Pond Boss Forums INTRO Questions & Observations Black Soldier Fly project

Hi -steve,

I don't think there is a material that would allow liquids to pass through and also block BSF larvae without quickly clogging. The smallest larvae are as thin as a thread and even the larger larvae are able to squeeze through small openings. It may be possible to keep them primarily in the upper layers by creating a (deep) lower layer without food, but I think it would become unnecessarily complex due to the powerful mixing action of the BSF. I say that because it's not a big issue if a few larvae escape through the bottom. I think that for most part the larvae move into and below the filter temporarily and that they eventually migrate back to the upper layer. It's difficult to contain BSF larvae so I think it's best to allow them to move freely when possible.

Most BSF systems I've tried don't have a drain outlet that is always open. They've either been capped or drained directly into the ground (which stops juvenile larvae). In these cases any larvae that venture down the drain pipe would simply return the way that they came. I have also operated systems where the drain pipe terminated above ground and would allow larvae to escape. I didn't notice larvae escaping in those systems but it's possible. Even if some larvae did exit via the drain pipe it would not have been a large percentage.

I looked at the matala filter material and I think it's ideal for a BSF unit except for the cost.


Yes, the larvae can crawl upside-down given enough moisture and a smooth surface, however, a lip can still be an effective barrier. The original BioPod (as opposed to the BioPod Plus) did an excellent job of containing BSF larvae because of the lip at the top of the main compartment. The larvae are able to crawl on the underside of the lip, but they can't easily make the 360º turn required to continue once they reach the end. The lip molded into the original BioPod was an upside-down "U" shape but a horizontal lip would probably work well also. The main problem is in building such a lip without the benefit of rotomolding which is how the BioPod is made.


Maybe. 1/8 inch might be too long and at the least I suspect it would allow medium sized larvae to pass. One important consideration with this and anything else you build for a BSF unit is the fact that BSF larvae will defeat many types of adhesives over time. Velcro has fairly aggressive adhesive and I've seen it fail several times due to BSF. Part of the problem is that many adhesives are weakened by moisture which is normally present in a BSF unit. The main issue is that the larvae will continually dig into any tiny gap. The most common failure I've had with Velcro is in the corners of units made from rectangular storage containers. The larvae congregate in the corners and when large numbers of them crawl up the sides most of them will concentrate at the Velcro in the upper corners. With a column of larvae beneath them, the higher larvae have something to brace against and they often loosen the Velcro. (This isn't strategy on their part, just programming to migrate and to dig into crevices.) The round shape of a bucket avoids this problem because there isn't a concentration at any one spot. A single larvae will still try to dig beneath the Velcro, but with nothing below it to brace against it just slides backwards without success.

Note that some types of Velcro are advertised to be moisture resistant and those are the types to use. I'm very careful when installing Velcro to clean the surface with alcohol and not to touch the sticky part of the Velcro with my fingers. I also work out every air pocket under the the Velcro and rub it firmly with the back of a spoon. Lately I heat the Velcro with a hair dryer after it's installed and before I rub it with the spoon. I don't use the "loop" side because larvae get tangled in it and will usually get stuck and die.


The "compost", for lack of a better term, should not be wet and pasty. "Too dry" for BSF larvae is below 70% humidity, and if you're processing household food scraps this won't be an issue. The problem with a pasty consistency is that it becomes oxygen starved therefore promoting the growth of anaerobic bacteria. If your compost smells like a sewer this is probably the cause.

I've been using sawdust, and more recently wood shavings in my units with good results. As opposed to making the compost too dry I believe the wood shavings work as moisture buffers; they absorb excess liquids and then retain them to help stabilize the moisture content. I theorize that the shape of the wood shavings help trap air in the compost, helping to limit the growth of anaerobic bacteria and also aiding the flow of liquids though it. I thought wood shavings were the best thing you could add to a BSF unit, but I think there might be something even better.

Yesterday it occurred to me that ground corn cob would work even better. It has a sponge-like structure that may hold oxygen and drain better than wood shavings. In addition I think it may be a great substrate for aerobic bacteria (good kind) which the BSF larvae feed on. I've added fresh corn cobs to my BSF units before and I've noticed that the larvae are attracted to it for several weeks, long after any obvious corn remains. Usually it's the smallest larvae that are attracted to the cobs. If this same behavior happened with commercially available ground corn cob then the larvae would carry oxygen with them as they work on the cob, and this would keep the pieces from becoming anaerobic. Ground cob is sold in pet stores as animal bedding and it's not too expensive.

-steve, thanks for the great questions. I hope to hear more from you as you experiment with BSF.