Influence of Rearing Substrate on Black Soldier Fly Larvae Nutritional Value for Animal Feed

The animal feed industry relies heavily on soybean meal and fishmeal as the main protein source, but these ingredients are costly and scarce. Therefore, finding alternative protein sources that are environmentally friendly and sustainable is crucial for producing meat, milk, and eggs. Insects such as the black soldier fly larvae (Hermetia illucens; BSFL) are being considered as sustainable, protein-rich feed ingredients to meet the increasing demand for meat (Van Huis et al., 2013; Van Huis and Oonincx, 2017; Gadzama and Ndudim, 2019). 

Black soldier fly larvae can convert organic food waste into high-quality protein and fat for livestock, aquaculture, and pet feed. Recent studies have shown that BSFL can be safely included in animal diets without negative impacts on growth or nutrient digestibility and may even offer immune benefits due to their chitin content. However, the nutritional value of BSFL as an animal feed ingredient is directly influenced by the substrates on which they are reared. Other factors that may influence the nutritional composition of BSFL include the methods of killing and drying (Larouche et al., 2019; Huang et al., 2019) or product fractionation (e.g., defatting). This review presents a comparative overview of BSFL-rearing substrates and BSFL crude protein, crude fat, and metabolizable energy content across different companies.

Sources of BSFL as reported in various studies

Black Soldier Fly Larvae have gained significant attention as a sustainable solution for organic waste management and as a source of protein and fat for animal feed (Gadzama et al., 2023; Silva et al., 2024). Companies across the globe are utilizing various substrates to rear BSFL (Table 1). The global distribution of these companies, from Europe to Asia, Australia, South America, and Africa, reflects the scalability and adaptability of BSFL farming. Some of the rearing substrates of BSFL, as reported by various companies, include:

• Food waste
• Chicken feed
• Kitchen waste
• Chicken manure
• Brewer's by-product
• Blend of flour (35%), water (65%), and vegetable waste
• Recycled pre-consumer food from farms, processors, and stores
• Pre-consumer vegetable and fruit waste with a bit of chicken feed

Proximate composition of BSFL rearing substrate

The optimal substrate moisture content for BSFL growth and development is between 60% and 80% (Dzepe et al., 2020). The figure below shows a typical proximate composition of BSFL rearing substrate: pre-consumer food wastes, vegetables and fruit wastes, and chicken starter mash (Gadzama et al., 2023).

Rearing Substates of Black Soldier Fly Larvae

Black soldier fly larvae can be reared on various organic materials, ranging from pre-consumer recycled food to chicken manure and brewery by-products (Table 1). Studies have shown that BSFL can also thrive on substrates such as cow manure, fish offal, restaurant waste, and sewage sludge (Spranghers et al., 2017; Meneguz et al., 2018; Lalander et al., 2019). In addition, BSFL can be easily reared and propagated on any nutrient-rich substrate, including plant residues, animal manure, and waste, food scraps, agricultural byproducts, or straws (Lu et al., 2022; Dou et al., 2024). This approach aligns with the principles of circular economy, where waste is transformed into valuable feed resources.

Nutritional composition of BSFL as influenced by rearing Substrate

The nutritional composition of BSFL is directly influenced by the substrates they are reared on For instance, larvae reared on substrates with higher protein content, such as chicken feed, tend to have higher crude protein levels (Table 1). Generally, BSFL typically contains about 40% protein and 30% fat on a dry matter basis (Barragan-Fonseca et al., 2017). However, recent studies have indicated variations in this nutritional composition based on different rearing substrates. For example, Table 1 shows that crude protein content ranges from 25.30% to 60.69% DM, and fat ranges from 6.84% to 40.99% DM across different rearing substrates as reported by various, surpassing the figures reported by Barragan-Fonseca et al. (2017). These ranges indicate that the choice of substrate is crucial in determining the nutritional value of BSFL, which can be tailored to specific needs based on the desired protein and fat content for various applications.

Similarly, BSFL reared on a mixture of pre-consumer food waste, including fruits and vegetables, supplemented with a small amount of chicken starter mash, was reported to have a crude protein content of 48.20% DM (Gadzama et al., 2023). While BSFL reared on fish waste, specifically Sardinella aurita, for a brief period of two days, it has an exceptionally high total protein content of 78.8% (Hopkins et al., 2021). In contrast, BSFL reared on various fruit and vegetable waste formulations was found to have a lower protein content of 12.9% (Hopkins et al., 2021). The correlation between the substrate's composition and the nutritional content of BSFL is further supported by Fuso et al. (2021), who found that the protein content in BSFL is mainly correlated with the fiber and protein content of the rearing substrate. Shumo et al. (2019) reported that the rearing substrate affects all the proximate nutritional parameters of BSFL except ash. Similarly, the fat content in BSFL can fluctuate based on the fat content of the rearing substrate. For instance, BSFL reared on brewer's by-product, which is inherently rich in fats, was reported to have high fat levels (Campbell et al., 2020). 

Studies have shown that the rearing substrate of BSFL can be manipulated to enhance specific nutritional components, such as essential fatty acids (Spranghers et al., 2017; Meneguz et al., 2018). For instance, when BSFLs are fed with fish offal and algae, they exhibit increased levels of EPA and DHA, which are crucial for the health of animals consuming the BSFL-based feed. The metabolic energy value, which indicates the energy available from the feed, also reflects the substrate's composition (Table 1). The ability to modify the fatty acid profile of BSFL through diet customization is pivotal for optimizing their utilization as animal feed ingredients. 

The data presented in Table 2 suggest a significant variation in the nutritional composition of BSFL when reared on different substrates and subjected to various oven-drying temperatures and durations. The data suggests that substrate selection and processing conditions are key to optimizing BSFL for use as a sustainable protein source in animal feed.

Conclusion

The rearing substrates used for BSFL cultivation significantly impact their nutritional profile, particularly the crude protein, fat, and metabolizable energy content. Selecting appropriate rearing substrate is essential for optimizing the nutrient content of BSFL, which can be tailored depending on the larvae's intended application, whether animal feed or other uses. The nutritional composition of BSFL positions it for use as a sustainable feed ingredient in the diets of various animals, such as cattle, poultry, pigs, rabbits, fish, dogs, etc.

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