1. Explanation of the life cycle of the black soldier fly
The black soldier fly (Hermetia illucens) comes from the order Diptera and belongs to the family Stratocedae, native to the subtropical and warm regions of the Americas. An adult black soldier fly has a black wasp-like appearance and can reach a length of up to 20 mm. When fully grown, BSF does not have a spongy feeding structure like common houseflies, which makes BSF unattractive to potentially hazardous waste; As a result, the risk probability of transmission of zoonotic diseases is reduced.
Flies maintain their bodily functions and survival mainly by depleting the fat reserves they accumulate during the larval feeding phase, making the feeding phase crucial for their longevity. BSF is considered a non-pest fly and spends most of its time on leaves without water, which have a life expectancy of 5 to 12 days and can survive in water for 14-21 days.
After 2 days of pupal emergence, the female and male begin the mating ritual, courting to fly to a vertical height of 1.5 meters and landing on the ground for mating. The gestation and oviposition periods occur 2 days after mating, not earlier.
Each female flies lay 400-900 eggs. When the eggs hatch after 3-4 days, the larvae begin feeding and go through 5 larval stages before reaching the 6th propupal stage. At this stage, BSFL stops eating and excretes the intestinal contents. The growth and viability of insects also depend on the nutrient bioavailability of insects, but unfortunately, we know very little about this (Lalander et al., 2019). As the larvae develop, their mouthparts undergo morphological changes, suggesting the development of a feeding Xi that consumes more feed from stages 1-4 and slows down by pupal stage, indicating nonlinear consumption.
An attractive feature of this species is that in its sixth stage, they naturally migrate to protected dry areas and are found to climb vertical surfaces that are moist enough to maintain water tension. This migratory behavior is known as "self-harvesting" (Wang and Shelomi, 2017). The physical appearance of the pre-pupa is distinguished by the hardened pigmentation of the exoskeleton, while the larvae remain white.
The difference between the propupal and pupal stages is that the pupal is completely immobile and inelastic in the last abdominal segment. Pre-pupation will take 7-9 days to reach the pupal stage, and another 6-9 days to complete metamorphosis and complete metabolism.
2. Protein and lipid content of black soldier fly
The larvae of the black soldier fly are able to reach a lipid concentration of 25-35% and a crude protein of up to 45%. Regarding crude protein, fish does not depend on the level of crude protein per fish, but on the amino acid profile. In order to be properly applied in the lowest cost formulation software, it is essential to understand the factors that contribute to changes in nutritional value.
The nutrient composition of black soldier fly pre-pupa is highly dependent on the feeding substrate provided and can significantly replace fishmeal (FM) in animal nutrition. Compared to fishmeal, soybean meal (SBM) has a lower concentration of essential amino acids, especially methionine, lysine and threonine, and lacks essential n-3 fatty acids; Eicosapentaenoic acid, docosahexaenoic acid (Cummin et al., 2017). As a result of this conflict, animal nutritionists are eager to find new sources of protein to feed non-ruminants with high protein quality and digestibility. In the Thomas Sprangers 2017 study, three waste stream matrices were used to raise black soldier fly larvae. The analysis concluded that feeding the substrate had no substantial effect on the amino acid profile. The Lalander et al. (2019) data using 11 feeds also showed the same conclusion reached by Spranghers et al. (2017), which had similar crude protein concentrations for both despite higher individual essential amino acid content.
Table 1: Average of crude protein % and lipid % dry matter of P. nigra pre-pupa in different diets
Spranghers et al. (2017) showed that the fat content and conventional protein used in feed formulations can be replaced by insect meal without adversely affecting product quality. Studies have also shown that insect crude protein increases to 42.5% when reared with fish manure (Schmitt et al., 2019), which may prove to contribute to the production of a micro-circular economy. The study also showed that substrate type had an effect on lipid content, but to a limited extent, consistent with Spranghers et al. (2017). Compared to n-6 PUFA concentrations (46-120 mg kg-1), n-3 PUFA levels ranged from relatively low (9-23 mg kg-1) and had high levels of lauric acid (436.5-608.9 mg/kg). St. Hilaire et al. (2007) demonstrated that the lipid concentration of n-3 PUFAs ranged from negligible to about 3% when BSF larvae were fed with fish offal. The same authors also proposed that the same level could be achieved in prepupal by adding a small amount of fish waste to the feed of the larvae 24 hours before pupation.
A special reason for the flight of soldiers is that in its last larval stage, their fat
A particular reason for the interest in black soldier flies is that in their final larval stage, they have a higher lipid content than other larvae. This makes them interesting for biodiesel production (Surendra et al., 2016), but the economic feasibility of using larvae to reduce waste depends on the nature of the input material.
3. Accumulation of mycotoxins
There is a great deal of knowledge about the interaction between the biological activity of mycotoxins and terrestrial animals, especially in animal husbandry, which can be a problem. Mycotoxins are secondary metabolites of molds that have significant effects on animal and human health (Zain, 2011). The results of ingestion of mycotoxin-contaminated feed in fish are similar to those of terrestrial animals, often resulting in stunted growth, impaired immune systems, severe poisoning, and death (Marijani et al., 2019). The most common compound in cereals is the aflatoxin Aspergillus flavus produced by fungi. Food waste can be used as feed for BSFL, but precautions must be taken to prevent fungal poisoning. For example, if aflatoxins are ingested by dairy cows, the harvested milk can be contaminated with chemical compounds that are transferred to humans. Similarly, toxic residues can be carried into the body through fish meat (Marijani et al., 2019).
Table 2: Mean mycotoxin concentrations in feeder substrates and bioaccumulation in BSFL
Gülsünoğlu et al. (2019) assessed the accumulation and nutrient recovery of deoxynivalenol (DON) using the black soldier fly and concluded that the toxin does not accumulate in the larvae, but rather passes through the larvae when nutrients are absorbed into the body, but its exact nature is not explained. From the perspective of disease transmission, an added benefit is that when the larvae reach the post-feeding stage, they excrete the intestinal contents, thus reducing their potential harm (Spranghers et al., 2017). However, more research is needed on the defecation period after the onset of the pre-pupal stage, as premature harvesting of pre-pupae may lead to the absorption of harmful substances by the animal, but it has been shown that it takes 1-3 days (Diener et al., 2015b). The study by Purschke et al. (2017) also showed that for various mycotoxins, concentrations were below the detection limit, but comparable levels of deoxyniferol were detected in the residual substrate, while zearalenone did did not differ significantly between postlarval and prelarval.
The continued growth of the fungus during the biological treatment of the larvae may also result in higher levels of residues. Another study by Bosch et al. (2017) is consistent with the findings of Purschke et al. (2017), in which BSFL has a high tolerance to aflatoxins B1 and M1 below 0.10 μg/kg and does not affect larval growth, which was also reported by Camenzuli (2018). The study also showed that BSFL larvae metabolized and excreted a mixture of aflatoxins B1, (DON), ochratoxin A, zeafenone and mycotoxins, but warned that additional research and toxicological testing were needed to understand possible contamination limits in feed. As far as I know, unfortunately studies on the accumulation of mycotoxins in the larval gut have not been well documented, as the earliest studies were in 2017 (Purschke et al., 2017; Bosch et al.， 2017)。 The existing research on the bioaccumulation of mycotoxins before pupal is promising, as the studies have come to the same conclusions.
Fourth, the accumulation of heavy metals
The production of the black soldier fly is an opportunity for farmers to generate additional income directly from their farms, but large amounts of heavy metals have been found in animal manure, some of which have exceeded feed heavy metal thresholds in some countries, such as China and the European Union (EU) (Qiao etal., 2017). For example, the typical concentration of cadmium in pig manure in the Northeast is 15.1 mg/kg dry weight (Qiao et al., 2017). This concentration far exceeds China's limit of 0.1 mg/kg and is 7.5 times that of the European Union. The highest concentration of chromium in livestock manure was 2402.95mg/kg, which exceeded the Chinese standard by 12 times. These levels are a problem because cadmium and chromium are often toxic to animals and humans, even at low levels. Studies have shown that certain heavy metals have negative developmental and survival effects on BSF, such as elevated zinc concentrations and strong fluctuations in daily pre-pupal harvests. Zinc-contaminated food has been shown to severely affect the survival of larvae (40%) and pupae (70%) of common houseflies due to reduced blood cell density, an indicator of the fitness of the fly's immune system.
Table 3: Comparison of initial heavy metal concentrations in feedstock with BSFL prepupal and residues
It is known that the accumulation of heavy metals does occur in pre-pupa (Table 3). However, few people have taken into account the accumulation of heavy metals in the pupal stage for the purpose of reproduction. In an interesting study, Joe et al. (2017) tested the presence of cadmium and chromium in different life stages and different parts of the body and found that larval and pre-pupal cadmium concentrations were significantly higher compared to diet, but chromium produced the opposite result, which was lower in concentration. During the larval stage and pro-pupal stage, these two metals are mostly found in the body rather than in the coat. However, in pupa, the content is higher in pupa and the result is the opposite. The results suggest that the different concentrations of these two heavy metals in different body parts at different life stages may be a strategy for BSF to tolerate and discard heavy metal stress through molting.
In terms of the effects of heavy metals on larval development, cadmium and chromium had no effect on the elution rate and larval survival rate. Cadmium had no significant effect on the cumulative number of pupae, whereas chromium had a significant effect on the cumulative number of pupa. One possible reason is that high chromium concentrations with low pupation rates may disrupt the metabolic activity of proteins, carbohydrates, and lipids, resulting in a lack of energy for life stage progression. Another possible explanation for the high accumulation of cadmium is that Cd2+ is similar to Ca2+ and can be absorbed into cells through calcium channels. Despite an 18-fold increase in cadmium concentrations in (2015b), which should cause toxicity, the development time from closed eggs to pre-pupae changed by only 1.3 days on average. One hsp70 family protein discovered by Braeckman et al. could explain the low effect on developmental time. This protein is induced by high cadmium levels in the cellular environment of Aedes albopictus (Diptera: Culex family) and acts as a defense mechanism against the effects of cadmium. On the other hand, as Ortel explains, heavy metals also contribute to the reduction of lipid accumulation in BSFL. A paper by Purschke et al. (2017) is consistent with Ortel's (1995) finding that postlarval quality is low.