What Feed Causes Size Variation in Black Soldier Fly (I)

              Effects of Different feedstock’s on BSF larval development

Insect farming can be a viable new source of animal protein. Insects can be farmed in high densities with small space requirements and they have a high bioconversion ratio.  Furthermore, many insects can be reared on waste streams, which keep the environmental footprint low and assists in recycling of refuse. Consequently, the use of fly larvae in waste management has started to gain attention. One species that has gained more attention than others is the black soldier fly (BSF).

 BSFL are known to feed and develop on a wide range of feed sources, such as kitchen waste, dairy manure, chicken manure and human faeces. The larvae of the black soldier fly are ferocious feeders on decomposing organic material and could be used as protein source in animal feed. One reason why this species is of particular interest is because the fly does not feed and thus is not a vector in disease transmission. Another reason is that, when the larvae stop feeding in the final larval stage, they are higher in fat than other fly larvae.

However, the economic feasibility of a BSF system depends, among other factors, on the larval biomass produced from a certain amount of waste, in other words, the waste-to-biomass conversion ratio. In this article we will investigate the effect of different substrates on fly larvae composting in terms of larval growth (biomass conversion ratio, final larval weight), larval development time, waste reduction and amino acid profile of the larvae, and to identify the substrate parameters that influence the fly larvae composting process.

Types of Substrates Used

• Food waste: Food waste was collected from the local restaurants.
• Fruit & vegetables: Lettuce (50%), apples (30%) and potatoes (20%) were minced in a food processor.
• Abattoir waste: Abattoir waste was collected from a sheep farm. It comprised 48% stomach contents, 16% blood (cattle blood bought in a retail outlet), 12% manure, 16% meat and 8% organs (lungs, heart).
• Abattoir-fruit & veg. waste: Abattoir waste was mixed with fruit and vegetables in a 1:1 ratio based on wet weight.
• Human faeces: Human faeces were collected fresh in plastic bags and stored at −20 °C.
• Poultry manure: Fresh poultry manure (laying hens) was collected from farm.

Feeding Set-up
The feeding set-up was established in triplicate in plastic containers (21 cm × 17 cm x 11 cm) with netted lids, kept at 28 °C. In each box, 200 larvae (0.2 cm in size, 10 ± 2 day old) were placed, giving a larval density of 0.6 larvae cm⁻². The substrates were applied every second or third day, with a feeding rate of 40 mg DM (dry matter) larva⁻¹ d⁻¹. The substrate portions were thawed and brought to room temperature before feeding. When 50% of the larvae had transformed into prepupae, feeding was stopped but the experiment continued until all larvae had either turned into prepupae or had died. The survival rate was determined at the end of the experiment by enumerating all emerging prepupae and dividing this number by the total number of larvae added at the start of the experiment.

Results
Below are the results of Process efficiency and larval development properties to the final larval stage prepupae (pp) in BSFL composting with different substrate.

The highest waste-to-biomass conversion ratio (BCR) was achieved with the abattoir waste (15% DM) and the highest material reduction occurred for poultry manure, of which 85% was reduced on a DM basis.

The average crude protein content of the prepupae in this study was 41.2%  and it did not vary greatly between the prepupae reared on the different substrates. The highest protein content was found for the prepupae reared on abattoir waste (44%) and the lowest for those reared on human faeces (39%). The combined amino acid component of the dry matter comprised 36 ± 0.6% . The differences in amino acid profiles in the prepupae reared on different substrates were not large, although some were significant. Below is tale of The crude protein content (% of DM) and amino acid profile of the larvae (g kg⁻¹ crude protein) reared in the different substrates. 

Factor affecting larval size and development
The larvae grew largest on abattoir waste (including that mixed with 50% fruit & vegetables). In all substrates, the larval weight gain appeared to be linear. The correlation between VS feeding rate and the final weight of the prepupae was very strong, and is likely the strongest factor controlling the final prepupal weight.
Volatile solids and protein content seemed to explain the variations in bio mass conversation ratio and larval development to a great extent. However, other factors are also likely to contribute. The fat content is likely to have an impact, since BSF larvae need to accumulate energy in the form of fat for the adult phase. A substrate too rich in fat could also be detrimental, e.g. a study found that blowfly larvae reared on a high fat diet lived for a shorter time and did not survive to adulthood.  In the present experiment, no factors affecting the survival rate were identified and quite a high survival rate was found for all substrates

Conclusion
The main substrate properties affecting BSFL composting were found to be VS and protein content of the substrate. The BSFL were effectively reared on many waste streams, including food waste, human faeces and abattoir waste.
The larvae of BSF are robust and can feed on a variety of substrates, provided that the VS and protein contents are sufficiently high to support larval development. Abattoir waste, food waste, human faeces and a mixture of abattoir waste – fruits & vegetables are highly suitable substrates for BSFL composting, while pure fruit & vegetable waste and different sewage sludges are less suitable.