We have had a home compost bin going in our backyard for a good twelve years now. We a started out with a bottomless, stacking, heavy-duty plastic bin that has three open square sections that fit one on top of the other with a jointed lid that opens halfway. The method of that system was to open the lid on its hinge and dump in whatever was being fed to the compost pile. As the level rose in the bin (let’s call it bin 1), you were supposed to remove the cover, take the top section, which is basically an open square, and place that on the ground. It would then become the bottom layer of bin 2.

We would scoop the composting matter out of bin 1 with a pitchfork or shovel, turning it over and dumping it into bin 2. The process was repeated until the level in bin 2 rose to the top of the bottom square and the amount in bin 1 decreased enough to remove the second level. This was then placed on the bottom tier of bin 2 and the shoveling process was repeated until the third tier could be placed onto bin 2 and then covered with the lid. Needless to say, it was a fair amount of work and in order to use any finished compost that it might have generated involved another step. In the process of transferring the pile to the new ‘bin’ we would screen it for any usable finished compost. What I liked about this system was that the open base sat directly on the ground which made it easy for the earthworms to get in there and do their thing. It’s still in use in our yard but more as a holding bin at this point.

We received the gift of a tumbling composter from our daughter and son-in-law a few years ago (it’s the same model that they have) and it has made the composting process much easier. The fact that it rotates means that we don’t need to continually turn it over with a shovel, we simply give a few good spins each time we add raw material to it. It also has two compartments, one for new material and one for compost that is in the works. When the new side is full the ‘in the works’ side is usually ready to be used. You simply place a tarp or low bin under the barrel, rotate it so that the opening is at the bottom, slide the door open, and the compost dumps out. OK, sometimes it doesn’t just fall out and needs a bit of encouragement, especially if it is still pretty moist. We will add this to the tiered bin if we aren’t ready to use it right away.

Our bins, side by side

Dog waste composterOur son and his fiancée returned to Connecticut this year and they have embraced all things gardening and environmentally friendly, including composting. They even have an in-ground dog waste composter as they have two adorable fur-babies who provide plenty of material for it. It’s basically a septic system for dog waste. Dog waste shouldn’t go into a home compost pile that is used in a vegetable garden as it doesn’t get hot enough to destroy the harmful pathogens that may be present. Additional info on composting do’s and don’ts can be found at the UConn Soil and Nutrient Laboratory’s Composting Basics fact sheet.

However, what I really want to tell you about is a beneficial visitor to Luke’s backyard compost bin. It started with an image that he sent to me of a little black flying insect that he said were all over the outside of their first compost bin which was simply a 30-gallon plastic storage bin that they had drilled holes into. What I saw was a little bluish-black fly with two wings so I immediately suspected that it was a member of the order Diptera.

Adult fly on storage bin, image by Luke Pelton.jpeg

Further investigation revealed it to be a black soldier fly, Hermetia illucens. I got to see it in person the next time that I stopped over and the first thing that I noticed was that its shiny body seemed metallic and almost wasp-like with a very narrow ‘waist’. It may remind you of the mud dauber wasp Trypoxylon politum with its shiny blue-black exoskeleton. And there is a good reason for that. The soldier fly is considered a mimic fly, using its appearance as a form defense as it tricks predators into thinking that it is capable of stinging them. But these harmless flies do not sting; in fact, they don’t even bite since they don’t have mouth parts or digestive organs. They subsist on nectar and water, living off of fat reserves from their larval period. The adult female will lay her eggs, up to 500, near decaying matter but not on it so that she does not transmit any pathogens from the waste. And then the really interesting stuff happens.

The larvae of the soldier fly are among some of the most efficient waste-consumers ever to exist. As soon as they hatch they begin to consume waste, double their body weight per day, a process that they can extend for up to 6 months if the conditions are unfavorable for their development. They start out 1 mm, .04 inches, in length and can reach up to 27 mm, 1.06 inches, within two weeks. The image below shows many different sizes cohabiting.

BSF larva 1 image by Jamie Zimmerman.png

Their diet can be any kind of organic waste; kitchen scraps, animal manure, and even coffee grounds, although coffee grounds alone aren’t good long-term as it boosts their metabolism and makes them overly active (does that sound familiar?).

As they eat they can actually raise the temperature of the compost pile, a benefit as higher temps also help to speed up the composting process. They eat waste before it has a chance to decompose and start to smell. Their frass, or poop, is an odorless residue.

BSF larvae 3 crop

When it is time to pupate, the larvae go through a stage known as prepupae wherein they stop eating, empty their guts of any of any undigested remains, and then their chewing mouth parts become climbing mouth parts as they head to drier areas to pupate for the next week or two before the adult fly emerges to begin the cycle again. Some of the larvae in Luke’s bin may be emigrating to a new home in our bin this weekend so that we can also employ their amazing abilities, at least until the temperatures descend below 32°F. Black soldier flies do not survive the extreme cold of a Connecticut winter so we can only hope that they will return to the area in the spring. As I tried to get images of them I learned two things about them: The larvae prefer to be out of sunlight as they moved very quickly into darker areas and the adult flies are even quicker!

Getting a good image was difficult as they do not sit still but fortunately Bugwood had some great ones, as seen below. The image on the left was taken by Marilyn Sallee, Bugwood.org and the image on the right by Johnny N. Dell, Bugwood.org.

The black soldier fly is an amazing little creature that is quickly emerging as an important economic and ecological member of our world. The grown larvae are high in protein and fat making them a desirable food source for poultry, some livestock and even humans. If they are fed a diet of fish-trimmings they can actually contain omega-3 fatty acids. Although Diptera species account for only 2% of the edible insects consumed worldwide as of 2012 this number may increase as the wonders of this little marvel are researched further. Just in case you want to start searching for recipes.

Susan Pelton

Images by Susan Pelton, Luke Pelton, & Jamie Zimmerman except where noted.

UConn Home & Garden Education Center