Farming Black Soldier Flies (BSF) | Managing Larval Migrationby Terry Green on 09/06/15
Black Soldier fly (BSF) larvae, on reaching the prepupae stage in their life-cycle, spontaneously self-harvest from waste in search of a dry and dark place to pupate (see The Life Cycle of the Black Soldier Fly). Younger larvae on occasion albeit prematurely self-harvest on encountering too much carbon dioxide accumulation in pockets of waste inadequately ventilated, or from waste that has overheated as a result of too much aeration. This blog describes a simple efficient larval trapping and retaining system designed to contain larvae exiting waste that is adaptable to a number of design layouts in farming BSF.
We earlier described a method of collecting larvae exiting waste processing bins (see Scaling Up Black Soldier Fly Food Scrap Processing | Phase III). Drain pipes used in the latter system had slots cut on the top side of the pipes longitudinally which served to catch larvae dropping from waste processing bins, and led them to drop sites where collection containers in harvesting the larvae were placed. Some larvae inevitably escaped however from the open pipes before reaching the drop sites. Larvae escaping capture bypassed the drop sites by crawling up and over the top of the slotted pipes. On reaching the floor beneath the waste bins they crawled in all directions to hiding places some distance from the waste.
This type of migrating activity underscores a well-known characteristic of BSF larvae and prepupae - that they are persistent and superb escape artists. They are photophobic (light fearing), and naturally seek out dark nooks and crannies. In seeking a dry dark hiding place, prepupae, for example, have no difficulty crawling up vertical walls several feet unaided, and they can travel horizontally long distances of upwards of several hundred feet. They push through tiny openings of less than 1/8th of an inch (~3.2 mm) width, lodge under concrete blocks, burrow beneath rocks, leaves, and boards, even wedge between stacked shingles, bricks and other tight crevasses in seeking cover from predators where they subsequently metamorphosize into adults.
Given the opportunity, prepupae and larvae will climb up and over the edge of a container or shallow channel system used to collect them by extending and catching the hook-like feature of their mouth part over the edge of the container or channel guide. Once they have secured a hold on the edge, they pull the rest of their body up and over the edge, freeing themselves to crawl away unimpeded in search of an appropriate place to begin the process of metamophosizing into an adult.
They cannot however maintain a grip on horizontal surfaces while in an upside down position. This limitation in their ability to crawl about is the key element that can be exploited in limiting their migration. All that is needed to block their migration up and over the edge of a container or channel guide is to secure a flat “drip” edge horizontally around the perimeter of a container or channel guide extending outward from the wall edges a distance of ~1.5 to 2 inches (~3.8 to 5 cm) at a right or even more obtuse angle. Larvae and prepupae fall vertically by the force of gravity into channel guides and containers lined with barriers of this design, and on landing inside, are no longer able to escape from units set up in this manner.
Fig. 1 illustrates the general principle in managing larval migration. As prepupae exit waste, they drop inside a collection trough (or container) where they remain confined by the “drip” edge barrier extending at right angles from its wall.
Fig. 1 Cross-sectional view illustrating the position of a right angle barrier blocking escape of BSF larvae and prepupae from a self-harvesting collection unit. In this example, the collection unit is fabricated out of concrete. The barrier is a wood strip fabricated as a roof “drip” edge extending approximately 2 inches out from the concrete wall. The barrier, itself, is firmly secured with bolts around the perimeter of the concrete unit.
Whereas some larvae attempt climbing up and over the walls of the collection unit, the right angle barrier blocks their escape, preventing them from reaching the outer edge of the barrier. Because prepupae never reach a length of more than ~ 3/4th inch (~19 mm) at most, they are too short to extend their hook mouth part far enough to catch hold of the outer edge of the barrier. Those that try end up tumbling back to the bottom of the unit.
Figs. 2 shows how the same “drip” edge barrier design can be incorporated into a 5 gallon larval collection bucket by cutting a hole in the bucket lid large enough to allow larvae to fall freely into the bucket through the center hole section of the lid. In this application, the lid can be snapped in place over the bucket to ensure larvae falling into the bucket remain inside until they are to be cleared from the bucket. Snapping the lid off of the bucket can be done quickly and easily in clearing the bucket of trapped larvae as needed.
Fig. 2. Picture of a larval “drip” edge barrier lid fabricated from a lid fitting over a 5 gallon plastic bucket used for collecting self-harvesting BSF prepupae.
The center section of the plastic lid was cut out leaving behind a perimeter barrier approximately 2 inches wide. Snapping the lid over the collection bucket blocks larvae and prepupae from escaping out of the bucket. Prepupae and larvae are easily cleared from the bucket on removal of the lid.
Fig. 3 is an example of a commercial gutter incorporating a “drip” edge barrier which blocks larvae from crawling up and out of the gutter. The right and obtuse angles and turns on the edges of the gutter block larvae from gaining access to the edge of the gutter.
This gutter can be secured against the side wall of a waste bin. Larvae and prepupae exiting the waste drop directly from the edge of the processing bin into the gutter (also referred to in this blog as the channel guide which leads larvae and prepupae to the drop site where they are subsequently collected in containers placed beneath the drop site of the channel guide).
Once in the gutter, or "channel guide", larvae and prepupae cannot escape laterally and are forced to migrate along its longitudinal axis. Collection containers lined with larval barriers (see, for example, Fig. 2) placed beneath the open ends of the gutters (the "drop sites") can be used in combination with the gutter channel guides to ensure efficient containment of larvae and prepupae exiting the waste.
Fig. 3. An example of a commercial gutter fabricated with right and obtuse angled ends scrolled in a design that blocks BSF larvae and prepupae landing inside the gutter from escaping laterally back out of the gutter. Larvae and prepupae migrating longitudinally along the gutter can be collected in collection containers quantitatively as they drop vertically off the open ends of the gutter.
As of the posting of this blog we have roughly three years of field experience year in and out using this design in farming BSF larvae. We have observed no significant losses of larvae escaping from barriers incorporating this design.
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