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Journal of Insects as Food and Feed, 2015; 1(4): 261-270 WageningenAcademic
Publishers
ISSN 2352-4588 online, DOI 10.3920/JIFF2015.0030 261
1. Introduction
Urban poverty is a fundamental challenge in low and
middle-income countries associated with rapid urban sprawl
(Moore et al., 2003). The urban poor suffer most from
inadequate sanitary services and deficient municipal solid
waste management leading to increased health risks and
impaired household resilience. While informal collection
and recycling systems of inorganic material with a market
value are currently available, the organic waste fraction
often remains uncollected and untreated. Indiscriminately
dumped organic waste accumulates along streets, clogs
stormwater drains, pollutes water bodies, rots, and attracts
disease-transmitting vectors (e.g. flies, rodents), thus posing
serious direct or indirect health risks to local residents.
Local authorities, community-based organisations, non-
governmental organisations and research institutions
have recognised this deficiency and identified the need
for simple, environmentally and economically sustainable
organic waste treatment solutions in urban areas (Fluitman,
2000; Zurbrügg et al., 2007).
In many low and middle-income countries, the mass of
organic waste may be substantially reduced using larvae of
the non-pest black soldier fly, Hermetia illucens L. (Diptera:
Stratiomyidae) (Diener et al., 2009). H. illucens larvae feed
Bioaccumulation of heavy metals in the black soldier fly, Hermetia illucens and effects
on its life cycle
S. Diener1*, C. Zurbrügg1 and K. Tockner1,2,3
1
Eawag: Swiss Federal Institute of Aquatic Science and Technology, P.O. Box 611, 8600 Dübendorf, Switzerland;
2
IGB,
Leibniz-Institute for Freshwater Ecology and Inland Fisheries, Mueggelseedamm 310, 12587 Berlin, Germany; 3Freie
Universität Berlin, Institute of Biology, Takustrasse 3, 14195 Berlin, Germany; stefan.diener@eawag.ch
Received: 25 December 2014 / Accepted: 22 May 2015
© 2015 Wageningen Academic Publishers
RESEARCH ARTICLE
Abstract
In developing countries, effective waste management strategies are constrained by high collection costs and lack
of adequate treatment and disposal options. The organic fraction in particular, which accounts for more than 50%
of the waste production, constitutes a great, yet mostly neglected, reuse potential. Concomitantly, the demand
for alternative protein sources by the livestock feed industry is sharply increasing. A technology that effectively
transforms organic waste into valuable feed is therefore a timely option. Larvae of the non-pest black soldier
fly, Hermetia illucens L. (Diptera: Stratiomyidae), may be used to reduce the mass of organic waste significantly.
Concurrently, larval feeding converts organic waste into prepupae (last larval stage) which is high in protein. In
combination with a viable market, this potential animal feed may cover the waste collection costs and thus promote
innovative, small-scale entrepreneurs to establish a profitable business niche. Organic waste, however, often contains
persistent pollutants, such as heavy metals, that may accumulate in the larvae and prepupae of black soldier flies and
consequently in the food chain. In this study, we fed black soldier fly larvae chicken feed spiked with heavy metals
(cadmium, lead and zinc at three concentrations each) to examine the extent of metal accumulation in the different
life stages and the effect of heavy metal concentration in the feed on the life cycle determinants of the flies. The
cadmium accumulation factor in prepupae (metal concentration in the body divided by metal concentration in the
food) ranged between 2.32 and 2.94; however, the lead concentration remained well below its initial concentration
in the feed. The bioaccumulation factor of zinc in prepupae decreased with increasing zinc concentration in the feed
(from 0.97 to 0.39). None of the three heavy metal elements had significant effects on the life cycle determinants
(prepupal weight, development time, sex ratio).
Keywords: bioaccumulation, developing countries, food security, organic waste management, protein
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S. Diener et al.
262 Journal of Insects as Food and Feed 1(4)
voraciously on decaying organic leftovers from markets
and restaurants, animal droppings and on human faeces.
Myers et al. (2008) and Sheppard et al. (1994) reported a
33-58% reduction in organic matter from cow manure and
50% from chicken manure respectively and Diener et al.
(2011) reached a dry matter reduction of 70% in municipal
organic waste.
The final larval instar of H. illucens is called the prepupa
and consists of 32-44% raw protein and 33-35% crude fat
(Booram et al., 1977; Diener et al., 2009; St-Hilaire et al.,
2007). Hence, larval feeding converts organic waste into
a highly valuable protein that may be used as a substitute
for fishmeal. The increase in aquaculture led to a growing
demand for feed for aquatic organisms and therefore to
increasing prices (Riddick, 2014). Fishmeal is becoming
less available and the production and the sale of insect
protein can thus contribute to cover the waste collection
costs as well as allow innovative, small-scale entrepreneurs
to establish a profitable business niche.
However, organic waste often contains persistent pollutants
such as heavy metals that may accumulate in larvae and
prepupae and therefore enter the food chain. The heavy
metals enter the waste stream in various ways, be it through
atmospheric emissions or inappropriate disposal of heavy
metal containing refuse. While terrestrial organisms
ingest contaminants orally (biomagnification), aquatic
organisms also enrich pollutants in their biomass through
diffusion (bioconcentration). Bioaccumulation refers to
both bioconcentration and biomagnification (Walker, 1990).
The bioaccumulation factor (BAF) thus is the concentration
of a pollutant in organisms divided by its concentration
in the diet.
A stable black soldier fly population generating viable
eggs and producing healthy offspring are prerequisites
for running a sustainable organic waste treatment facility
using black soldier flies. However, heavy metals in organic
waste may influence life history traits. For example copper-
and lead-contaminated host plants negatively affected
fecundity and intrinsic rate of natural increase (rm) in the
cabbage aphid, Brevicoryne brassicae L. (Görür, 2006).
Reduced bodyweight in the offspring of the carabid beetle,
Pterostichus oblongopunctatus, inhabiting a metal-polluted
environment has been observed by Lagisz and Laskowski
(2008) and Moroń et al. (2014) found a clear relation
between increasing metal concentrations in the soil layer
and the increased negative impact on life cycle determinants
(e.g. population growth rate, number of brood cells, survival
rate) for wild bees, Osmia rufa.
In this study, the larvae of the black soldier fly, H. illucens,
were fed with chicken feed contaminated by different levels
of cadmium, lead and zinc to investigate the following
research questions:
•
To what extent do cadmium, lead and zinc – fed at
different concentrations – accumulate in the prepupae
of the black soldier fly?
•
Does heavy metal in the food influence the life cycle
determinants of the flies? (i.e. development time, body
weight, sex ratio)?
2. Materials and methods
Animals
Black soldier flies, H. illucens L. (Diptera: Stratiomyidae),
were obtained from a laboratory colony grown in an indoor
cage (1.5 m × 1.5 m × 2.0 m) at constant temperature
(26.5±0.05°C, 60.8±0.8% RH). The room was fitted with
two windows as direct sunlight is crucial for successful
mating (Tomberlin and Sheppard, 2002).
The newly hatched larvae used for the experiments were
reared on chicken feed (UFA 625, digestible energy:
11.7 MJ/kg, 60% moisture). A detailed description of the
rearing and hatching facility is given in Diener et al. (2009).
The experiments conducted in Switzerland did not violate
Swiss law (e.g. Animal Protection Law, Animal Husbandry
Act) or any of the provisions or regulations stipulated in
these laws. The experiments also met the International
Guiding Principles for Biomedical Research Involving
Animals as issued by the Council for the International
Organizations of Medical Sciences (CIOMS, 1985).
Experimental setup
Larvae were fed with chicken feed pellets moistened (final
moisture level: 60%) with either pure deionised water
(control) or a solution of deionised water containing heavy
metal ions (three concentration levels for each metal).
The 2% HNO
3
solutions used for feedstock preparation
contained cadmium (1000 mg/kg), lead (1000 mg/kg) or
zinc (10,000 mg/kg). The nominal concentrations in the
food were: 0.0 μg/g (control), 2.0, 10.0, and 50.0 μg/g Cd;
0.0 μg/g (control), 5.0, 25.0, and 125.0 μg/g Pb; 0.0 μg/g
(control), 100, 500, and 2,000 μg/g Zn. Low concentrations
corresponded to the metal concentrations typical for
market vegetables in India or Bangladesh (Alam et al.,
2003; Marshall et al., 2003; Sharma et al., 2007). Middle
concentrations for cadmium and lead corresponded to
concentrations typical for organic waste in Bangladesh or
Sweden (Eklind et al., 1997; Rytz, 2001) (Table 1).
Metal concentrations in the control groups were
derived from the chicken feed itself. Unfortunately, the
experiment series with the low concentrations of zinc were
contaminated and the results had to be discarded. However,
as the chicken feed itself had similar concentrations of zinc
(145.3 mg/kg, standard error (SE) = 10.1) as what was used
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Bioaccumulation of heavy metals in black soldier fly
Journal of Insects as Food and Feed 1(4) 263
in the experiment with low concentrations (177.4 mg/kg,
SE=3.8) we were able to utilise the control series (only
chicken feed and its respective zinc concentration) as
indication of results for the low zinc concentration series.
Thus the control series could be considered as the low zinc
concentration series and was compared to the medium
and high zinc series.
Each replicate (three replicates per treatment) contained
200 7-day old larvae placed in plastic containers (14.0 × 7.5
× 7.0 cm) and covered with nylon tulle held in place by the
lid of the box. The lids with nine holes (ø 15 mm) allowed
air circulation. The food and larvae were covered with a
re-sealable polyethylene bag containing a piece of black
cardboard to shield larvae from light. The pre-prepared
meal portions were packed into separate polyethylene bags
and kept frozen until use. The quantity of the diet was
calculated based on 100 mg food (wet weight) per larva
and day. The larvae were fed three times a week. Feeding
stopped when 50% of the larvae in the box metamorphosed
into prepupae to avoid overfeeding the remaining larvae.
Sampling and analysis
The samples (larvae, larval exuviae, prepupae, pupal exuviae,
and adults) were washed with deionised water, weighed,
lyophilised to measure dry weight, and ground in an agate
mortar for heavy metal analysis. The food samples and the
remains at the end of the experiments, the so-called residue,
were treated the same way except for the washing. Larvae
and prepupae were killed by freezing (-10°C), while adults
were killed with ethyl acetate. To prepare the samples for
the analyses, ~50 mg of the ground material was digested
in polytetrafluoroethylene beakers (HPR-300/10; MLS
GmbH, Leutkirch im Allgäu, Germany). The material was
moistened with deionised water. Approximately 4 ml HNO
3
and 1 ml H
2
O
2
were added before the sample was heated in
a laboratory microwave digester (MLS 1200 MEGA; MLS
GmbH). The clear solution was diluted with deionised
water (10× for Cd and 100× for Pb and Zn) and analysed
with the high resolution inductively coupled plasma-mass
spectrometer (HR-ICP-MS, Element II; Thermo Fisher
Scientific, Waltham, MA, USA). The standard solutions
were made using Merck ICP multi-element standard
solution IV (Merck Millipore, Darmstadt, Germany): 10,
100, 1000, 5,000 and 10,000 ng/l. The natural river water
standard SLRS-4 and the TM-28.3 trace elements fortified
calibration standard (National Research Council Canada,
Ottawa, Canada) were used as a control. The detection
limit for these elements was 10 ng/l.
BAF was calculated according to Walker (1990) as:
concentration in organism (Ci)
BAF = (1)
concentration in food and/or water ingested (Co)
In the present case, Co consisted solely of the heavy metal
concentration in the food.
Table 1. Heavy metal concentration in municipal solid waste and vegetables (based on dry weight) compared to the legal maximum
threshold level allowed in animal feed, human food and compost.1
Cadmium (mg/kg) Lead (mg/kg) Zinc (mg/kg) Reference
Heavy metal concentration in municipal solid waste
Sweden 0.16-0.6 2.4-26 49-165 Eklind et al., 1997
Dhaka, Bangladesh 5.0 n/a 226 Rytz, 2001
Heavy metal concentration in vegetables
Garden vegetables, rural village, Bangladesh 0.05-0.4 0.2-1.7 11-54 Alam et al., 2003
Market vegetables, Delhi, India 1.0-5.5 0.3-2.2 41-150 Marshall, 2003
Field vegetables, Varanasi, India 0.5-4.3 3-16 3-41 Sharma et al., 2007
Heavy metal limits in animal feed
European Union 2 10 n/a EC, 2002
Heavy metal limits in human food
European Union 0.05-1.0 0.02-1.0 n/a EC, 2001
India 0.1-1.5 0.2-10 5-100 Government of India, 1954
Heavy metal limits in compost from household waste
European Union 0.7 45 200 EC, 1991
Proposed standard in LMIC 3 150 300 Hoornweg et al., 1999
1 LMIC = low and middle-income countries; n/a = not applicable.
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S. Diener et al.
264 Journal of Insects as Food and Feed 1(4)
Statistical analyses
Statistical analyses were performed using SPSS Statistics
17.0 software (SPSS Inc., Chicago, IL, USA). For some
data, a violation of the Levene homogeneity of variances
was calculated. However, as the groups are equal in size,
ANOVA is very robust to this violation.
3. Results
Heavy metal accumulation
In all development stages (larvae, prepupae, and adults),
the metal concentration generally increased significantly
with increasing metal concentration in the food (Table 2-5).
However, BAF, i.e. the ratio of the amount of metal in
the body compared to that in the food varied among the
different metal elements and development stages (Table 6).
In prepupae, the BAF ranged from 2.32 to 2.94 for cadmium,
independent of the concentration in the food, while the
BAF remained <1 (0.25-0.74) for lead. In adults, the BAF
was very low for both cadmium and lead concentrations
(BAF=0.12-0.21). For zinc, the BAF decreased with
increasing concentration in the food (prepupae: from 0.97
to 0.39; adults: from 0.98 to 0.19). The EU threshold value
for cadmium (2 mg/kg) in animal feed was exceeded in
prepupae even at low cadmium concentration (7.9 mg/kg,
SE=0.6). Only prepupae from the low lead concentration
group (1.5 mg/kg, SE=0.7) met the EU concentration limit
for lead (10 mg/kg) in animal feed (EC, 2002).
Table 2. Cadmium concentration in soldier flies, Hermetia illucens, at different life stages based on dry weight, in digested material
(residue) and in food source. Larvae fed with chicken feed (100 mg/larva/day, 60% moisture) spiked with cadmium (three different
concentrations).1,2
Control Low cadmium Medium cadmium High cadmium
Mean (mg/kg) SE Mean (mg/kg) SE Mean (mg/kg) SE Mean (mg/kg) SE
Food 0.2 a 0.02 2.7 b 0.2 13.3 b 0.7 61.5 b 2.3
Residue 0.2 a 0.01 2.9 b 0.1 16.0 bc 0.4 89.8 c 2.1
Larvae 0.2 a 0.02 7.0 d 0.3 32.5 d 0.6 170.5 d 8.5
Prepupae n.d. – 7.9 d 0.6 36.2 d 1.9 142.9 e 8.3
Adults n.d. – 0.6 a 0.04 1.9 a 0.2 7.8 a 0.4
Larval exuviae 0.1 a 0.03 2.2 ab 0.3 18.8 bc 4.1 54.2 b 3.7
Pupal exuviae 0.5 b 0.1 5.2 c 0.7 22.9 c 2.6 94.1 c 12.1
1 Mean values followed by the same small letter in the same column do not vary significantly (P>0.05).
2 SE = standard error; n.d. = not detected.
Table 3. Lead concentration in soldier flies, Hermetia illucens, at different life stages based on dry weight, in the digested material
(residue) and in food source. Larvae fed with chicken feed (100 mg/larva/day, 60% moisture) spiked with lead at three different
concentrations.1,2
Control Low lead Medium lead High lead
Mean (mg/kg) SE Mean (mg/kg) SE Mean (mg/kg) SE Mean (mg/kg) SE
Food 1.1 ab 0.4 5.9 bc 0.3 34.3 c 1.8 142.9 b 2.9
Residue 0.1 a 0.01 7.8 cd 0.6 53.2 d 3.3 267.9 c 12.8
Larvae n.d. – 3.8 ab 0.4 22.8 b 1.6 141.7 b 17.2
Prepupae n.d. – 1.5 a 0.7 25.3 bc 1.9 40.1 ab 3.7
Adults n.d. – n.d. – 5.9 a 0.57 17.3 a 1.36
Larval exuviae 5.9 c 1.3 11.3 e 0.01 87.7 e 4.8 312.9 c 74.1
Pupal exuviae 3.7 bc 0.1 9.3 de 1.0 24.2 bc 2.1 66.7 ab 3.6
1 Mean values followed by the same small letter in the same column do not vary significantly (P>0.05).
2 SE = standard error; n.d. = not detected.
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Bioaccumulation of heavy metals in black soldier fly
Journal of Insects as Food and Feed 1(4) 265
Effects of heavy metals on life cycle determinants
Prepupae treated with cadmium were significantly heavier
than the control group. No significant effects were found in
prepupae treated with lead and zinc (Table 7). Development
time from hatching of the larva to the prepupal stage
generally increased with heavy metal concentration
although the increase was statistically insignificant (Table 8).
Average development time until pupation amounted to
15.2 days (SE=0.1) and did not differ significantly between
treatments or sexes. Heavy metals had no influence on
the sex ratio of adults (average males/females ratio: 0.98,
SE=0.02).
4. Discussion
The black soldier fly, H. illucens, fed with cadmium, lead
and zinc, exhibits different accumulation patterns. Larvae
and prepupae accumulated cadmium, yet the incorporation
of lead and zinc was suppressed as concentrations found in
the body were lower than in the food. These findings are
consistent with literature data (Figure 1). In the literature
reports, the BAF of cadmium uptake by detritivorous insects
averages 2.86 (SE=0.30, range 0.46-6.09) (Gintenreiter et
al., 1993; Kazimirova and Ortel, 2000; Kramarz, 1999;
Lindqvist, 1992; Maryanski et al., 2002; Ortel, 1995).
Cellular cadmium uptake probably occurs through Ca2+
channels. Due to their very similar ionic radii, Cd
2+
ions
can easily enter the cell via Ca
2+
channels, independent of
endocytosis or an ATP requiring ion pump (Braeckman
et al., 1999). Moreover, Braeckman et al. (1999) found a
protein of the HSP70-family induced by elevated cadmium
concentrations in the environment of Aedes albopictus
(Diptera: Culicidae) cells. Production of this protein, which
protects other proteins from denaturation, may also explain
the low effect of contaminated food on life-cycle parameters
such as development time or fluctuating asymmetry despite
the observed bioaccumulation of cadmium (cf. present
study).
Table 4. Zinc concentration in soldier flies, Hermetia illucens, at different life stages based on dry weight, in the digested
material (residue) and in food source. Larvae fed with chicken feed (100 mg/larva/day, 60% moisture) spiked with zinc at three
different concentrations. Samples from the series ‘Low zinc’ were contaminated during the experiment and could not be used
for interpretation.1,2
Control Low zinc Medium zinc High zinc
Mean (mg/kg) SE Mean (mg/kg) SE Mean (mg/kg) SE Mean (mg/kg) SE
Food 145.3 b 10.1 177.4 3.8 616 b 37.8 2,044 c 16.2
Residue 192.3 b 13.1 n/a – 1,196 c 66.7 3,313 d 240.9
Larvae 165.8 b 19.9 n/a – 596 b 88.0 866 b 141.9
Prepupae 138.9 b 24.3 n/a – 513 ab 44.5 801 ab 32.1
Adults 141.4 b 9.2 n/a – 272 ab 22.8 389 ab 37.6
Larval exuviae 275.5 c 13.5 n/a – 1,514 c 240.2 1,883 c 104.8
Pupal exuviae 35.1 a 4.1 n/a – 145 a 39.2 334 a 56.5
1 Mean values followed by the same small letter in the same column do not vary significantly (P>0.05).
2 SE = standard error; n/a = not applicable.
Table 5. Kendall’s tau rank correlation (r) between the heavy metal concentration in food and the concentration values in larvae,
prepupae, larval exuviae, and adults of the black soldier fly, Hermetia illucens.
Larvae Prepupae Larval exuviae Adults
r P N r P N r P N r P N
Cadmium 0.778* 0.004 12 0.833* 0.002 9 0.722* 0.007 9 0.741* 0.000 18
Lead 0.778* 0.004 9 0.833* 0.002 9 0.778* 0.004 9 0.671* 0.001 17
Zinc 0.667* 0.012 9 0.611* 0.022 9 0.833* 0.002 9 0.647* 0.000 18
* P<0.05
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S. Diener et al.
266 Journal of Insects as Food and Feed 1(4)
Table 6. Bioaccumulation factor (BAF) for larvae, prepupae and adults of the black soldier fly, Hermetia illucens, fed heavy
metal contaminated food at three different concentrations (low, medium and high; Table 2, Table 3 and Table 4). BAF for ‘low
concentration, zinc’ was calculated using data from the control group (see explanation in text). Because the concentrations of
cadmium and lead in the control group were so low, the analytical error had the effect of providing inaccurate BAFs, and are
therefore not shown here.1
Low Medium High
BAF SE BAF SE BAF SE
Cadmium
Larvae 2.65 0.10 2.46 0.11 2.79 0.24
Larval exuviae 0.86 0.19 1.41 0.31 0.88 0.06
Prepupae 2.94 0.09 2.75 0.25 2.32 0.09
Adults 0.21 0.01 0.15 0.01 0.13 0.01
Lead
Larvae 0.66 0.09 0.67 0.07 0.99 0.10
Larval exuviae 1.9 0.11 2.56 0.01 2.21 0.56
Prepupae 0.25 0.12 0.74 0.03 0.28 0.03
Adults n/a – 0.17 0.01 0.12 0.01
Zinc
Larvae 1.14 0.09 0.97 0.14 0.42 0.07
Larval exuviae 1.92 0.19 2.45 0.32 0.92 0.04
Prepupae 0.97 0.20 0.84 0.09 0.39 0.01
Adults 0.98 0.08 0.45 0.04 0.19 0.02
1 SE = standard error; n/a = not applicable.
Table 7. Prepupal dry weight of Hermetia illucens fed with chicken feed (100 mg/larva/day, 60% moisture) spiked with three different
heavy metals at different concentrations (low, medium and high).1,2
Control Low Medium High
Mean (mg) SE Mean (mg) SE Mean (mg) SE Mean (mg) SE
Cadmium 55.9a 2.3 96.2c 10.0 75.3b 2.0 83.6bc 3.3
Lead 55.9ab 2.3 61.5b 6.8 51.3a 2.1 59.1ab 0.6
Zinc 55.9a 2.3 n/a – 64.8a 5.7 59.2a 4.8
1 Mean values followed by the same small letter in the same row do not vary significantly (P>0.05).
2 SE = standard error; n/a = not applicable.
Table 8. Effects of heavy metal concentration (low, medium and high) in food on development time (eclosion from egg to prepupa)
of Hermetia illucens larvae.1,2
Control Low Medium High
Mean (days) SE Mean (days) SE Mean (days) SE Mean (days) SE
Cadmium 18.4ab 0.5 18.0a 0.5 18.8ab 0.4 19.3b 0.6
Lead 18.4a 0.5 18.8ab 0.3 19.4b 0.4 20.7c 0.2
Zinc 18.4a 0.5 n/a – 18.9a 0.5 20.1b 0.6
1 Mean values followed by the same small letter in the same row do not vary significantly (P>0.05).
2 SE = standard error; n/a = not applicable.
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Bioaccumulation of heavy metals in black soldier fly
Journal of Insects as Food and Feed 1(4) 267
In contrast to cadmium, the BAF for zinc decreased with
increasing zinc concentration in the food, which suggests
active regulation of zinc within the body (Table 6). Similarly,
larvae of the house fly, fed with zinc-contaminated food
(from 61 to >7,000 mg/kg) accumulated zinc only up to a
maximum level of 216 mg/kg (Kramarz, 1999; Maryanski
et al., 2002). Even though the mean zinc concentration
in the literature data for Musca domestica is lower than
that found in prepupae of H. illucens of the present study
(484 mg/kg, SE=97.5), it is possible that the two organisms
possess a similar regulation mechanism.
Active regulation of zinc in insects has been described
previously (Lindqvist, 1995; Mason et al., 1983). Zinc is
an essential, yet potentially toxic element. Therefore, it
is not surprising that its intracellular uptake is actively
regulated. Especially the metal-responsive-element-binding
transcription factor-1 (MTF-1) is a key regulator in higher
4
3
2
1
00.1 1 10 100 1000
Bioaccumulation factor (BAF)
Cadmium
0 100 200 300 400 500
200
150
100
50
0
500
400
300
200
100
0
0 50 100 150 200
Cadmium
Concentration sample (mg/kg)
Concentration sample (mg/kg)
4
3
2
1
0
110 100 1000
Bioaccumulation factor (BAF)
LeadLead
0 500 1000 1,500 2,000
2,000
1,500
1000
500
0
Concentration food (mg/kg)
Zinc
Concentration sample (mg/kg)
4
3
2
1
0
10 100 1000
Concentration food (mg/kg)
Bioaccumulation factor (BAF)
Zinc
Figure 1. Concentration and bioaccumulation factor (BAF) of heavy metals in black soldier fly larvae (○) and prepupae (●), which
were fed with heavy metal spiked food (current study). Crosses show data from literature which originates from similar studies about
heavy metal concentrations in various insect larvae (Cd: Kazimirova and Ortel, 2000; Kramarz, 1999; Lindqvist, 1992; Maryanski
et al., 2002; Ortel, 1995; Pb: Gintenreiter et al., 1993; Kazimirova and Ortel, 2000; Ortel, 1995; Zn: Kramarz, 1999; Maryanski et al.,
2002). 1:1 line shown for reference. Missing BAF values are attributed to undetectable concentrations in control groups.
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S. Diener et al.
268 Journal of Insects as Food and Feed 1(4)
eukaryotic cells. It is responsible for the activation of several
genes involved in intracellular zinc sequestration and
transport (Laity and Andrews, 2007).
Though larvae and prepupae contained low lead
concentrations in the present study, the larval exuviae
accumulated lead. Lead tends to be stored in granular,
metal-containing structures of the cells before it is
transported to and immobilised in the exoskeleton (Hare,
1992). Similar to terrestrial insects, lead is most likely
disposed of during moulting (Roberts and Johnson, 1978).
Heavy metal concentration in adults was significantly lower
than in prepupae. We assume that this phenomenon occurs
mainly because animals defecate before pupation or shortly
after adult emergence. Yet Sheppard et al. (1994) reported
without supporting data that prepupae had an empty gut
when migrating. Conversely, Aoki and Suzuki (1984)
describe an over 50% loss of the larva’s cadmium content
due to defecation in newly emerged flesh flies within the
first two days following emergence. In the present study,
prepupae were collected 1-3 days after transformation. We
assume that defecation had not occurred during this period,
and cadmium was removed during the later prepupal phase,
i.e. during pupation, or after emergence. Therefore, toxic
substances and pathogens present in the waste may remain
in the gut of the harvested prepupae and in this way may be
taken up by fishes or poultry fed with the prepupae. Future
studies have to determine the period between initiation of
the last larval instar (prepupa) and defecation, including the
potential loss of feedstuff energy due to such a protraction.
The effective elimination of heavy metals by defecation
has been described for larvae of the social paper wasp
Polistes dominulus (Hymenoptera: Vespidae) (Urbini et
al., 2006). However, even if heavy metals accumulate in the
cells lining the alimentary canal, they may be rejected after
a short time. For example, Tenebrio molitor (Coleoptera:
Tenebrionidae) discards cells of the midgut epithelium
after four days (Lindqvist and Block, 1995; Thomas and
Gouranto, 1973). Heavy metal accumulated in these cells
will therefore be rejected with defecation.
5. Conclusions
Our studies reveal that the concentrations of lead and zinc
in larvae or prepupae remain below the initial amounts in
the food. Furthermore, the three heavy metal elements
examined had only minor effects on the development of
the black soldier fly even at very high concentrations. Yet,
since cadmium accumulated in the prepupae, it could
potentially limit the use of prepupae in the production of
animal feed. In the case of lead and zinc, concerns about
the use of prepupae in animal feed are less critical. The
waste treatment technology using black soldier flies may
contribute to reducing the burden of an animal protein
shortage in the animal feed market and provide new income
opportunities for small entrepreneurs in low and middle-
income countries.
Acknowledgements
We wish to express our thanks to Simone Blaser and
Mauro Esposito for assisting during the experiments,
David Kistler for supporting us in lab analysis and Alejandra
Teresita Arroyo for her statistical advice. We appreciate
the comments by Janet Hering on an early version of the
manuscript, and are grateful to Sylvie Peter for linguistic
editing support. The authors would like to acknowledge the
financial support by the Velux Foundation, Eawag: the Swiss
Federal Institute of Aquatic Science and Technology, the
Swiss National Centre of Competence in Research (NCCR
North South) and the Swiss Agency for Development and
Cooperation (SDC).
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