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What are the consequences of mixing worm species? To increase "processing" throughput rate, would it be better to have two worm bins of x m³, where each on uses a different specie (i.e Eisenia fetida and Eisenia hortensis), or one huge bin (2x m³) with both species mixed?

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Quick answer:

It depends on the circumstances in your bin and your abilities to keep those circumstances ideal for a particular type of worm. Assuming this is no problem, my best bet would be that a mix of Eisenia Fetida and Eisenia Hortensis has the highest throughput rate.

Long answer:

Most research on vermicomposting focuses on reproduction and grow rates of different worms species. It is assumed that higher reproduction rates means more juvenile worms which in turn means a higher processing rate. I'm not entirely sure if this assumption is true but since we have nothing else to go on I'll stick with reproduction and grow rates for my answer.

Mixing worm species can be done just fine as long as you can keep the conditions in your bin suited for all species involved. Chances are however that the conditions will be more favorable for one particular type of worm and that this worm will flourish and the population of the other worms will decline, so not all combinations of worm species will result in an effective mix.

Most research on vermicomposting worms favor Eisenia Fetida (a.k.a red worm) for a number of reasons.

  1. Fetida worms have fast reproduction and grow rates,
  2. they tolerate and can survive under a wide range of circumstances,
  3. they don't mind disturbances of their enviroment (at least not so much as other worm species like the Eudrilus Eugeniae).

It is for these reasons that the Fetida worm has become the most popular one for vermicomposting. A bin with only Fetida worms certainly is a good choice to get a good throughput rate under different circumstances. The only problem with Fetida worms is that they tend to stay close to the surface. They don't eat food that is buried deep into a bin. That's why I think you might be able to get a slightly higher throughput rate if you add Eisenia Hortensis (a.k.a. Dendobaena Veneta or European nightcrawler) worms. Hortensis worms like it a bit more wet and colder than Fetida worms, but both species will do ok in circumstances that are a bit less than ideal. Additionally, Hortensis worms like to spread out all over the bin so if you add food both at the top as well as deeper into the bin you'll get a larger surface of active worms.

Note that there has been some discussion about Hortensis worms. Research done in the late 80s and early 90s showed that the Hortensis had a rather low reproduction and grow rate, but in the late 90s this has been questioned by a number of worm farmers. Several people have reported that they found that the Hortensis worm can grow and reproduce just as fast as the Fetida and claim the different results can be explained by the difference in bin conditions or in the food supplied to the worms (see here, here and here). AFAIK the high reproduction rate of the Hortensis has not been proven scientifically yet but the fact is that the Hortensis worm has gained quite a bit of popularity in the last 10 years. If the Hortensis worm indeed has a similar reproduction and grow rate then it may outperform the Fetida worm when it comes to throughput rates.

Like I said in my quick answer, ultimately it will depend on the circumstances in your bin(s) and your abilities to keep those circumstances ideal for one or more types of worms. If you have a rather wet bin, it might be best just to use only Hortensis worms. If you have a lot of temperature changes the Fetida worm may be better. If your bin is kept at a constant temperature of around 25 Celsius (77 F) a bin with only Eudrilus Eugeniae might be the better choice. The best way to find out what works for you is simply to experiment.

EDIT:

I just found this very interesting answer of Robert Moore at the bottom of this post

What I have found is that combined breeding does increase the breadth of microbial species versus that found in single-species breeding.

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It is mainly the saprophytic microbial activity that composts, not the earthworm per se.

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The rate of decomposition is therefore a matter of the diversity and density of the microbial community along with how many earthworm mouths are present for a given volume of bedding which should be at C:N of 30:1 for maximum microbial reproduction within newly-prepared bedding, and harvested at 25:1

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