Black Soldier Fly Larvae vs. Red Worms - Who is the Better Composting Worm?

Posted by Critter Depot on November 22, 2020

Black Soldier Fly Larvae Composting vs Red Worm Composting

Two methods of worm composting are becoming increasingly popular: using black soldier fly larvae (BSFL) and using redworms (earthworms). While both of these methods are faster than traditional composting without added organisms, each method has its own advantages and disadvantages.

In this article, we will compare and contrast these two methods of composting, discuss which is more efficient, and look at some of the pros and cons of each method to help you choose which form of composting is right for you!

The Basics of Worm Composting with Added Organisms

Composting, in general, is the breakdown of organic waste products that creates a useable fertilizer for gardens and crops. Organic waste products like kitchen scraps, manure, and agricultural byproducts are loaded with nutrients like nitrogen and phosphorus that plants need to grow. However, these nutrients are trapped in the cells of the waste products and are bound in chemical forms that plants cannot use. Composting - whether with insects, worms, or microorganisms - breaks down cells and the molecules in waste products into forms that plants can readily incorporate and use.

Traditional composting (or composting without added organisms) involves putting kitchen scraps, yard waste, manure, or any other plant or animal waste products into a bin and letting them decompose over time. Traditional composting relies on microorganisms like bacteria and fungi to break down complex molecules into molecules that plants can use. There is no need to add organisms to a traditional compost pile because this method is most commonly done outdoors where these organisms are already abundant. However, the process can take months or years for the material to fully decompose.

By adding macroorganisms that specialize in composting waste (called decomposers), this time can be drastically reduced. Decomposers like redworms and BSFL are often chosen because they are very efficient composters and create compost that is extremely nutrient-dense. However, these organisms have more complex requirements than simple microorganisms. So, conditions must be kept just right to keep the worms or maggots alive and composting efficiently. Many people find that the added speed and efficiency of worms or maggots is worth the extra effort it takes to maintain these decomposers.

Comparing BSFL and Redworm Composting

There are many aspects of BSFL and redworm composting that are different. The categories below represent many of the different aspects of decomposer care that must be practiced in each of these forms of composting. This section can help you figure out which decomposer is right for you!

The Composting Bins

Each of these organisms is traditionally added to a bin that can help congregate the organisms around the waste products that you want to decompose. Each organism has a slightly different life cycle and requirements, which dictates the need for different types of bins. Let’s take a look at each one:

Redworm Bins

Redworms are typically kept in either an above-ground bin with multiple trays that the worms can move upward through or in a below-the-ground bin that allows worms to leave the compost bin to tunnel through other parts of your garden. The above-ground versions are essentially small storage totes with drawers. The bottom of the drawers have holes cut in them, so the worms can move from the bottom to the top of the bin as they continuously work on composting materials added to the higher drawers. The bottom drawers can be removed when they are full of fully-composted material, making it easy to harvest compost when it is ready.

By contrast, below-ground bins have a number of holes cut in the sides and bottom of the chamber that allow worms to come and go as they please. These chambers are great because they allow worms to come and go freely. Since worms aerate the soil and help plants thrive, planting a composting bin like this in the middle of your garden is a great way to spread nutrients throughout your garden and keep your soils aerated. With this form of redworm composting bin, you simply add compostable materials to the top of the bin and let the redworms do your work. When the bin is full of good compost, you can remove some of the compost and spread it around your garden.

Black Soldier Fly Larvae Bins

Black soldier fly larvae require slightly different composting bins than you use with redworms. The main difference is that black soldier fly bins are typically always maintained above ground. A BSFL bin typically has a square bottom with sloping sides. Attached to the edges of the bin are small troughs. Interestingly, black soldier fly larvae are “self-harvesting” simply due to behaviors they show at different times in their life cycle. When black soldier fly larvae are full-sized and ready to undergo metamorphosis into flies, they naturally leave the compost bin to find a safe place to enter pupation. So, these sloping sides and collection troughs can catch the “escaping” larvae very easily.

Since these maggots become flies after metamorphosis, most BSFL composting operations keep their bins within a screen-in area. This helps keep the flies in one area and ensures that they will lay their eggs back onto the compost bin. When a bin becomes full of compost, you simply stop giving the adult flies access to the bin. The remaining larvae will grow to full size and leave the compost, leaving you with ready-to-use compost that can be added to soil, mixed into a nutrient “tea”, or dried and sprinkled onto the surface of your garden. A new bin is typically rotated in and adult flies are allowed to lay eggs once again to continue the process.

What Wastes can be Composted?

Another consideration that is very important to composting is what sorts of materials you plan on composting. While BSFL will eat about anything, redworms have a much more restrictive diet. (It should be noted that you should never add foil, plastics, oil, solvents, paint, soap, or any other non-organic chemical substance to any composting bin, as it can kill off not only the worms or maggots but the microorganisms as well.)

Foods for Redworms

Redworms can eat most food waste and yard waste including plant materials, bread, cardboard, grass clippings, and leaves. There are many organic items beyond this list that redworms can decompose. That being said, there are few things that you should never feed your redworms:

Pet waste, manure: These wastes have high amounts of ammonia, which can kill redworms. These materials can be composted in a separate bin, but no redworms will be able to survive in any sort of highly-concentrated animal waste.

Meat and Dairy: Meat and dairy cannot be digested by redworms, since these organisms thrive mostly on plant materials. Meat and dairy can also attract other pests and will rot over time. Not only does this create a terrible odor, but it can actually lead to infectious diseases killing off your redworms.
Hot or Heavily Spiced Foods: Redworms have a very low tolerance for some spices like paprika or turmeric. Many spices are made with spicy peppers, which contain the chemical capsaicin. Redworms will not eat these materials, as they are toxic. So, no peppers or other “spicy” substances can be added to a redworm compost bin.
Citrus foods: Like spicy foods, citrus fruits (oranges, lemons, limes, grapefruits) contain essential oils that are toxic to redworms. Adding these substances to your composting bin will kill off all of your redworms, even in small amounts. You should not even add substances that have citrus juice added, since this is enough to harm your redworms.

Foods for BSFL

Compared to the restrictive diet of redworms, black soldier fly larvae can eat about anything. More and more research is showing that BSFL are capable of decomposing all sorts of waste. They can repurpose fish byproducts into fish food. They can decompose chicken, pig, and cow manure into a much more concentrated compost. They also have no known restrictions. For instance, they will readily consume citrus fruits, heavily spiced food, meat, dairy, and oily or fatty foods.

While you can feed your BSFL about any organic waste product, they cannot digest everything. For instance, they will leave behind the rinds of fruit and corncobs because they either contain substances that are unpleasant or they are too hard to consume. However, indigestible objects like rinds and corncobs most often make their way to the top of the compost pile as the maggots discard them. Then, you can easily pick them off the top and throw them away. Some scientists have raised concerns that BSFL will accumulate heavy metals if fed certain toxic materials, though most household wastes and agricultural wastes do not have high amounts of heavy metals.

The Environmental Conditions

Both of these organisms require specific environmental conditions in order to survive and reproduce. In general, redworm composting is slightly more forgiving as redworms are typically happy at lower temperatures and since they stay underground they have no requirements for light or air humidity. Here’s a quick comparison of the environmental conditions that need to be maintained for each species:

Redworm Environment

Since redworms live their entire life underground, there are really only two conditions that you must monitor. First, you have to keep the temperature of the soil between 55° and 80° F. Though worms can survive temperatures nearing freezing and temperatures slightly higher than this range, this is the optimal range for efficient composting. Second, you need to maintain the moisture level of the soil. In general, you simply need to keep the soil wet enough that the worms can easily move through it. This is slightly drier than mud but contains more moisture than dust.

Another factor to consider is soil pH, with the optimal range between 6 and 7. However, if you are simply composting yard and kitchen waste it is unlikely that the soil will change drastically beyond this range. However, you can get a cheap soil pH test to conduct testing if you are having trouble keeping worms alive.

BSFL Environment

Within the compost pile, black soldier fly maggots have very similar needs to redworms. They prefer a neutral pH and relatively high moisture content. However, BSFL do well at much higher temperatures. In fact, BSFL will continue composting up to 100° F!

That being said, in order for adult black soldier flies to reproduce they need a more specific set of environmental conditions outside of the compost pile. Specifically, the flies do best with an air temperature of about 86° F. As for humidity, the flies need quite a bit of humidity in their air in order to be active and not die prematurely. The humidity level in the air should be around 70% for optimal mating and egg-laying behavior. Interestingly, black soldier fly larvae also require UV light and several other specific frequencies of light in order for mating and egg development to occur. If you keep your BSFL composting operation outdoors, natural sunlight will provide all of these frequencies with no problem. If you are composting indoors, you will need to get a light that emits the specific frequencies that the adult flies need to initiate and complete the reproductive process.

You can read more about maintaining a black soldier fly composting operation in our article on Black Soldier Fly Breeding.

The Bottom Line: The Pros and Cons

Each of these organisms has a different set of pros and cons, and which organism you choose depends mostly on your personal preferences and needs.

Redworm Pros and Cons

Pros

Easy to maintain - redworms require a lot less monitoring, cannot escape enclosures easily, and do not have very specific requirements to be successful.
Can aerate your garden and produce compost - worms are natural soil aerators that will not damage garden plants. So, if you are producing more worms than can fit in your compost bin, simply transfer them to your garden!
No flies - redworms do not undergo metamorphosis into any other form. Redworms simply produce more redworms, so you don’t have to worry about screening or other forms of containment.
Incorporate directly into the ground - the best redworm composting bins can be installed directly into the ground of a garden, allowing worms to come and go freely. This not only makes the compost bin less stinky, but the worms can aerate your garden and spread nutrients as they tunnel through the soil.

Cons

Complex diet - redworms have a much more restrictive list of foods that they can decompose. In general, this is restricted to plant matter. Leaves, grass, and paper are optimal, but worms will die if given citrus, meats, dairy, or even spicy foods.
Slower decomposition - compared to BSFL, redworms are very slow eaters. This means it will take much longer to decompose the same amount of food.

Black Soldier Fly Larvae Pros and Cons