Separating Black Soldier and House Fly Larvae from One Anotherby Terry Green on 05/14/15
Black Soldier fly larvae (BSFL, Hermetia illucens) efficiently scavenge food scrap and manure (see Black Soldier Fly Processing of Biodegradable Wastes and The Black Soldier Fly, Hermetia Ilucens, as a Manure Management / Resource Recovery Tool). Larvae from the common housefly (Musca domestica), though distinct from H. illucens, grow on the same wastes and, like BSFL, self-harvest. Often wastes used in farming BSFL are contaminated with larvae of the common housefly. This blog describes a simple, rapid and inexpensive method of separating harvested BSFL and prepupae contaminated with housefly larvae and pupae from one another.
Both fly larvae exit waste on which they grow as they approach their pupa stage seeking a dry and dark place where they can pupate and metamorphosize into adults. It is relatively straightforward to collect larvae using gutters or troughs designed to collect the larvae as they climb out of the waste on which they are growing.
Fig. 1 Separation of BSFL larvae and prepupae (H. illucens) from the common housefly (M. domestica). Upper Left, self-harvested housefly larvae and pupae recovered from food scrap waste contaminated with fly larvae; Upper Right and Lower Left, BSFL prepupae and housefly pupae floating in water suspension (Upper Right) and denser housefly larvae settling out (Lower Left), respectively; Lower Right, Elliptical stainless steel wire screen with longitudinal holes between wire mesh strands spaced at approximately 2 mm apart through which fly larvae and pupae are passed in separating out BSFL from housefly larvae and pupae.
Because there is such an enormous difference in the size and density of BSFL larvae and prepupae relative to that of housefly larvae and pupae, the fly larvae and pupae can be separated from one another by simply suspending the harvested larvae and pupae in water (see Fig. 1). Most BSFL prepupae, and housefly pupae, for example, float on the surface of water. By submerging, lifting and agitating an elliptical wire mesh screen with a longitudinal gauge opening of approximately 2 mm width in a scooping motion across the surface of the suspension, BSFL prepupae are quickly and easily skimmed off from the larval suspension leaving behind housefly larvae and pupae, and younger BSFL that have sunk beneath the surface due to not having yet built up sufficient fat reserves to float on the surface of the water.
The same elliptical screen can be used to capture the younger BSFL too dense to float on the surface by swirling the remaining larvae back into suspension and then netting them in a second pass of the screen as they swirl up from the bottom. The critical element in capturing and separating BSFL from housefly larvae and pupae is the gauge width of the elliptical wire screen. It must be wide enough to allow housefly larvae and pupae to freely pass through the screen, yet narrow enough to block BSFL from similarly passing through the screen.
The elliptical screen works well in rapidly separating BSFL from housefly larvae and pupae because of the marked differences in width and lengths of their larvae and pupae which can be exploited in separating them from one another after they have self-harvested. Water used at this stage in processing them serves to also clean up the larvae from residual waste carried over on their exoskeleton at the time they self-harvested, and can be recycled in growing and processing additional BSFL.
Residual housefly larvae and pupae left behind in the water wash can be easily recovered once BSFL have been separated off by simply passing the wash suspension through a stainless steel mesh screen having holes of 1 mm or less, too small for housefly larvae and pupae to easily wash through. Even so, if the housefly are left idle on the screen, the larvae will attempt to crawl through the small pores of the screen making cleanup of the screen tedious. To get around this latter problem, cold water can be used in the wash step. Additionally, scraping housefly larvae free of the screen without delay after filtering them off from the suspension works well.
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