Black soldier fly (BSF) larvae are attracting the attention of researchers and livestock industry as an alternative source of protein. These larvae can convert organic waste into protein for animal consumption, whilst also changing waste into a product that can be used, for example, as compost (Dortmans, 2017). Their use in animal feeding could be a step into carbon footprint reduction and sustainable agriculture (Van Huis, 2013). This article explores the process of rearing BSF larvae, its nutrient composition, as well as the factors affecting such composition.
The increase in food demand of an ever-growing human population, as well as the need to reduce the carbon footprint of agriculture, is fueling the search for new sources of protein for animal feeding. Insect larvae seem to fulfil the requirements for a new low-cost, ecofriendly ingredient.
Main positive points of growing insect larvae for feeding livestock
So far, house fly (Musca domestica), yellow mealworm (Tenebrio monitos), and black soldier fly (Hermetia Illucens) have been singled out as having the greatest potential to become industrialised (Shumo et al, 2019; Sogari et al., 2019; Spranghers et al. (2017). In fact, several companies around the world are already producing them at a commercial scale.
This article looks at the cycle of larvae production, focusing on black soldier fly (BSF), currently most popular species being grown to feed livestock. The focus is on the life cycle, the composition of the final product, and the factors affecting variability in composition.
In the production of black soldier fly larvae (BSF larvae), the flow of organic waste transformation runs parallel to the cycle of BSF (Dortmans et al, 2017). After a certain period in which waste being transformed by BSF larvae, the process renders pre-pupae larvae for animal feeds, as well as processed organic waste that can have many uses (e.g. compost, biogas production, etc.).
A range of organic waste can be used as substrate for growing larvae, as long as they fulfil three premises (Dortmans et al., 2017; Purschke et al., 2017):
Dortmans et al., (2017) classified the sources of organic waste into municipal waste (municipal waste, restaurant waste, market waste), agro-industrial waste (e.g. food processing, slaughterhouse, grain wastes), and manure/faeces. If the particle size and the moisture are beyond the expected range, they should be corrected. Contaminated organic matter should be rejected before entering the processing plant.
Spranghers et al. (2017) reared BSF larvae on chicken feed, biogas digest, vegetable waste, and restaurant waste, measuring chemical composition of pre-pupae larvae. Larvae in chicken feed grew significantly faster (12 days to the first harvest, P<0.05), with a significantly higher yield (P<0.05), and more efficient utilisation of substrate (P<0.05). Larvae in restaurant waste grew slower, probably due to the substrate’s high fat content, which is difficult for the larvae to digest (19 days to first harvest, P<0.05). Biogas digest gave the lowest larvae yield (P<0.05).
The total cycle of the fly lasts, approximately, 5 to 6 weeks (Fig. 1). It starts with the oviposition of, approximately 400 to 800 eggs, after which the female dies. Four days later, the first larva stadium hatches; they are a few mm long and voracious. They continue growing for 12 to 14 days, provided they are under suitable environmental conditions and with a good organic substrate. At the end of this period, the larva enters the pupa stadium, which ends in 2 or 3 weeks-time, with the emergence of a fly (Dortmans et al., 2017).
In their single week of adult life, females look for a partner, mate, lays eggs and die. Sixty-nine percent of mating occur 2 days after eclosion, significantly affected by light intensity (Tomberlin & Sheppard, 2002). Four days after eclosion, 70% of adult females had already laid their eggs (Tomberlin & Sheppard, 2002).
By synchronizing the mating activity based on the management of light intensity, females are induced to lay their eggs at, approximately, the same time (Dortmans et al., 2017). Five days after the larvae had hatched, they are inoculated into the organic matter and the process of biotransformation begins. The temperature should be between 26 and 30 ºC, shaded and with a substrate moisture between 60 and 90%. Two to five percent of the larvae are left to grow and become adults (Dortmans et al., 2017).
They are harvested at day 12 post-inoculation, before they become prepupae, and when the nutritional properties are at their maximum point (Dortmans et al., 2017; Liu et al., 2017). The final weight is determined by the quality and amount of substrate, as well as by the environmental conditions and the level of light. In suboptimal conditions, the attained weight will be lower and the time to achieve the final stage will be longer (Dortmans et al., 2017).
After harvesting, the transformed organic matter and the larvae are processed separately. The larvae are normally dried or frozen, to be then commercialized as an animal feed ingredient. The organic product can be dried to be used as compost, or being fermented for biogas, among other uses (Dortmans et al., 2017).
Erickson et al. (2004) reported that BSF larvae secrete substances that repel other insects and potential disease vectors, such as common house fly (Musca domestica).
Regarding the transmission of pathogens, BSF larvae have been reported to reduce the levels of E. coli and Salmonella enterica in cow manure (Erickson et al., 2004; Shumo et al. 2019). Similarly, Liu et al. (2017) reported a reduction of the count of E. coli count in chicken manure treated with BSF larvae.
Purschke et al. (2017) found that rearing larvae in substrates contaminated with cadmium and lead reduced their growth rate and feed conversion. Larvae also accumulated cadmium and lead in their tissues (accumulation factors of 9 and 2 respectively), whilst other heavy metals remained at a lower concentration than in the substrates.
Larvae growth is not affected by pesticides or mycotoxins, and these toxins were not found in larval tissues (Purschke et al., 2017). Shumo et al. (2019), using LC-Qtof-MS analysis, also reported not finding traces of aflatoxins when examining BSF larvae grown on spent grains, kitchen waste and chicken manure.
The composition of BSF larvae can be affected by the substrate where the larva has been reared, as well as by the larval stadia. Some components are more affected than others.
The dry matter of fresh larvae at harvest is quite high, between 35 and 35 % (Ewald et al., 2020; Makkar et al., 2014; Spranghers et al., 2017).
In general, BSF larvae contain high levels of crude protein (above 33 %) (Makkar et al., 2014; Spranghers et al., 2017; Shumo et al., 2019). Compared to a soybean meal (SBM) of the same protein content (44%), the amino acid profiles are quite similar (Spranghers et al., 2017). Shumo et al (2019) found that the amino acid profile of BSF larvae was superior to the FAO specifications for SBM and sunflower, with methionine levels also above the FAO standards for fishmeal. If, as SBM, BSF meal was to be defatted, the percentage of protein will increase up to 60%, with a superior amino acid composition (Spranghers et al., 2017).
Larvae are high in lipids, and their content depend largely on the quantity and types of lipid in the substrate. Liu et al. (2017) reported a lipid content of 28% at 14 days, whilst Makkar et al. (2014) reported up to 36 % before the larvae becomes pre-pupae (Makkar et al., 2014). Although the level of unsaturated fatty acids is high in housefly meal (60-70%), it is quite low in black soldier fly (19-37%) (Makkar et al., 2014; Ramos-Bueno et al., 2016). Regarding saturated fatty acids, BSF has a high proportion of lauric acid of their own synthesis (Ewald et al., 2020), as well as low levels of cholesterol (Ramos-Bueno et al., 2016).
The high content of medium chain fatty acids (MCFAs), especially lauric (C12:0) it is of great importance in pig and poultry. Lauric acid has a probiotic action, especially against Clostridium perfringens, with lowest effect of Lactobacilli, helping to maintain in healthy microflora in the proximal small intestine (Spranghers et al., 2017). Its effect against enveloped viruses, other bacteria, and protozoa was also reported (Shumo et al., 2019).
NDF and ADF content of BSF they are affected by NDF and ADF contents of the substrate. Shumo et al. (2019) reported values of 21%, 20% and 29 % NDF for BSF larvae reared in chicken manure, kitchen waste and spent grains, respectively. ADF values for larvae in the same substrates were 12.6%, 13% and 15 %, respectively.
The content of Calcium is highly depending on the composition of the substrate (Spranghers et al., 2017). Phosphorus content is more consistent, varying between 0.6-1.5% DM (Makkar et al., 2014).
The presence of chitin, between 5% DM (Nafisah et al, 2019) and 8.5% DM (Spranghers et al., 2017), is of importance. Chitin has been attributed some prebiotic effects (Selenius et al., 2018). However, care should be taken of its antinutritional properties, since it has a negative effect on nutrient digestibility in animals, even at low concentrations (Nafisah et al., 2019; Spranghers et al., 2017). Fermentation of larvae with chitinolytic bacteria (Bacilllus subtilis) has resulted in reduction of chitin quality and content (Nafisah et al., 2019).
Shumo et al. (2019) reported the existence of 5 flavonoids in BSF larvae meal, two of them, apifenin and kaempherol being dependent of the concentration in the substrates.
Different factors produce modifications on the composition of larvae. Among them, developmental stage and substrate composition have an important effect. The following sections analyse their effect, focusing mainly on the effect of substrate, since the larvae are always processed at pre-pupae stage.
Regarding vitamins, Shumo et al. (2019) found pro-vitamin D, alpha-tocopherol, and gamma-tocopherol, although in lower levels than reported by
There is a change in composition as the larva develops into the different instars. Liu et al. (2017) showed that larvae rapidly increased their crude fat content between 4 and 14 days, time at which they reach the highest concentration of 28.4% DM. Between early pupae and late pupae there is a drop from 24.2% to 8.2%. The differences in fatty acid profile at different stages were attributed to the modulation of different genes involved in fat metabolism (Giannetto et al., 2020).
Studies have also performed on the variation of protein over time. Crude protein drops in the larval stage until day 12 down to 38%, thereafter increasing to 46% in early pupae, being at its highest in the adult state (57%) (Liu et al., 2017).
Since larva are harvested at just before stage, the following sections will focus mainly on the effect of different substrates in the composition of BSF at that time. Greatest attention is paid to crude protein, ether extract, and mineral fractions.
CP fraction is, in most cases, above 30 % DM (Table 1). Spranghers et al (2017) did not find significant correlation between CP in substrate and CP in larvae, whilst Shumo et al (2019) reported a strong correlation when using chicken manure and kitchen waste as a substrate.
BSF larvae consistently achieve CP values above 30 % in a wide variety of substrates. However, their efficiency is not the same for all substrates. For example, larvae reared in restaurant waste showed one of the highest protein contents, however larvae took longer in develop because of their low ability of degrading substrates with high oil content (Spranghers et al. 2017).
Regarding amino acid composition, Shumo et al. (2019) did not find any significant effect of substrate on lysine, methionine, isoleucine, leucine. Spranghers et al. (2017) reports very small variations in amino acid composition of larvae reared in different substrates. Liland et al. (2017) also reported minimal variations when increasing the levels of brown algae added to a processed wheat-based diet, from 0 to 100%.
The EE fraction is more affected by substrate composition and larvae weight (Ewald et al. 2020; Makkar et al. 2014; Shumo et al., 2019; Spranghers et al., 2017). However, the predominance of saturated fatty acids, especially lauric (C12:0), was consistent throughout experiments and treatments (Ewald et al. 2020; Gianetto et al, 2020; Makkar et al. 2014; Shumo et al., 2019; Spranghers et al., 2017).
Ewald et al. (2020) found that there is an increase in the content on PUFAs, especially eicosapentanoic (EPA) and docosahexaenoic (DHA) omega fatty acids, when feeding fresh mussels and mussel silage. Similar results have been observed by St-Hilaire et al (2007) when feeding fish offal. Although the differences are significant respect of substrates with low or inexistent PUFAs, the ability of modifying the profile of fatty acids based on dietary changes seem to be rather limited (Ewald et al., 2020). As the larvae become heavier. As the larvae became heavier, the concentrations on EPA and DHA tended to decrease irrespectively (Ewald et al., 2020).
Ash is the most affected fraction by substrate characteristics. Sprangers et al (2017) reported high correlations between the ash content of the substrate and of the larvae. The same authors found that Calcium levels were very viariable (66 g/kg for larvae grown in biodiesel digestate vs. 1g/kg for larvae fed in restaurant waste).
Shunno et al. (2019) found a lower range of concentrations when rearing larvae in chicken manure and kitchen waste (1.94 % for both) and spent grain (3.5%), although the variability of the calcium concentration in larvae reared in chicken manure was much higher than for larvae reared in kitchen waste. Makkar et al. (2014) reported higher average values of calcium (mean: 75.6 +/- 17.1 g/kg; min: 50 g/kg; max: 86.3 g/kg). Calcium concentration may further increase after fat extraction (Sprangers et al, 2017). Care should be taken with substrates high in calcium since that translates directly into high concentration of the mineral in BSF larvae meal.
Phosphorus varied between 4 and 6 g/kg, and it has been reported to be more consistent between larvae reared in different substrate (Shumo et al., 2019; Spranghers et al., 2017). However, Makkar et al (2014) reviewed higher values for phosphorus (mean: 9.0 g/kg +/- 4; Min: 6.4 g/kg; Max: 15.0 g/kg).
As mentioned above, Shumo et al (2019) reported that flavonoids apifenin and kaempherol are affected by substrate composition. Shumo et al. (2019) also found that the levels of pro-vitamin D, alpha tocopherol and gamma tocopherol were not significantly affected by rearing substrate (spent grain, chicken manure, and kitchen waste).
The production of BSF larvae as a protein source for animal feeds is certainly a promising, lower carbon footprint option to the more traditional sources. The high protein content and quality, as well as the high content of lauric acid, are very positive traits.
One of the possible disadvantages is the high variability of some of the components. Differences in composition depend on the rearing substrate and other factors such as rearing microclimate, differences in rearing methods, harvesting times and techniques, and possibly due to genetical heterogeneity (Shumo et al., 2019). The variability can be difficult to control, especially considering the heterogeneous and inconsistent character of biological.
Currently, several groups around the world are running projects to better understand and finetune the production of BSF larvae and its inclusion in animal feeds. In upcoming articles, we will discuss the utilisation of BSF larvae in different species and production types.
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