Harvesting Black Soldier Fly Prepupae in Your Home Gardenby Terry Green on 06/08/13
Black Soldier flies are non-pest, beneficial, outdoor insects which silently and efficiently recycle food scrap and agricultural debris (see Black Soldier Fly Processing of Biodegradable Wastes, Processing Waste | Let the Black Soldier Fly Have At It!, and Recycling Food Scrap? | What's With the Stink?). Using these remarkable insects in your food scrap recycler/compost bin can help save you money while also reducing the work and drudgery of having to turnover compost (see Composting Getting You Down? Let Black Soldier Flies Do the Job!). The leachate (liquid) fraction released during the decomposition process also improves your garden soil’s fertility (see Black Soldier Fly Processed Food Scap | Foilant and Soil Applications, Amending Soil With Black Soldier Fly Processed Food Scrap Leachate, and Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soil). Additionally, Black Soldier fly larvae grown from food scrap and agricultural debris are an excellent feedstock for domesticated chickens and grub eating native birds. Fish (tilapia, trout, catfish, bluegills, etc.), reptiles and amphibians also thrive on the larvae. This blog describes a trouble-free and inexpensive way to simultaneously collect the leachate fraction and larvae as they carry on with the work of recycling food scrap and compost waste added to your home food scrap recycler or composting bin.
Figure 1 shows the general setup. Depending upon the type of recycler or bin you are using, variations in this design can be made.
Figure 1. All in one collection basin used for simultaneous collection of food scrap and yard debris leachate fraction and Black Soldier fly larvae recovered from recycler/compost bins. Fig. 1(a), food scrap recycler housed inside 30 gal. plastic collection bin. Recycler bin has a drain system near its bottom which channels leachate from waste processed by Black Soldier fly larvae into an empty 1 gallon milk jug resting on the basin floor beneath the recycling bin. Larvae dropping from the top edge of the bin fall into the collection basin and crawl out the side hole cut in the wall of the basin and fall into a separate collection container. A lid, with a hole cut in its center area for larvae to fall through, attached over the latter container prevents larvae from escaping the container; Figs. 1(b), 1(c) and 1(d), close up view of details of the collection basin. Copyright(c) 2013, Terry Green, All rights reserved.
To collect leachate released during the recycling process, drill a hole near the bottom of your recycler bin if it lacks a drainage system. Install a simple drain system to passively drain your bin of leachate as it forms. The drain does not need to be placed at the very bottom of the bin, but instead should be placed about 3 to 4 inches from the bottom of the bin.
By having the exit port for the drain above the bottom of the bin, fluid accumulating in the bottom section of the bin also gets processed by anaerobic and facultative anaerobic bacteria which further accelerate decomposition of the waste and enhance synergism with larvae feeding off of these microbes and the byproducts they produce during the processing of the waste. We have also compiled an eBook which can be purchased at nominal cost for anyone interested in building their own from readily available materials found in most hardware stores (see Building Your Own Black Soldier Fly Food Scrap Recycler) .
PVC or ABS pipe with right angle fittings and a couple of flat gaskets works well in constructing the drainage system for your recycler bin. To keep the drain pipe free of particles which otherwise could clog the drain, wrap it in mosquito netting. Your bin should be elevated on concrete blocks, or other alternate supports, so that you can insert a leachate collection container under the exit drain port. That’s all it takes to collect the leachate fraction.
To collect Black Soldier larvae, obtain a plastic basin (these can be purchased inexpensively from most hardware stores) or build one out of wood or other convenient material of sufficient diameter so that larvae crawling up and out of the bin, on reaching its top edge, will drop into the basin (for example, see Figs. 1a – 1c). Larvae, on reaching their prepupae stage, the stage in their life cycle where they spontaneously choose to exit the waste, are very efficient in crawling up and away from waste, and on reaching the edge drop by gravity into the bottom of the basin.
A common misconception regarding the ability of larvae at the prepupa stage is that they need help in the form of sloped ramps or other devices to find their way out of waste bins. They are, contrary to this misconception, quite capable of climbing several feet vertically up any wall. They will crawl up and over the lip of your bin without any help, and, in the absence of a basin, drop by gravity to the ground, seeking on the ground a dark, dry place out of sight from predators to pupate. Since the larvae are photophobic, most of the crawling and exit activity occurs during the night.
With the basin in place, larvae dropping from the top of your bin fall directly into the basin where they remain dry and clear of decomposing waste. To herd them into a collection container, drill a hole through one wall of the collection basin and place a small plastic or other collection container beneath the exit hole in the basin (see Figs. 1a – 1d). Larvae will wander around the basin, find the exit hole, and drop through it into the collection container.
Remember in harvesting the prepupae to leave at least 10 to 15% behind for propagating and maintaining the population in your recycling bin. Just toss them aside around the garden. They will find hiding places under leaves, rocks, etc., and as adults find their way back to your bin in maintaining new offspring in the bin.
To keep rain water from catching in the basin and flooding into the collection container, drill small 1/32nd to 1/8th inch holes spaced around the bottom of the basin. These latter holes are two small for the larvae to exit but will allow the water to freely escape the bottom basin area and consequently not find its way into the larvae collection container (Fig. 1d).
If you don’t wish to collect the larvae, place a layer of rocks, bark dust, decorative bricks or similar cover material around and under your bin. The larvae (prepupae) exiting your recycler or bin during the night will drop to the ground and burrow under the cover of the bark dust, rocks, etc., staying out of sight of predators where they will pupate. In two to three weeks they will emerge from their hiding places as adults and hide out in nearby trees and shrubs. Females mate within four to five days of emergence from their puparia, and will easily locate your recycler or bin, and lay clutches of upwards of 400+ eggs nearby in starting the cycle over again for the next generation.
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