International Journal of Tropical Insect Science Vol. 33, No. 1, pp. 71–81, 2013
q icipe 2013
doi:10.1017/S1742758412000458
Post-harvest insect infestation in maize
grain stored in woven polypropylene and
in hermetic bags
Kukom Edoh Ognakossan1,2, Agbéko Kodjo Tounou1,
Yendouban Lamboni2 and Kerstin Hell2*
1
Ecole Supérieure d’Agronomie, Université de Lomé, BP 1515, Lomé,
Togo; 2International Institute of Tropical Agriculture (IITA), BP 08-0932
Tri Postal Cotonou, Republic of Benin
(Accepted 24 November 2012)
Abstract. Maize was artificially infested with either 10 or 25 individual Prostephanus
truncatus (Horn) and Sitophilus zeamais (Motschulsky) or a mixture of both, and stored in a
hermetic grain bag (HGB) or a woven polypropylene bag (WPB) for 150 days. Population
growth of P. truncatus and S. zeamais during storage was low in HGB, while in WPB, the
insect population increased significantly with storage duration. Mortality rate during
storage was significantly higher in HGB than in WPB. After 60 days of storage, the average
mortality rate of 99.50% was observed in HGB infested with 25 P. truncatus, and 100% for
S. zeamais at the same infestation density after 90 days of storage. Grain losses were
significantly lower in HGB compared with WPB. Less than 0.5 and 6.0% losses were
obtained, respectively, for S. zeamais and P. truncatus in HGB infested with 25 individual
insects after 150 days of storage, whereas losses of 19.2% (infestation with S. zeamais) and
27.1% (infestation with P. truncatus) were observed in WPB. HGB seems to be resistant to
the perforation of S. zeamais, but not to P. truncatus. The moisture content of maize grains
stored in HGB remained practically the same during storage, compared with the levels in
WPB, which reduced with storage time. WPB could be used for maize storage, protecting
it against insect infestation without the need for insecticide use.
Key words: hermetic grain bags, hermetic storage, population dynamics, grain losses,
mortality rate, S. zeamais, P. truncatus
Introduction
Maize (Zea mays L. [Poaceae]) is the most
important cereal food crop in sub-Saharan Africa
(SSA) and an important source of energy and
protein (Zia-Ur-Rehman, 2006). Yields are low in
SSA, and there have been no significant increases
over the last 50 years (PACN, 2010). Maize yields
are being affected by multiple abiotic and biotic
stresses, such as low soil fertility, water stress,
weeds, diseases and insects. In some cases, losses
due to pests and diseases, both pre- and post*E-mail: k.hell@cgiar.org
harvest, far outweigh any reasonable hope for
increases in productivity through improved germplasm and pre-harvest management (Chabi-Olaye
et al., 2005). The most damaging maize pests in SSA
are lepidopterous stem and cob borers, post-harvest
weevils and the larger grain borer. Losses
as high as 36% have been recorded on stored
maize in Benin, due principally to infestation
with Prostephanus truncatus (Horn) (Coleoptera:
Bostrichidae) (Lamboni and Hell, 2009). These
losses have an impact on food security and income
generation and contribute to high food prices by
removing part of the supply from the market
(Abebe et al., 2009; Yuya et al., 2009).
72
K. Edoh Ognakossan et al.
In recent years, maize production and storage
systems have undergone several changes. Higher
yielding, improved varieties were introduced, but
these varieties also proved to be more susceptible to
post-harvest insect pests, when stored in traditional
stores used by West African farmers (Meikle et al.,
1998). Extension agents and private sector pesticide
input dealers have promoted the use of storage
insecticides, but often these inputs are not available
at the right time, leaving farmers no option than to
treat with pesticides that are available, but not
recommended for use on food crops (Meikle et al.,
1999). Likewise, access to insecticides in SSA is not
always simple and its use has an impact on the
environment and human health (Obeng-Ofori and
Reichmuth, 1999). Biological control of P. truncatus
based on the use of Teretrius nigrescens Lewis
(Coleoptera: Histeridae), the most effective natural
antagonist of this pest, has been used in Africa as
well as alternative control methods (Golob, 2002).
However, the conclusions of the various studies
(pheromone trapping, field experiments, gut analysis grain store surveys, simulation modelling,
statistical and economic analysis) carried out by
different research groups to measure the effect of
the natural enemy on P. truncatus have been largely
equivocal (Meikle et al., 2002). Therefore, the use of
complementary alternative control methods, which
are economically viable, safe to humans and the
environment and can minimize qualitative and
quantitative maize grain losses in storage, is
needed. Hermetic grain bag (HGB) storage technology seems to have a great potential. The bags
have a low air permeability, so that after 1 week to
10 days at room temperature, oxygen levels are
modified by the respiration and metabolism
of insects, fungi and the grain itself to a level
of 1 – 2% O2 while the CO2 levels rise substantially, causing the death of insects and
microorganisms by asphyxiation (Varnava et al.,
1995; Murdock et al., 2012).
Based on this principle, simple hermetic storage
technologies have been developed, and include
triple-layer bag storage (Purdue Improved Cowpea
Storage (PICS) bags) mainly used for cowpea
storage (Murdock et al., 2003) and the International
Institute of Tropical Agriculture (IRRI) Super bag
used to control post-harvest problems on rice in
Asia (Rickman and Aquino, 2007). Because of its
effectiveness and ease of use, small-scale farmers
and other organizations have quickly adopted the
PICS technology (Baributsa et al., 2010). This
technology offers three advantages: (i) reducing
direct losses from attack by cowpea weevils, (ii)
farmers are not obliged to sell at harvest when
prices are generally low and can sell later at a much
higher price and (iii) farmers avoid price discount
for cowpea grain damaged by weevils (NGI, 2009).
Studies carried out with the PICS technology for the
control of stored pests on rice in Asia have shown
its effectiveness (Ben et al., 2006; Rickman and
Aquino, 2007). In Vietnam, no change in moisture
content was observed on rice grains stored in the
IRRI Super bag and the number of live insects was
reduced to 1 per kg, while in traditional storage,
living insect levels increased to an average of 53 per
kg (Ben et al., 2006). This simple hermetic
technology might be attractive for transfer to Africa,
but the effect of this technology on the main stored
maize insect pests Sitophilus zeamais (Motschulsky)
(Coleoptera: Curculionidae) and P. truncatus needs
to be tested prior to its use. Prostephanus truncatus
is a wood-boring insect, which has been known
to bore through plastics (Nansen and Meikle, 2002),
so testing whether hermetic storage is effective
in controlling this pest is especially pertinent.
Materials and methods
Maize and moisture content
Maize grains of the downy mildew-resistant
improved variety were sorted to eliminate
damaged grains and impurities, and then kept at
2 18.58C for 2 weeks to destroy any insect
infestation before bagging. Moisture content was
determined initially and at each sampling date
according to the ISO 712:1979 routine method. From
20 samples of 50 g each, three separate subsamples
of about 10 g of maize grains were drawn, milled
and transferred into metal containers, and weighed
and dried in an oven for 2 h 15 min at 1308C and
then reweighed. Moisture content was calculated
by the following formula:
Wi 2 Wd
£ 100;
mc ð%Þ ¼
Wi
where mc is the moisture content; Wi, the initial
weight; and Wd, the dry weight.
S. zeamais and P. truncatus rearing
Adults of S. zeamais and P. truncatus were
obtained from a laboratory culture. Fifty unsexed
adults of each species were reared on 250 g of maize
in 1-litre glass jars closed with a lid covered with a
fine wire mesh (150 mm) at 27.26 ^ 0.68 8C and
80.66 ^ 1.32% relative humidity and a day length of
12:12 h. After 2 weeks of incubation, the adult
insects were removed from jars by sieving. At the
end of 5 weeks, the newly emerged generation was
used in the experiments.
Storage bags
A HGB (27 cm long £ 20 cm wide) and a woven
polypropylene bag (WPB) of about 25 mm in
Insect population dynamics in hermetic storage
thickness, commonly used by farmers and merchants in Benin, were used for maize storage. The
WPB was cut the same size as the HGB, but the
edges were sewn to close.
73
were determined by the count and weight method
described by Boxall (1986):
weight loss ð%Þ ¼
Storage, sampling and data collection
ðW u £ N d Þ 2 ðW d £ N u Þ
£ 100;
W u ðN d þ N u Þ
where Wu is the weight of undamaged grains; Nu,
the number of undamaged grains; Wd, the weight
of damaged grains; and Nd, the number of
damaged grains.
Maize storage started on 16 December 2008 and
ended 21 May 2009; and the trial overlapped two
seasons in Benin: the long dry season (November to
mid-April) and the long rainy season (mid-April to
mid-July). The experiment was set up in a split-plot
design with three replications per experimental
unit. The whole-plot factor was the storage
technology (HGB and WPB) and subplot factors
were as follows: infestation level (10 and 25), type of
insects (P. truncatus, S. zeamais and mixture
(P. truncatus þ S. zeamais)) and storage duration
(60, 90, 120 and 150 days). Each bag was filled with
1 kg of grain and artificially infested with either 10
or 25 sexed S. zeamais or P. truncatus adults or with a
mixture of S. zeamais þ P. truncatus with the two
density levels. After filling, the bags were compressed to remove excess air and tightly sealed. All
bags were stored in a room at ambient temperature
and relative humidity, and monitored with destructive sampling at 60, 90, 120 and 150 days. Weekly
mean temperatures and relative humidity were
recorded. At each sampling, bags were opened,
grains sieved and dead and living P. truncatus or
S. zeamais adults counted. After this, 30 g of grains
were drawn for moisture content determination
according to the ISO (1980) method described
above. Number and weight of damaged and
undamaged grains on the basis of three random
samples of 1000 grains and the number of holes on
the bags were also recorded. Grain weight losses
Data analysis
SPSS for Windows version 16.0 (SPSS Inc.,
Chicago, IL) was used for statistical analysis using
the general linear model (SPSS, 2007). The number
of insects and holes was log(x þ l)-transformed.
Mortality rate, losses and moisture content were
arcsine square root (x/100)-transformed to normalize the data. Analysis of variance was used to
compare means between the two storage technologies. The Student – Newman – Keuls test was used
to separate averages at 5%. Correlations were
computed for the HGB technology to establish the
interactions between the number of holes and insect
density, mortality rate and losses.
Results
Temperature and relative humidity during storage
The weekly mean temperatures and relative
humidity in the storage room during the trial were
28.90 ^ 0.69 8C and 79.44 ^ 3.63% (Fig. 1). The
highest temperatures were recorded in March/
April and the highest relative humidity at the end of
the trial in May.
87
Relative humidity (%)
Mean relative humidity (%)
85
Temperature (˚C)
Mean temperature (˚C)
Mid-April
32
31
81
79
30
77
Long rainy season
75
29
73
Long dry season
71
28
69
First sampling
67
65
Temperature (°C)
Relative humidity (%)
83
0
1
2
3
4
5
6
7
8
9
10
Second sampling
11
12
13
14
Fourth sampling
Third sampling
15
16
17
18
19
20
21
22
27
Weeks
Fig. 1. Weekly average relative humidity and temperature in storage during the trial (16 December 2008 to 21 May 2009)
K. Edoh Ognakossan et al.
74
C
13
Moisture content (%)
Moisture content (%)
A
12
11
10
9
13
12
11
10
9
0
60
90
120
150
0
Storage duration (days)
Moisture content (%)
B
60
90
120
150
Storage duration (days)
HGB (10 S. zeamais)
WPB (10 S. zeamais)
HGB (mixture (10 S. zeamais + 10 P. truncatus))
HGB (10 P. truncatus)
WPB (10 P. truncatus)
WPB (mixture (10 S. zeamais + 10 P. truncatus))
HGB (Mixture (25 S. zeamais + 25 P. truncatus))
13
WPB (Mixture (25 S. zeamais + 25 P. truncatus))
12
11
10
9
0
60
90
120
150
Storage duration (days)
HGB (25 S. zeamais)
WPB (25 S. zeamais)
HGB (25 P. truncatus)
WPB (25 P. truncatus)
Fig. 2. Evolution of moisture content in hermetic grain bag (HGB) and in woven polypropylene bags (WPB) during
storage
Grain moisture content
Moisture content in HGB showed the same trend
irrespective of the level of infestation and species of
insect. The average initial moisture content was
12.05 ^ 0.16%.
In bags infested with S. zeamais, significant
differences were obtained between maize stored in
HGB and WPB after 120 days when infested with 10
S. zeamais and after 90 days in maize infested with
25 S. zeamais (Fig. 2A and B). In HGB, moisture
Table 1. Number (mean ^ SE, n ¼ 3) of holes caused by Sitophilus zeamais, Prostephanus truncatus and mixture of insects
(S. zeamais þ P. truncatus) on hermetic grain bag (HGB) and woven polypropylene bag (WPB)
S. zeamais
Days after storage
Infestation with 10 insects
60
90
120
150
Infestation with 25 insects
60
90
120
150
HGB
P. truncatus
WPB
Mixture
(S. zeamais þ P. truncatus)
HGB
WPB
HGB
WPB
0.00 ^ 0.00Aa 1.00 ^ 0.00Ab
0.00 ^ 0.00Aa 5.33 ^ 0.88Bb
0.00 ^ 0.00Aa 18.33 ^ 4.91Cb
0.00 ^ 0.00Aa 23.00 ^ 5.00Cb
2.33 ^ 1.20Aa
2.67 ^ 1.76Aa
5.50 ^ 1.50Aa
0.50 ^ 0.50Aa
8.67 ^ 3.28Aa
39.33 ^ 6.43Bb
94.67 ^ 8.66Cb
81.67 ^ 9.59Cb
0.00 ^ 0.00Aa 1.33 ^ 0.66Aa
0.00 ^ 0.00Aa 6.00 ^ 2.08ABb
0.00 ^ 0.00Aa 23.00 ^ 10.44Bb
0.00 ^ 0.00Aa 21.67 ^ 3.18Bb
0.33 ^ 0.33Aa 30.67 ^ 5.81Ab 0.00 ^ 0.00Aa 31.00 ^ 4.72Ab
3.33 ^ 1.76Aa 74.00 ^ 13.89Bb 15.00 ^ 2.88Ba 46.00 ^ 10.69Ab
9.33 ^ 2.02Aa 109.00 ^ 6.65Bb
3.00 ^ 3.00Aa 104.33 ^ 27.14Bb
4.00 ^ 3.05Aa 118.67 ^ 11.26Bb
–
–
0.00 ^ 0.00Aa
4.33 ^ 0.66Ab
3.67 ^ 1.76Aa 40.67 ^ 7.17Bb
2.33 ^ 2.33Aa 61.33 ^ 10.99Bb
0.00 ^ 0.00Aa 113.33 ^ 12.12Cb
Mean (^SE) values within a column (row) followed by the same uppercase letter (lowercase) are not significantly
different from each other at the 5% probability level.
Insect population dynamics in hermetic storage
HGB (S. zeamais)
WPB (S. zeamais)
HGB (P. truncatus)
WPB (P. truncatus)
B
Log (numbers/kg)
Log (numbers/kg)
A 4
3
2
4
75
HGB (S. zeamais)
WPB (S. zeamais)
HGB (P. truncatus)
WPB (P. truncatus)
3
2
1
1
0
60
90
120
150
0
60
Storage time (days)
90
120
150
Storage time (days)
Fig. 3. Population growth of Sitophilus zeamais and Prostephanus truncatus adults in HGB and WPB during storage (n ¼ 3).
A, infestation with 10 S. zeamais or 10 P. truncatus. B, infestation with 25 S. zeamais or 25 P. truncatus.
content did not vary significantly during the whole
storage period (P ¼ 0.350 (10 S. zeamais); P ¼ 0.408
(25 S. zeamais); Fig. 2A and B). In WPB, moisture
content was significantly different at 120 days of
storage in bags infested with 10 S. zeamais
(P ¼ 0.0001; Fig. 2A) and after 60 days of storage
in bags infested with 25 S. zeamais (P , 0.0001;
Fig. 2B).
In bags infested with P. truncatus (Fig. 2A and B),
moisture content differed between the HGB and
WPB after 60 days with 10 individuals and at all
sampling dates with 25 individuals. No significant
differences were observed for moisture content
in HGB (P ¼ 0.206 (10 P. truncatus); P ¼ 0.060 (25
P. truncatus)).
Under mixed infestations (Fig. 2C), significant
differences were observed between the HGB and
WPB after 60 days of storage in both levels of
infestation. In HGB, no significant differences
A
4
regarding moisture content were observed, while
in WPB, significant differences were observed at
60 days of storage in both the levels of infestation
(P , 0.0001).
Number of holes on storage bags
No holes were observed on HGB during the
whole experimental period, when only S. zeamais
was present. Significant differences were observed
for the number of holes on WPB compared with
HGB, except for the infestation level of 25 S. zeamais
at 60 days of storage (Table 1). The number of holes
increased significantly until 120 days of storage
in WPB under both infestation levels (P , 0.0001
(10 S. zeamais); P ¼ 0.006 (25 S. zeamais)).
When P. truncatus was present, the number of
holes was lower in HGB than in WPB (Table 1). For
10 P. truncatus, significant differences were
B 4
HGB (mixture (S. zeamais + P. truncatus))
HGB (mixture (S. zeamais + P. truncatus))
WPB (mixture (S. zeamais + P. truncatus))
Log (numbers/kg)
Log (numbers/kg)
WPB (mixture (S. zeamais + P. truncatus))
3
2
1
3
2
1
0
60
90
120
Storage time (days)
150
0
60
90
120
Storage time (days)
Fig. 4. Growth of the mixture of insects (Prostephanus truncatus þ Sitophilus zeamais) in HGB and WPB during storage
(n ¼ 3). A, infestation with 10 P. truncatus þ 10 S. zeamais. B, infestation with 25 P. truncatus þ 25 S. zeamais.
K. Edoh Ognakossan et al.
76
B
2.5
y = 0.4273x + 1.4475
R2 = 0.692
P = 0.001
2.0
1.5
y = 0.2329x + 1.2391
R2 = 0.4033
P = 0.049
1.0
0.5
y = 0.1909x + 1.7876
R2 = 0.7678
P = 0.004
2.5
Infestation with 10 P. truncatus
Infestation with 25 P. truncatus
Log (density of mixture+1)
Log (density of P. truncatus+1)
A
2.0
1.5
y = 0.2693x + 1.6428
R2 = 0.3529
P = 0.054
1.0
0.5
Infestation with mixture (10 S. zeamais + 10 P. truncatus)
Infestation with mixture (25 S. zeamais + 25 P. truncatus)
0
0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0
0.5
Log (number of holes+1)
1.0
1.5
Log (number of holes+1)
Fig. 5. Relationship between insect numbers and the number of holes made in HGB. A, relationship between Prostephanus
truncatus numbers and the number of holes. B, relationship between mixture (P. truncatus þ Sitophilus zeamais) density and
the number of holes.
observed between the number of holes in HGB and
WPB after 60 days of storage. When bags were
infested with 25 P. truncates, significant differences
between the HGB and WPB were observed during
the whole storage period. No significant differences
were obtained between the different sampling dates
for the number of holes observed in HGB (P ¼ 0.387
(10 P. truncatus); P ¼ 0.092 (25 P. truncatus)). The
number of holes increased significantly in WPB up
to 120 days of storage when infested with 10 P.
truncatus (P , 0.0001), but with 25 P. truncatus, a
significantly lower number of holes were only
observed up to 60 days of storage (P ¼ 0.001).
Under mixed infestation (S. zeamais þ P. truncatus), the number of holes recorded in HGB was
significantly lower than those recorded in WPB at
both levels of infestation (Table 1). In HGB infested
with 10 insects, no significant differences between
the sampling dates were obtained for the number of
holes (P ¼ 0.173) independent of insect species.
However, at higher levels of infestation, the number
of holes at 120 days was significantly higher than
the numbers recorded at 90 and 60 days (P ¼ 0.009).
In WPB, the number of holes increased significantly
with storage duration with 10 S. zeamais þ 10
P. truncatus (P , 0.0001) and at higher infestation
levels (P ¼ 0.033).
Population growth of S. zeamais and P. truncatus in
HGB
The population growth of S. zeamais in HGB and
WPB is presented in Fig. 3A and B. The number of
S. zeamais recorded was higher in WPB than in HGB
with both infestation levels. The Sitophilus zeamais
population in WPB increased significantly with
storage time (P , 0.0001), while no significant
difference was observed in HGB.
Table 2. Per cent mortality (mean ^ SE, n ¼ 3) of Sitophilus zeamais, Prostephanus truncatus and mixture of insects
(S. zeamais þ P. truncatus) stored in hermetic grain bag (HGB)
S. zeamais
Days after storage
HGB
Mixture
(S. zeamais þ P. truncatus)
P. truncatus
WPB
HGB
Infestation with 10 insects
60
96.30 ^ 3.70Aa 5.28 ^ 0.19Ab 83.45 ^ 8.31Aa
90
98.04 ^ 1.96Aa 7.94 ^ 2.98Ab 92.70 ^ 7.29Aa
120
100 ^ 0.00Aa 10.38 ^ 1.24Ab 85.06 ^ 5.24Aa
150
100 ^ 0.00Aa 10.64 ^ 3.05Ab 100 ^ 0.00Aa
Infestation with 25 insects
60
92.48 ^ 6.02Aa 7.59 ^ 0.84Ab 99.52 ^ 0.47Aa
90
100 ^ 0.00Aa 10.93 ^ 0.13Bb 87.97 ^ 6.03Aa
120
100 ^ 0.00Aa 14.01 ^ 0.99Cb 73.04 ^ 16.33Aa
150
100 ^ 0.00Aa 17.74 ^ 1.16Db 82.92 ^ 14.38Aa
WPB
HGB
WPB
7.74 ^ 1.14Ab 92.05 ^ 1.45Aa 7.90 ^ 1.98Ab
10.80 ^ 2.38ABb 87.80 ^ 6.76Aa 9.06 ^ 2.04Ab
24.29 ^ 5.76Bb 92.63 ^ 7.36Aa 19.82 ^ 1.86Bb
25.51 ^ 5.43Bb
100 ^ 0.00Aa 24.13 ^ 2.15Bb
7.77 ^ 0.66Ab
10.51 ^ 1.47Ab
19.41 ^ 1.82Bb
21.83 ^ 1.57Bb
92.75 ^ 4.46Aa 9.70 ^ 1.42Ab
89.00 ^ 2.54Aa 16.32 ^ 1.26Bb
90.81 ^ 9.18Aa 22.27 ^ 2.00Bb
—
—
Mean (^SE) values within a column (row) followed by the same uppercase letter (lowercase) are not significantly
different from each other at the 5% probability level.
Insect population dynamics in hermetic storage
B
1.8
1.6
y = –0.5796x + 1.5942
R2 = 0.7214
P = 0.0001
1.4
1.2
1.0
y = –0.6002x + 1.6113
R2 = 0.7275
P = 0.002
0.8
0.6
0.4
Infestation with 10 P. truncatus
0.2
Infestation with 25 P. truncatus
Arcsin (√(mortality rate/100))
Arcsin (√(mortality rate/100))
A
1.8
77
y = –0.1518x + 1.3916
R2 = 0.2749
P = 0.182
1.6
1.4
1.2
1.0 y = –0.4064x + 1.4695
R2 = 0.5424
0.8
P = 0.010
0.6
0.4
Infestation with mixture (10 S. zeamais + 10 P. truncatus)
Infestation with mixture (25 S. zeamais + 25 P. truncatus)
0.2
0
0
0
0.2
0.4
0.6
0.8
1.0
1.2
0.5
0
1.4
1.0
1.5
Log (number of holes+1)
Log (number of holes+1)
Fig. 6. Relationship between the mortality of insects and the number of holes made in HGB. A, relationship between
Prostephanus truncatus mortality and the number of holes. B, relationship between the mortality of the mixture of insects (P.
truncatus þ Sitophilus zeamais) and the number of holes.
Significant differences between the HGB
and WPB for P. truncatus were observed at 90, 120
and 150 days, but not at 60 days of storage
(P ¼ 0.835 (10 insects); P ¼ 0.596 (25 insects); Fig. 3A
and B). The number of P. truncatus was lower in
HGB than in WPB and increased in WPB with
storage duration. In HGB, significant differences
were only observed at 120 days of storage
(P ¼ 0.028 (10 insects); P , 0.0001 (25 insects)) for
the two levels of infestation.
In the mixture of S. zeamais þ P. truncatus, the
total number of both insects was significantly
higher in WPB compared with HGB at both levels
of infestation (Fig. 4A and B). In HGB, infested with
10 S. zeamais þ 10 P. truncatus, the total number of
insects at 60 days of storage was significantly lower
than the levels on the other sampling dates
(P ¼ 0.009). In maize infested with 25 S. zeamais þ 25
P. truncatus and stored in HGB, significant
differences for the number of insects were only
observed between 60 and 90 days (P ¼ 0.041). In
WPB, the total number of insects increased
significantly with storage duration for the two
levels of infestation (P , 0.0001).
The number of holes increased with the
number of P. truncatus in HGB (r ¼ 0.64, P ¼ 0.049
(10 P. truncatus); r ¼ 0.83, P ¼ 0.001 (25 P. truncatus);
Fig. 5A). In HGB infested with 10 S. zeamais þ 10 P.
truncatus, the number of holes did not increase with
the number of insects (r ¼ 0.594, P ¼ 0.054), while
under higher infestation levels, the number of holes
increased with the number of insects (r ¼ 0.876,
P ¼ 0.004) (Fig. 5B).
Effect of HGB on the mortality rate of S. zeamais and P.
truncatus
The mortality rate of S. zeamais was significantly
higher in HGB than in WPB during the whole
storage period under both levels of infestation
Table 3. Percentage of grain losses (mean ^ SE, n ¼ 3) in hermetic grain bag (HGB) caused by Sitophilus zeamais,
Prostephanus truncatus and mixture of insects (S. zeamais þ P. truncatus)
S. zeamais
Days after storage
Infestation with 10 insects
60
90
120
150
Infestation with 25 insects
60
90
120
150
HGB
P. truncatus
WPB
HGB
WPB
Mixture
(S. zeamais þ P. truncatus)
HGB
WPB
0.07 ^ 0.03Aa 1.19 ^ 0.37Ab
0.45 ^ 0.37Aa 3.55 ^ 0.29Bb
0.13 ^ 0.03Aa 8.83 ^ 1.05Cb
0.12 ^ 0.02Aa 16.88 ^ 1.32Db
0.46 ^ 0.19Aa 2.18 ^ 0.51Ab
0.44 ^ 0.10Aa 6.57 ^ 2.56ABb
2.46 ^ 0.47Ba 11.41 ^ 0.71BCb
0.47 ^ 0.08Aa 19.22 ^ 4.77Cb
0.81 ^ 0.08Aa 6.48 ^ 0.67Ab
1.60 ^ 0.52Aa 17.16 ^ 2.87Bb
2.12 ^ 0.54Aa 21.10 ^ 2.53Bb
1.10 ^ 0.11Aa 29.93 ^ 2.02Cb
0.14 ^ 0.01Aa 2.04 ^ 0.13Ab
0.34 ^ 0.06Aa 4.74 ^ 1.53Bb
0.38 ^ 0.12Aa 12.70 ^ 1.07Cb
0.45 ^ 0.09Aa 19.21 ^ 0.75Db
0.62 ^ 0.19Aa 5.38 ^ 1.34Ab
1.77 ^ 0.73Aa 13.53 ^ 1.35Bb
5.48 ^ 2.16Aa 18.90 ^ 1.09Bb
2.02 ^ 1.16Aa 27.06 ^ 2.76Cb
0.84 ^ 0.23Aa 8.40 ^ 1.48Ab
5.15 ^ 1.65Ba 21.64 ^ 1.20Bb
1.69 ^ 0.22Aa 30.04 ^ 3.23Cb
—
—
Mean (^SE) values within a column (row) followed by the same uppercase letter (lowercase) are not significantly
different from each other at the 5% probability level.
K. Edoh Ognakossan et al.
78
(Table 2). In HGB, no living S. zeamais was recorded
from 120 days of storage with 10 S. zeamais and from
90 days of storage with 25 S. zeamais. In WPB
infested with 10 S. zeamais, no significant differences existed for mortalities (P ¼ 0.254) but with 25
S. zeamais, mortality rate increased significantly
with storage time (P , 0.0001) to reach 17.74 ^ 1.16
at 150 days.
The mortality rate of P. truncatus was significantly
higher in HGB than in WPB (Table 2). The mortality
rates of P. truncatus in HGB did not differ
significantly with the level of infestation (P ¼ 0.382
(10 P. truncatus); P ¼ 0.460 (25 P. truncatus)). In WPB,
the mortality rate increased significantly with
storage time (P ¼ 0.022 (10 P. truncatus); P , 0.0001
(25 P. truncatus)); however, this mortality rate did not
reach one-third of the mortality rates detected in
HGB infested with 10 P. truncatus and a fourth of the
rates in HGB infested with 25 P. truncatus.
When infested with S. zeamais þ P. truncatus, the
mortality rate was significantly higher in HGB
than in the control under both levels of infestation
(Table 2). In HGB, the mortality rate remained
practically the same during the whole storage
period for both levels of infestation (P ¼ 0.443
(10 S. zeamais þ 10 P. truncatus); P ¼ 0.740
(25 S. zeamais þ 25 P. truncatus)), while it increased
in WPB (P ¼ 0.002 (10 S. zeamais þ 10 P. truncatus);
P ¼ 0.004 (25 S. zeamais þ 25 P. truncatus)).
Insect mortality in HGB infested with P. truncatus
decreased when the number of holes increased
(r ¼ 2 0.85, P ¼ 0.002 (10 P. truncatus); r ¼ 2 0.84,
P , 0.0001 (25 P. truncatus); Fig. 6A). The same was
obtained with 10 S. zeamais þ 10 P. truncatus
(r ¼ 2 0.73, P ¼ 0.010), whereas with 25
S. zeamais þ 25 P. truncatus, the number of holes
did not significantly influence mortality (r ¼ 2 0.52,
P ¼ 0.182) (Fig. 6B).
Arcsin √(losses/100)
B
Infestation with 10 P. truncatus
Infestation with 25 P. truncatus
0.35
0.25
0.20
0.15
y = 0.0787x + 0.0482
R2 = 0.4418
P = 0.036
0.10
0.05
Infestation with mixture (10 S. zeamais + 10 P. truncatus)
Infestation with mixture (25 S. zeamais + 25 P. truncatus)
0.30
y = 0.1528x + 0.0589
R2 = 0.7817
P < 0.0001
0.30
Losses lower than 0.5% were observed in HGB
during the whole storage period, compared with
WPB, where losses of 16.88 ^ 1.32 and
19.21 ^ 0.75% were observed when infested with
10 S. zeamais and 25 S. zeamais, respectively (Table 3).
Losses were significantly higher in WPB than in
HGB and remained practically the same in HGB
during the whole period of storage (P ¼ 0.610
(10 S. zeamais); P ¼ 0.105 (25 S. zeamais)). In WPB,
losses increased significantly with storage duration
under both levels of infestation (P , 0.0001).
In bags infested with P. truncatus, losses were
significantly higher in WPB than in HGB (Table 3).
At 90 days, losses in WPB were practically three
times higher than in HGB. Losses in HGB infested
with 25 P. truncatus did not differ significantly
during the whole storage period, whereas in
bags infested with 10 P. truncatus, losses were
significantly higher at 120 days of storage. In WPB,
losses increased with storage time (P ¼ 0.004
(10 P. truncatus); P , 0.0001 (25 P. truncatus)).
In bags infested with a mixture of S. zeamais and
P. truncatus, losses in WPB at 60 days of storage
were practically double that of the highest losses
recorded in HGB (Table 3). In HGB infested with 10
S. zeamais þ 10 P. truncatus, no significant difference
was observed during the whole storage period
(P ¼ 0.152), but at higher infestation levels, a
significant difference was observed at 90 days of
storage (P ¼ 0.044). Losses in WPB increased
significantly with storage duration.
Grain losses in HGB infested with P. truncatus
increased with the number of holes in the bags
(r ¼ 0.66, P ¼ 0.036 (10 P. truncatus); r ¼ 0.88,
P , 0.0001 (25 P. truncatus); Fig. 7A). This was
0
Arcsin √(losses/100)
A
Effect of HGB on grain losses caused by S. zeamais and
P. truncatus
y = 0.1055x + 0.0914
R2 = 0.7418
P = 0.006
0.25
0.20
0.15
0.10
y = 0.0631x + 0.1013
R2 = 0.6063
P < 0.0001
0.05
0
0
0.5
1.0
Log (number of holes+1)
1.5
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Log (number of holes+1)
Fig. 7. Relationship between maize losses and the number of holes made in HGB. A, relationship between losses caused
by Prostephanus truncatus and the number of holes. B, relationship between losses caused by mixture of insects
(P. truncatus þ Sitophilus zeamais) and the number of holes.
Insect population dynamics in hermetic storage
also observed under both levels of mixed
infestation (r ¼ 0.77, P ¼ 0.010 (10 S. zeamais þ 10
P. truncatus); r ¼ 0.86, P ¼ 0.006 (25 S. zeamais þ 25
P. truncatus); Fig. 7B).
Discussion
This study demonstrated a major reduction of
progeny number, grain weight loss and a high
mortality of S. zeamais or P. truncatus when maize
was stored in hermetic storage bags (HGB).
Temperature and relative humidity in storage
during our trial were close to those favourable
for optimum development of S. zeamais and
P. truncatus, but in HGB, low insect development
rates and high mortalities were observed compared with the normal farmer practice, e.g.
storage in WPB. These effects could be due to
the low oxygen permeability of HGB, and its
function as a hermetic storage system when
properly sealed (Rickman and Aquino, 2007). In
such an environment, oxygen levels drop sharply
while CO2 accumulates via the respiration of
pests, microorganisms and grains (Varnava et al.,
1995; Moreno et al., 2000).
High insect mortality rates were observed in
HGB compared with WPB. As with all aerobic
organisms, development and survival of insects is
strongly correlated with oxygen concentration, so
that in hermetic storage, all insect development
ceases (Donahaye and Navarro, 2000) and insects
perish if levels fall below 2 – 3% (Oxley and
Wickenden, 1963; Moreno et al., 2000). Bailey and
Banks (1980) stated that hermetic condition delayed
insect development, impaired metamorphosis and
altered fecundity.
The impact of insect pest destruction on stored
grain is commonly expressed in terms of the
percentage of grains damaged or the percentage of
grain weight loss (Boxall, 2002). However, crop
storage efficiency depends on storage duration and
losses during storage (including quality deterioration; Thamaga-Chitja et al., 2004). Losses observed
in the bags are linked to the infestation level and
mortality rate of insects in the bag. Storage of maize
in HGB minimized grain weight losses to less than
1% when infested only with S. zeamais and less than
6% for infestation with P. truncatus or the mixed
insect population. These levels were much lower
than those reported by Boxall (2002) who observed
losses of 5 –10% when maize was only infested with
Sitophilus spp., while losses in P. truncatus-damaged
maize ranged from 15 to 45%.
According to Compton et al. (1998), insect
damage has virtually no effect on price at levels
below 5– 6% damaged grains, but above this, the
percentage of damaged grains has a strong, quasilinear relationship with price. For every 1% increase
79
in grain damage, the expected mean price
decreased by between approximately 0.6 and 1%,
so that any increase in grain losses would directly
reduce the retail price. If maize can be kept nearly
insect-free through the use of HGB, such grains
would retail for the highest price.
Higher numbers of S. zeamais than P. truncatus
were observed during the whole storage period in
control bags; this could be due to S. zeamais
increasing more on maize grain, while P. truncatus
proliferating more easily on maize cobs than on
grains. Cowley et al. (1980) reported that P. truncatus
development in maize grains was much slower than
on cobs, but Sitophilus spp. had a higher development rate on maize grains than on cobs. In mixed
populations, the number of individual insects tends
to be much lower than the number of insects in
single-species populations, which could be due to
inter-specific competition limiting population size.
Several authors (Cowley et al., 1980; Giga and
Canhao, 1993; Meikle et al., 1998; Danho et al., 2000)
have reported that the presence of S. zeamais
negatively influences the development of P. truncatus. Traditional farmers’ storage ecosystems are
usually infested with numerous species, so that the
mixed infestation with S. zeamais and P. truncatus
illustrated more closely their challenges.
There was a strong correlation between the
number of holes observed on HGB and insect
numbers, mortality rate and losses, so that the
conclusion was drawn that holes negatively
affected the efficiency of HGB. Since P. truncatus is
a wood-boring insect, this species has a remarkable
ability to tunnel through all kinds of material
including plastics (Borgemeister et al., 2003). The
higher the number of holes, the higher the number
of insects, grain weight losses and the lower the
mortality rate. These results suggest that holes
created by perforation would favour gas exchange
and eventually HGB would no more be a hermetic
storage system. However, HGB offers a superior
resistance to insect damage than WPB, which is
probably related to its layer structure.
In this trial, it was observed that P. truncatus
makes more holes on the bags than S. zeamais,
allowing the larger grain borer to exit easily from
the bags. Sitophilus zeamais apparently did not make
any holes on HGB and a much lower number of
holes were observed on the control bag when
infested only with this species, than with
P. truncatus. Li (1988) reported that P. truncatus has
a remarkable ability to tunnel through hard
materials and was found to have penetrated plastic
of 35 mm in thickness. HGBs are only 0.078 mm
thick. The possibility of perforation of hermetic
storage plastics such as HGB by some insects such as
Rhyzopertha dominica (Coleoptera: Bostrichidae) has
been reported by De Dios et al. (2001) and Rickman
K. Edoh Ognakossan et al.
80
and Aquino (2007), while Baoua et al. (2012) stated
that Callosobruchus maculatus Fabricius (Coleoptera:
Chrysomelidae) was able to perforate HGB used for
cowpea storage in Niger. Most of the holes were
located near the seams of the plastic (De Dios et al.,
2001). Also, intermittent opening and closing for
sampling had led to reinfestation, and the insects
were then able to pierce the polyvinyl chloride
(PVC) liner, causing the system to fail (Rickman and
Aquino, 2007).
When maize was stored in HGB, moisture
content remained practically the same during the
whole storage period, inferring that exchanges
between HGB and the external environment were
limited since the bags were hermetic. However, this
also means that no further drying is possible within
this structure, so that grains have to be well dried
prior to storage. On the contrary, grain moisture in
polypropylene bags followed the evolution of the
ambient relative humidity due to the permeability
of these bags. This confirms that in hermetic bags,
initial moisture content remains largely unchanged
during storage.
Conclusion
Minimizing post-harvest losses and extending
shelf-life through the use of improved storage
structures are not only very effective ways of
improving farm income through crop storage and
sale at premium prices when demand outstrips
supply later in the post-harvest period, but also
offer the opportunity to smooth hunger between
staple crop harvests. The results obtained in this
study show that HGB potentially promises to be
one of the key technologies for effective, safe postharvest management of maize grains, and improving food security of small-scale farmers. The
tested technology does seem to be very promising,
especially as it was done using small-sized bags
that have a much bigger surface area-to-volume
ratio, making it more difficult to attain hermetic
conditions. Bigger bags would be much more
effective. We observed holes bored by P. truncatus
on the bag; however, this did not affect the
hermetic storage principle of the bags when maize
was stored for 150 days. Further studies are
needed (1) to assess the potential of P. truncatus
for entering bags and (2) for the reuse potential of
bags, e.g. for how many seasons and also the
economics.
Acknowledgements
We thank Joe Rickman from IRRI Mozambique
for supplying us with the IRRI Super bags for this
trial.
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