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How many BSFL for a Compost Pile
Larvae of Hermetia illucens (L.) (black soldier fly, BSF) are great natural feeder insects for your pets, such as reptiles and chickens. Larvae of BSF (BSFL) also can efficiently consume and metabolically process your composted plant-based food scraps and convert them into usable products, like frass (insect feces). It has been estimated that BSFL can reduce food scraps and bio-material by up to 80% within two weeks under optimal conditions.
The feces your BSFL produces is beneficial not only as a biofertilizer but can also help protect your plants from pathogens, as discussed in our article about the plant’s response to chitin (the primary component of an insect’s exoskeleton). As you get more experience with BSFL composting, you might wonder how efficient this species is at producing nutrient-rich compost.
BSFL in Commercial Settings
Extensive research has been conducted that evaluates the benefits of BSFL with regard to lowering waste treatment costs by adding them to compost. Currently, BSFL is being researched as a means of reducing a large volume of biowaste—waste that is organic and biodegradable; solid or liquid—and converting it into nitrogen-rich fertilizer with larval casings that will help protect plants from pathogens.
What is not so widely discussed with this species, at least on the individual level, is the conversion of biowaste. For example, research estimated that of a metric ton of plant-based food waste, nearly 13% was converted into usable metabolic products by BSFL (for growth) and 25% was converted into insect feces. Another experiment reported that from 10 metric tons of food waste inputs, nearly 33% of it was converted into frass. However, the research mentioned did not quantify how many BSFL are needed to obtain these percentages.
Unfortunately, the above articles did not cite the exact composition of food that BSFL fed upon, which can have a significant impact on the health of your larvae and their outputs. For example, a study was conducted that used BSFL frass as fertilizer on maize plants, the difference between the frass used was what the larvae fed upon. One type of frass was derived from brewery waste (spent grain, protein sediment [hot trub], and residual yeast), and the other was derived from chicken droppings. It was found that after feeding on brewery waste, BSFL converted 2.5% for itself and 33.8% into feces. This is compared to chicken droppings, in which BSFL consumed and processed 6.4% for itself while 62.9% was converted into frass biofertilizer. All to say that the type of compost you feed your BSFL matters, as they will metabolize some materials better than others. This is especially important if you are interested in feeding BSFL to your pets, such as chickens.
How many BSFL for my compost?
But how many larvae should you use in your compost? Our research has led us to recommend densities of 4 larvae/cm2; equivalent to 3,716 larvae per square feet of compost. These evaluations of BSFL frass outputs found that when 10,000 BSFL were fed 15–20 lbs. of food scraps in an area of 0.25 m2 (2.7 ft2), the larvae produced 1.5 times their body weight in frass. Scientific research suggests higher densities of larvae are acceptable. For example, one study tested two different densities of BSFL (7 and 10 larvae/cm2) on different mixtures of food waste. What the researchers found was that the larval density and feed composition had a great impact on the reduction of the compost, the biomass of larvae, and their production of frass. But what also was important, was the mixture of food waste. The study indicated that when compost was 20% coffee grounds, survival rates of BSFL decreased. This is contrasted with a compost mixture of 80% food waste and 20% high fiber residue, which was found to be optimal for larvae. This compost mixture increased larval biomass and reduced compost faster than the other mixtures. The frass output for each larva using optimal compost was about 60 mg.
If you have not tried using BSFL in your compost, you are missing out on an ecologically sound method to convert your food scraps into nutrient-rich compost. If you are raising chickens, using BSFL to compost provides them with food and opportunities to forage. Although higher densities of larvae will reduce volumes of compost faster, it appears that the makeup of your compost is the most important aspect in this regard. Research suggests that BSFL can convert nearly 30% of your compost into nitrogen-rich material. This is to say that for your compost pile, you can likely expect nearly a third of your compost to be converted into insect frass by BSFL; especially if you use higher densities of larvae. What this means for you is that your compost pile will be primed to promote disease resistance in your garden, helping you produce a healthy harvest.
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