Farming Black Soldier Fly (BSF) Larvae | Managing Feedstock and Avoiding Colony Collapse : The Life and Times of BSF (Black Soldier Flies)
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Farming Black Soldier Fly (BSF) Larvae | Managing Feedstock and Avoiding Colony Collapse

by Terry Green on 01/31/16

In an earlier blog we described essential technical steps that need to be addressed in scaling up and optimizing larval yields using food scrap as larval feedstock (see Farming Black Soldier Flies (BSF) | Scaling Up & Sustaining Larval Production). In this blog we describe some observations on larval behavioral traits and corrective troubleshooting strategies effective in amplifying larval yields while at the same time averting colony collapse.

It is not uncommon in farming larvae, especially on scaling up production, at one time or another to run up against a perplexing problem - the sudden die off of the larval colony. This can be costly and frustrating. In some cases it may take several weeks, even months, to rebuild the colony population back to where it was before the colony collapsed. Colony collapse can however be traced inevitably to a shift in one or more environmental variables away from those required in farming and harvesting healthy larvae. It is avoidable.

Raising larvae off of nutrient wastes, for example food scrap, vegetal pomace, or other agricultural waste byproducts, is a biological process involving competition for nutrients. Microbes of all sorts, common soil invertebrates, predatory insects and dentritivores, etc., all compete for nutrients and niches, each doing what it can to knockoff or gain a toehold over its competitors. In situations where the environmental balance tips away from that needed in farming BSF larvae, the larvae lose out to competitors. Without taking decisive corrective measures in restoring proper growing conditions necessary in maintaining healthy growth and propagation of the larvae, this imbalance can spiral out of control leading directly to colony collapse. Let’s face it – “It’s a jungle out there!” – only the fittest survive!

Whereas larvae are well-adapted in competing for nutrients, dynamic physiochemical changes in the composition of the decaying waste are ongoing (see Recycling Food Scrap? | What’s with the Stink?). Local pH gradients and temperature gradients occur while it resides in the bioreactor and undergoes degradation, its moisture content can fluctuate widely depending upon drainage conditions, its nutrient quality may vary, etc.  No matter how well controlled the external room temperature is maintained, the humidity, the load rate of feedstock deposited into a larval bioreactor, etc., managing larval production is never a static process.

To sustain larval production over the long haul, you must first learn by observation the most favorable conditions in maximizing larval yield. Using this knowledge, you must proactively manage the larval environment in keeping the competitive balance for nutrients as much as is practical tipped favorably toward the conditions best suited in growing and propagating larvae. At the same time, you must keep competitors in check over the full life cycle of larval growth on waste added to the bioreactor.

Consider what must be done using food scrap as your source of larval feedstock. To maximize the growth, it must first be shredded or macerated. Why? Observe closely and you will see that larvae need help in penetrating the outer cuticle layer of many lignocellulosic plant products making up food scrap wastes. They simply lack the physical attributes and enzymes needed in helping to break through this out layer. They cannot, for example, readily gain access to nutrients encapsulated inside oranges, melons, apples, bananas, cauliflowers, and other fruit and vegetal produce unless the outer skin or peeling is first pierced, cracked or split open.

Shredding and/or macerating food scrap markedly increases its overall surface area giving the larvae much freer access to nutrients. It also makes available an abundant supply of aqueous nutrients not as readily accessible in bulk chunks of food scrap. This simple step of macerating or shredding food scrap before adding it to a larval bioreactor can markedly improve larval yields.

The physical appearance and texture of food scrap however also changes after its addition to a larval bioreactor. Its density, its viscosity, and its porosity change in a very dynamic manner. The diffusion and permeation of oxygen, and the off gassing of carbon dioxide accompanying these metabolic and physical changes likewise change as it degrades. These changes must also be addressed in order to sustain higher larval yields over the long run. In maximizing larval yields in response to these latter changes, the challenge is to identify effective corrective measures which will accommodate the ongoing changes in the physiochemical nature of the decaying food scrap that are best suited and effective in keeping the processing train running efficiently.

Based on experience observing larvae grown off of food scrap at our DipTerra LLC test facility, we have compiled a set of working guidelines linking larval behavioral traits and yields with root opposing forces working against increased larval yields. Paying attention to larval behavioral traits, and understanding the message conveyed by a specific behavioral trait, can pay off in higher larval yields. For convenience in analyzing and linking larval behavioral traits with yields, we define those environmental conditions causing a negative drop in larval yield “stressors”.

More than one “stressor” may knock down efficient growth and propagation of larvae in a farming operation. In troubleshooting the root causes impeding larval production, start by identifying what “stressor(s)” are in play. Then proactively eliminate each in working through the farming operation. If you stick with this strategy, you should soon realize a marked improvement in larval yields in your farming operation. Keep it up and you will furthermore escape the dreaded “colony collapse”!

Here in no particular order are some tell-tale larval behavioral responses to known stressors, and accompanying proactive corrective strategies worth paying particular attention to:

STRESSOR INDICES IN LARVAL BEHAVIOR TO ATTEND TO IN MANAGING LARVAL PRODUCTION

Premature exit of young larvae from a bioreactor

Root Causes - Environmental stress
  • Generally associated with inadequate aeration/gas exchange of feedstock resulting in excess anaerobic activity, drop in pH accompanying excess accumulation of organic acids (pH below 5), and/or accumulation of excess CO2 inside the bioreactor – improve aeration and gas exchange and also render the food scrap in the bioreactor more penetrable to larvae by mixing bulking agent in with feedstock and/or check liquid released as the food scrap degrades is draining freely from the residue retained in the bioreactor (see Fig. 1); or
  • Excessive aeration and overheating of feedstock in bioreactor (feedstock temperature exceeding 40+ C) caused by the presence of too much bulking agent or insufficient retention of water draining free of feedstock allowing for aerobic oxidation to dominate degradation of waste with accompanying production of excess heat – reduce  volume of bulking agent mixed in with feedstock, add water to feedstock in bioreactor on which larvae feed to slow air exchange and verify feedstock temperature drops back into a range between 25 and 40 C for proper maintenance and sustained production of larvae; or
  • Overpopulation of larvae in bioreactor or, alternatively, insufficient feedstock (scrap waste) in bioreactor required in supporting growth and maturation of young larvae up through the prepupae stage of their life cycle – add additional feedstock to accommodate increased population of larvae growing in bioreactor or, if over population is occurring, move larvae self-harvesting prematurely to another bioreactor less populated with larvae.
Absence or noticeable decline in population of young larvae in bioreactor

Root cause - Operation not properly setup to ensure and sustain larval propagation
  • Improper management of feedstock in bioreactor leading to environmental stress – see above, Premature exit of young larvae from bioreactor; or
  • Inadequate mating conditions – verify that the temperature where BSF adults are mating is not less than 25 C, preferably 30+ C, verify that natural light bathes mating area during the day (see Fig. 3), confirm by visual inspection that adult BSF are mating, visually confirm that female adults are depositing egg clutches in vicinity of feedstock, and ensure that eggs are hatching (egg clutches should disappear within approximately 4 days of deposit unless eggs were killed by fungi or having desiccated because the humidity is too low to sustain eggs laid in clutches before new larvae can hatch from the eggs); or
  • Not enough mating events occurring because adult BSF population is too low due to an insufficient number of prepupae getting set aside to ensure emergence of an adequate number of mating adult BSF – increase the number of prepupae set aside from the harvest set aside in reserve to be used for mating and larval propagation of bioreactors; or
  • Insufficient drainage of feedstock in the bioreactor or alternatively too little bulking agent mixed in with feedstock retained in the bioreactor leading to anaerobic fermentation of residual feedstock, formation of heavy, viscous and relatively impenetrable feedstock which traps and suffocates young larvae buried in it resulting in a sharp die off larvae, especially younger larvae too small to push and burrow through the heavy and anaerobic feedstock residue inside the bioreactor (see Fig. 1) – add more bulking agent to the feedstock retained in the bioreactor to render it more penetrable and/or improve drainage (see Figs. 1 and 2).
Bulking agents mixed in with food scrap can be prepared by shredding tree limbs and leaf debris into small fragments. Shredded bulking agents should be premixed with food scrap and stored under anaerobic conditions ready to load into larval bioreactors as needed (see Figs. 1 and 2). Premixing bulking agents with food scrap ahead of time is of considerable advantage. It simplifies loading and distribution of the food scrap in the larval bioreactors.

images of food scrap decaying in BSF bioreactor
Fig. 1.  Effects of physio-chemical changes in food scrap composition on larval survival and harvest. Fig. 1a, partially decomposed food scrap lacking bulking agent near colony collapse having the physical attributes of a heavy, viscous, larval impenetrable mass. Larvae are trapped in the heavy mud-like texture and will soon die off in ever increasing numbers.  Fig. 1b, food scrap drawn from a bioreactor treated with bulking agent is lighter in density, open and free flowing. Larvae are abundant and move freely through it while feeding and growing off of nutrients. In this example, on a wet weight basis the bulking agent (shredded wood chips, See Fig. 2) was mixed with food scrap in a ratio of approximately 1:15. Copyright © 2016 Terry Green, All rights reserved.

image of shredded wood chips used as bulking agent in BSF bioreactors
Fig. 2. Images of wood chip bulking agent used in farming BSF larvae grown off of food scrap. Fig. 2a, image of bulk wood chip pile; Fig. 2b, close up view of wood chip bulking agent. Fragments in the shredded pile vary but on average have a length ranging from about 1 to 2 inches (approx. 2.5 to 5 cm). Copyright © 2016 Terry Green, All rights reserved.

The composition of food scrap can vary widely. Hence tests should be done to ascertain drainage and larval penetrability following the addition of food scrap into larval bioreactors in farming BSF. The ratio of food scrap and bulking agent needs to be worked out empirically depending upon the nature of the food scrap in a particular setting sense many environmental variables (changes in temperature during the day and night, humidity, bioreactor design, drainage layout, etc.) all have an effect on how the food scrap degrades as larvae feed on it. From a practical perspective, it is best to settle with a particular food scrap source and to adjust the mixing ratio of bulking agent stick with it in simplifying the farming operation in going forward.

The importance of sustaining an efficient system for propagating larvae simply and in high numbers must also not be underestimated in reaching higher larval yields on a sustained basis. Larval propagation can be managed simply using plastic reservoirs designed to attract and induce female adults to lay egg clutches inside the units, leaving the egg clutches untouched. In this system larvae spontaneously hatch and grow off of small amounts of feedstock deposited inside the units which served to attract female adults to the units in the first place, and which subsequently sustain larval growth pending transfer of the newly hatched larvae into bioreactors dedicated for raising and harvesting the newly hatched larvae (see Farming Black Soldier Flies (BSF) | Scaling Up & Sustaining Larval Production).  Avoid using cardboard flutes, or any other such egg collection placards that are susceptible to biodegradation. These latter materials are very prone to picking up fungal spores which readily infect and kill off larval egg clutches deposited on their surfaces before new larvae emerge free of the egg clutches. 

Lastly, don’t forget to set aside some prepupae for mating and propagation. You can even allow adults to mate and propagate your bioreactors free of a nursery operation by simply giving them direct access to spaces housing your bioreactors (see Fig. 3).

image of BSF adults hanging out and mating above BSF bioreactor
Fig.3. Image of adult BSF hanging out and mating inside BSF processing site housing larval bioreactors. Adult males tend to hang out on the walls and ceilings near skylights and mate with females in midflight above larval bioreactors. Females lay egg clutches directly in and about the bioreactors, seeding the bioreactors on a continuous basis with a fresh supply of newly hatching larvae. Copyright© 2016 Terry Green, All rights reserved.

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Comments (2)

1. Bernardo Kostich said on 11/8/18 - 05:52AM
Hello Terry. Is there something that could be done with the dead adult flies? Could they be put in the residue? Greetings from Chile, Bernardo.
2. Terry Green said on 11/8/18 - 05:02PM
Bernardo, the NPK ratio of dead adult flies is approximately 10:2.4:1.3, so they could be mixed with spent residue as a means of enriching the latter marketed as a soil amendment. Dead adult flies also are quite rich in eumelanin and melanin pigments, polyphenolic byproducts of significant value having both antioxidant and, in combination with chitosan and fats, broad spectrum antimicrobial and antifungal activities (see http://www.insectum.eu/wp-content/uploads/2018/06/Melanin-properties-at-the-different-stages-towards-life-cycle.pdf). Recycling dead adult flies in larval bioreactors may also be of some advantage in suppressing the growth of pathogenic bacteria and fungi finding their way into waste fed to larvae. The dead flies may also prove with more research to be a rich source for the discovery and isolation of novel antimicrobial agents with significant medical applications.


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