Organic Value Recovery Solutions LLC
© Organic Value Recovery Solutions 2010
© Organic Value Recovery Solutions 2010
Insect Digestion of Manure
D. Craig Sheppard and G. Larry Newton
An Excerpt from
Manure Management Strategies/Technologies
Submitted to the National Center for Manure and Animal Waste Management
Jeffery Lorimor, Iowa State University
Ron Sheffield, North Carolina State University
Ted Funk, University of Illinois
Charles Fulhage, University of Missouri
Ruihong Zhang, UC Davis
D. Craig Sheppard, University of Georgia
G. Larry Newton, University of Georgia
Insect Digestion of Manure
Insects, especially various fly larvae (maggots) and beetles readily feed on fresh manure,
converting residual protein and other nutrients into biomass, which is a high quality animal feedstuff.
Considerable research has been conducted to understand and exploit this activity for manure
management. Scientists in China, USSR, USA, Mexico, Eastern Europe, Israel, Australia and Central
and South America have studied manure digestion with insects to produce high quality feedstuff.
Lately, the emphasis has shifted from feedstuff production to potentially using insects to solve the
problems associated with the large amounts of manure produced at CAFO’s. While incorporating and
concentrating nutrients from manure into more valuable biomass (animal feedstuff), insect larvae
reduce the nutrient concentration and bulk of the manure residue, thus reducing pollution potential 50-
60% or more. Because of its much higher value ($500/ton), this feedstuff can be economically hauled
significant distances to relieve local nutrient overloads. Also, while occupying the manure the insects
aerate and dry it, reducing odors. Maggots modify the microflora of manure, potentially reducing
harmful bacteria. The high-value insect feedstuff, reduction of the manure mass, moisture content,
offensive odor and pollution potential are the returns for good management of such a system.
All published work known to the authors on insect digestion of manure relates to maggot
production. High reproductive and growth rates make flies (maggots are the larval, growth stage) the
best candidates. Flies that have been used experimentally to process manure include house flies
(Musca domestica), face flies (Musca autumnalis), blow flies (usually Sarcophaga sp.) and the black
soldier fly (Hermetia illucens). Except for the black soldier fly (Furman et al. 1959), all of these are
considered pests as adults due to their disease vector potential, behavior and preferred habitats.
Most current research is being done with the house fly and black soldier fly, so the following
discussion will be primarily limited to these.
Papp (1974) reported an 8% conversion of pig manure to house fly larvae (dry matter, d.m.
basis). Chiou and Chen (1982) found that blow flies converted 50 to 60% (depending on feeding rate)
of swine fecal mass to larval mass, including recovery of up to 55% of the manure organic carbon as
larval carbon. Black soldier fly larvae converted manure in a 460 hen facility to self-collected prepupal
biomass at a 7.8% (d.m. basis) rate (Sheppard et al. 1994), which would represent 58 tons from
100,000 hens in 5 months. House flies under optimum laboratory conditions converted poultry
manure to pupae at a 7.6% rate (Miller et al. 1974). In a recent study with swine, the authors
observed 15% d.m. conversion of manure to black soldier fly prepupae. Research by Engineering,
Separation and Recycling (L.L.C.) of Washington, LA found a 24% d.m. conversion of food waste to
soldier fly prepupal biomass. The mouth parts of soldier fly and blow fly larvae allow them to macerate
solids, apparently resulting in utilization of greater proportions of solid resources than is possible with
house fly culture. Manure-fed maggots convert feed to weight gain with at least the efficiency of our
most efficient concentrate fed domestic animals.In all successful published trials fresh, aerobic
manure was used. Beard and Sands (1973) and Morgan and Eby (1975) reported that anaerobic
manure was lethal or at best “unsuitable” for fly larvae development. Even aerobic manure a few days
old supported significantly less survival and growth of house fly larvae. This reduced performance
may be due to depletion of nutrients by microorganisms or development of conditions directly
detrimental to larval development. Beard and Sands (1973) wrote that established house fly larvae
slowed bacterial growth. Black soldier fly larvae also require fresh manure. Old or stockpiled manure
has supported poor larval growth.
Because of the need for very fresh manure, insect digestion is best done as a continuous
process directly under the poultry or livestock. This allows for immediate consumption before any
competitive bacterial action and saves expense of handling. The manure is reduced in-place and less
hauling is required. Serial batches started at short intervals have been successful with house flies
(Morgan and Eby 1975, Eby and Denby 1978), but require much more labor and a separate facility.
The black soldier fly is well suited to continuous digestion directly under the animals (Sheppard et al.
Twenty or thirty years ago dense soldier fly populations were common in open-sided poultry
and swine housing where they were held on wire or slats. Here soldier fly larvae were present by the
millions in a layer covering the manure bed. Fresh manure was consumed immediately. Adults were
hard to find since only the ovipositing females returned to the facility. These situations were common
in the southeastern US and west to California (Furman et al. 1959). These unmanaged populations
eliminated house fly breeding and reduced manure residue (Sheppard et al. 1983) but feedstuff
harvest was never attempted. A simple ramp and pipe system has been developed (Sheppard et al.
1994) which directs migrating prepupae to collection containers. Modern, environmentally controlled
animal housing makes the manure inaccessible to soldier fly adults, even though house flies often
flourish there. Recently developed rearing techniques for black soldier flies (Sheppard et al. in
manuscript) allow for introduction of immatures to digest manure in these modern facilities.
Bacteriological interactions associated with manure digestion by maggots are favorable.
Maggots are competitors with bacteria for nutrients and often reduce bacterial numbers greatly, or
eliminated them altogether (Beard and Sands, 1973; Sherman, 2000). Maggots may consume and
digest microorganism, and produce antibacterial and/or fungicidal compounds (Landi, 1960; Hoffmann
and Hetru, 1992; Levashina et al., 1995 and Landon et al., 1997). As maggots reduce pathogens in
manure they may make it safer for organic vegetable production. Foodborne illness associated with
fresh produce has doubled in the last 20 years and is associated with the increased use of animal
manure as fertilizer. E. coli: 0157:H7 cases are eight times more likely among people who consume
organic foods than those who do not. A preliminary study with black soldier fly larvae indicated a
reduction in E. coli 0157:H7 in an artificial medium innoculated with larvae. Numerous studies using
dried, rendered and fresh maggots as animal feed have revealed no health problems resulting from
this practice. Bacterial culturing of self-collected soldier fly prepupae from a recent swine trial
revealed no pathogens.
House flies have been the agent of choice in most studies because of their high reproductive
rate, short life cycle (2 weeks at 25C), well understood biology, and the ability to flourish in virtually
any animal manure. El Boushy (1991) reviewed the use of house flies to convert manure to high
quality animal feed. With any fly species to be used as feed, mature larvae or pupae are the
preferable stage for harvest. Losses of biomass by migrating larvae and in pupal development cause
the adult to be about half the weight of the mature larvae (Papp 1974) and the more chitinous
exoskeleton of the adult may reduce nutrient availability.
Feeding studies with house fly and blow fly based feedstuffs have shown them to be generally
equal to soybean meal (and other conventional ingredients) in feed value when fed to chicks, rats,
pigs, or trout (Calvert et al, 1969; Dashefsky et al., 1976; Finke et al., 1989; Gawaad and Brune, 1979;
Too et al., 1980; Teotia and Miller, 1974; Poluektova et al., 1980; Khan et al., 1999). Soldier fly
prepupae have been fed experimentally to several animals, replacing soybean or fish meal, along with
added fat, in formulated diets. The prepupae used in these trials contained 41-42% crude protein, 31-
35% ether extract, 14-15% ash, 4.8-5.1% calcium, and 0.60-0.63% phosphorus, on a dry basis.
These feeding tests utilized chicks (Hale 1973), pigs (Newton et al., 1977), catfish and tilapia (Bondari
and Sheppard, 1981, 1987), or frogs (Newton and Sheppard, unpublished data). The general
conclusion of each of these studies was that soldier fly larvae or larval meal was a suitable
replacement for a high proportion of conventional protein and fat sources. A recent replicated catfish
fingerling feeding study demonstrated that BSF prepuape can replace high quality fishmeal with
essentially the same growth, in spite of losses apparent with the fresh chopped prepupae. Fish on
both diets tripled weight in 55 d. The menhaden fishmeal fed as the standard in this study is valued at
about $500 per ton on the commodity market (2/02). Separation of the protein and fat in the larval
meal by rendering would have allowed for more precise diet formulation.
Experimental rendering of soldier fly prepupae produced a meal with 62% crude protein and an
oil which was high in desirable medium chain and monounsaturated fatty acids (Sheppard and
Newton, 1999), but house fly pupae are reported to be considerably higher in unsaturated fatty acids
(Calvert et al., 1969) than soldier fly larvae. In addition, it may be possible to extract other potentially
valuable products, such as chitosan or antimicrobial peptides, from fly larvae or pupae.
Insect digestion of manure as a primary treatment is promising. This process reduces manure
mass, nutrient content, moisture and odor, while producing high quality feed. Most studies have been
conducted with the house fly because it is easily reared and well known. High energy and equipment
costs probably explain why house fly systems have not been adopted. Better understanding of the
biology and culture techniques for the black soldier fly are supporting development of a very low cost
system to exploit the advantage of insect digestion of manure. Factors relating to the use of these two
insects are given in Table 1. There is still considerable interest in using house flies for manure
digestion, notably in Israel and Chile. A better understanding of how to utilize the black soldier fly is
generating interest in the US, Vietnam, Israel and Columbia. Engineering, Separation and Recycling
(L.L.C.) from Washington, Louisiana has developed and patented innovative manure handling
equipment to compliment soldier fly manure digestion by removing the oldest manure from the bottom
of the basin. This facilitates the continuous process.
Current research needs include developing practical commercial scale systems, determining
optimum utilization of the high value insect based feedstuffs, determining bacterial interactions,
including safety of feedstuffs and developing more efficient culture techniques for the black soldier fly.
Nutrient depletion of manure residue following maggot digestion has been variable. Studies on
maggot culture and environmental conditions are needed to maximize nutrient reduction, especially
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