- or by-products to achieve a sustainable production of food and feed.

Mealworms and black soldier fly (BSF) larvae are two of the most reared insects at an industrial scale. Both may feed on by-products from agricultural and food industries. Feed efficiency is one of the most important aspects of such processes and varies between species and feed substrates and depends on the metabolic performance of the larvae. Compared to each other, both species may hold advantageous capabilities affecting their feed efficiency, likely depending on the feed substrate. We reared mealworms and BSF larvae on a diverse selection of by-products from agricultural and food industries, quantified major metabolic rates across their life spans, and compared their performances. The type of feed substrates had stronger effects on the growth of mealworms than on black soldier fly larvae. Generally, BSF larvae were advantageous in terms of the highest maximal specific growth rate (0.50–0.77 day⁻¹) and feed assimilation rate (0.81–1.16 day⁻¹) and shortest development period (23–25 days) but disadvantageous in terms of metabolic maintenance cost (0.07–0.21 day⁻¹). In mealworms, the maximal specific growth rates were 0.02–0.11 day⁻¹, the highest feed assimilation rates were 0.16–0.37 day⁻¹, and the development period was at least 65–93 days, while maintenance was only 0.02–0.05 day⁻¹. In contrast to the BSF larvae, the specific maintenance rate was weight dependent in the mealworms and lowest in the largest individuals. The combined outcome of these metabolic rates resulted in an average carbon net growth efficiency, NGE*_avg of 0.16–0.40 in mealworms and 0.33–0.56 in BSF larvae across their life span. It thus seems that BSF larvae are more versatile and somewhat more efficient at converting diverse feed substrates into growth than mealworms. Differences in NGE*_avg affected the substrate conversion efficiencies (i.e., the ratio of the weight gain of the larvae to the reduced weight of feed substrates) and may thus impact the overall outcome of insect farming.

Increasing human population size and income growth are causing an increasing demand for food and feed. Insects are a more sustainable alternative to conventional animal source proteins, as they can convert waste and by-products from the agricultural industry into biomass for commercial feed for livestock and, potentially, serve as a food source for human consumption. Moreover, insects together with their microorganisms have been shown to play a pivotal role in the development of insects and in the breakdown of complex growth substrates, and are, therefore, closely tied to insect production. This study aims to determine if the removal of egg- and substrate-associated microorganisms impacts larval performance (growth, final biomass, and the survival rate) of M. domestica Linnaeus. Four treatments are tested: disinfected eggs and non-autoclaved substrate, non-disinfected eggs and autoclaved substrate, disinfected eggs and autoclaved substrate, and a control without any removal of microbiota. The results show a significant decrease in the final biomass of larvae subjected to the treatments with only disinfected eggs, only autoclaved substrate, and both compared to the control, and a significant decrease in survival rate for non-disinfected eggs and autoclaved substrate and disinfected eggs and autoclaved substrate compared to the control group. Moreover, larval growth shows a significant difference across days within all treatments. Together, this suggests that the microorganisms of housefly eggs and the growth substrate play an important role in biomass, which is critical in commercial insect production. Together this suggest, that more studies are needed to examine these parameters with respect to more commercially relevant substrates.

In a world with an increase in human population, food consumption, and the generation of organic waste, insects are emerging as a promising tool to convert organic waste material into human food or animal feed. The insect microbiome is known to play a key role in the degradation of organic substrates, but little is known about the metabolic potential of the microbiome of industrially reared fly larvae. We investigated the microbial composition and metabolic potential of the house fly ( Musca domestica ) larva gut microbiome from larvae grown on three different waste and by-product-based substrates. We found that bacteria associated with the larval gut were enriched for functions related to microbial stress mechanisms, indicating strong selection of the gut microbiome by house fly larvae. In addition, the gut microbiome of larvae reared on sludge-based substrate had higher diversity when weighting for rare species and a higher coverage of “carbohydrate transport and metabolism” genes compared to brewery by-product-based substrate. A positive correlation between coverage of “pyridoxal-P synthesis” and larval survival and substrate conversion efficiency suggests that microbial synthesis of vitamin B ₆ could enhance larval performance. Additionally, a negative correlation between coverage of the “Entner-Doudoroff pathway” and “homoprotocatechuate degradation” and substrate conversion indicates microbial competition for sugars and aromatic amino acids. Together, these results reveal how the host selects on gut microbiomes with metabolic potential that is optimized toward the conversion of substrates that may be ultimately valuable for commercial insect production. IMPORTANCE: Fly larvae are expected to play an important role in future food and feed production through the conversion of low-value biomass into high-quality protein. The gut microorganisms of fly larvae are expected to play an important role in bioconversion and could potentially be manipulated to improve biomass conversion. In this study, the importance of the gut bacteria of house fly larvae for bioconversion was investigated by metagenomic sequencing, which provided information on the bacterial abundance and potential functional roles in the larval gut. The results reveal that the functional potential of gut bacteria is affected by larval feed and correlates with larval performance, highlighting the importance of the gut microbiome for efficient biomass conversion.

Increasing population size and income growth are causing and increasing demand for food and protein of animal origin. Insects are an interesting alternative to conventional animal source proteins as they can convert by-products from the agricultural industry to biomass for commercial feed for livestock and potentially as a food source for human consumption. Microorganisms have been found to affect insects and can be accumulated via horizontal and vertical transmission. This study aimed to identify if the removal of egg- and substrate-associated microorganisms impact larval performance through the following parameters: development of biomass, final biomass, and the survival rate of house fly (Musca domestica) larvae. Four treatments were tested on substrate consisting of 10.81% alfalfa (Medicago sativa), 21.62% wheat bran (Triticum aestivum), and 67.57% water: (A) disinfected eggs and non-autoclaved substrate, (B) non-disinfected eggs and autoclaved substrate, (C) disinfected eggs and autoclaved substrate, and (D) a control without any removal of microbiota. The results showed a significant decrease in final biomass for the treatments with only disinfected eggs, only autoclaved substrate and both when comparing to the control, and a significant decrease in survival rate for B and D. Moreover, the development of biomass showed a significant difference between days within all treatments. Together this suggest that microorganisms of housefly eggs and the growth substrate plays an important role for growth, which is critical in commercial insect production. Further studies must be performed to examine these parameters in more commercially relevant substrates.