In West Africa, rice is a highly strategic cereal due to its importance as
food for the households and for domestic economy. In Benin, following on good
rice production levels recorded after improved, high-yield, short cycle varieties
were introduced and adopted, grain storage has gradually become an ingrained
habit for the population. Therefore, the past decade has seen increasing amounts
of rice stored to meet the populations subsistence needs, as well as for
marketing and use as seed. As a result, although for a long time in Benin not
very much was known about storage damage caused by insect pests, it began to
pose a serious threat to reserves stored by producers, processors and traders.
Nowadays, insect pest infestation has been observed in many on-farm paddy or
hulled rice storage units. Sightings have been recorded of the rice weevil Sitophilus
oryzae Linnaeus (Coleoptera: Curculionidae), the maize weevil Sitophilus
zeamais Motschulsky (Coleoptera: Curculionidae) and the lesser grain borer
Rhyzopertha dominica Fabricius (Coleoptera: Bostrichidae) on producer
stocks in the country. Similarly, Togola et al.
(2010) recorded infestations by the Angoumois grain moth Sitotroga cerealella
Olivier (Lepidoptera: Gelechiidae) in many rice-producing zones in the country,
causing an estimated 3-18% damage to grain, depending on the area and length
of storage. As a general rule, S. cerealella, S. oryzae, S.
zeamais, R. dominica and Prostephanus truncatus (Horn) (Coleoptera:
Bostrichidae) are the primary insect pest species that make up the core of the
parasitical complex that has a major economic impact on grain stocks worldwide
(Grenier et al., 1994; Hansen
et al., 2004). They are cosmopolitan pests (Cotton,
1960; CABI, 2005; Bamaiyi
et al., 2007; Plague et al., 2010; Arthur
et al., 2012), able to attack many types of cereal grain (Sedaghat
et al., 2011; Hamed and Sajid, 2012). Their
dispersal over large areas has come not just from changes to population feeding
habits caused by drought-making farmers abandon their dryland crops to grow
rice and maize (Meikle et al., 1998; Hansen
et al., 2004) but also from uncontrolled cross border movements of
grain produce fostered by regional or international trade (Youm
et al., 2011).
As in many African countries, studies in Benin focused on drawing up an inventory
of the entomofauna of stored grain (Ngamo and Hance, 2007;
Gueye et al., 2011) but, as far as rice is concerned,
very little research has focused on loss assessments, in general and even less
on economic loss, in particular. Rice production and storage is growing rapidly
and has become almost unavoidable for smallholders in the sector. It is therefore,
necessary to carry out a study on post-harvest loss evaluation on rice taking
into consideration stock predators, in general, with particular attention to
insects. This is all the more essential since any grain attacked by an insect
suffers not just losses in nutritional quality but also becomes unfit for marketing
and for use as seed.
Prior to any pest control operation, it is useful to assess the extent of quantitative
and economic losses that have occurred or are likely to affect the crop during
storage in order to better appreciate the need, the time and the place for an
intervention against target predators. This article aims to make such an assessment,
taking into account weight and economic loss incurred as result of insect infestation
on paddy rice stored on-farm in Benin. The study will permit an assessment of
the extent of loss in the southern, central and northern regions of the country
in order to seek solutions for improved stored rice protection and to safeguard
incomes for the various actors in the post-harvest sector.
MATERIALS AND METHODS
Site selection and sample collection: During 2011 and 2012 agricultural
seasons, a series of entomological prospections were carried out throughout
the territory of Benin. During these prospections, 65 farmer stocks were inspected
and samples taken from 28 of the prospected sites. Site selection was primarily
in major rice production zones from the South to the North of the country, including
the Center. Temperatures, relative humidity as well as the geographic coordinates
of the storage units were recorded.
Sampling methodology: In each site, 2 to 3 storage units were inspected,
taking care to avoid treated stock. Village-based resource persons (village
head, village guide or president of the rice growers association) helped to
identify the producers involved. For each farmers stock, where quantities
ranged from some ten kilos to several dozen tons of paddy, a series of random
samples were taken using the sampling technique recommended by the world food
program (Walker and Farrell, 2003) in order to obtain
an initial representative sample. For bagged rice (the most frequently encountered
mode of storage on the visited sites), a representative number of bags were
randomly sampled. As for bulk or grain in bins, or in earthenware pots (a minority
of the producers using this method), all or part of the stock was sampled using
the technique referred to above. Finally, for rice stored in sheaves (the least
frequently encountered method), a representative sample of sheaves were collected.
In each of the samples, the data was stratified until a sample of one kilogram
(1 kg) of paddy was retained for future investigations. Figure
1 describes the sampling technique applied.
|| Collection and sample analysis procedure, Broken rice: Damaged
grains by insects
Sample analysis methodology: For each farmers
sample, a subset of 100 g was measured and the infested grain was floated away
from the healthy grain. The different grain batches were dried for 25 min in
an oven set at 40°C. Finally, the healthy grain batch was separated again
under the microscope to detect the presence of any remaining infested grain,
especially any with insects inside the grain. After sorting, the different grain
batches were counted and weighed and glumes of the infested grains were removed
by hand to extract the broken grains. Only infested grains by insects were investigated
because several factors can contribute to damage and break the grains. These
grains were then separated out from impurities and weighed to know their respective
proportions in each sample. This analysis technique is described in Fig.
1. The weight of the impurities were not taken into account in this study
as they hardly amounted to much.
Loss evaluation methodology
Quantitative losses: This is the weight loss incurred by the different stocks
in the farming environment. The average percentage of infestation (A%) was assessed
as a function of the number of infested grain and the volume of healthy grains
based on the formula below (Harris and Lindblad, 1978):
Nd = No. of damaged grains
Nu = No. of undamaged grains
As for weight loss (B%), this was calculated by dividing the percentage of
infestation (A%) by a conversion factor C, according to the formula:
Conversion tables are used to determine the C factor per grain type. For rice,
C is equal to 2 (Harris and Lindblad, 1978). As reported
by the authors, this conversion method is suitable for an estimation of on-farm
storage insect pest damage. It is practical, fast and provides fairly accurate
Mention should be made of the fact that some qualitative losses from insect
pests attack were not analyzed in this study and therefore, were not used for
economic loss calculations. For example, loss of grain germination capacity,
loss of organoleptic quality and loss in color that could be contributory factors
to a loss in product marketing value. Research in the future will take them
Economic losses: Total economic loss attributable to insect pests was
calculated on the basis of the monetary value of the weight loss, on the one
hand and price depreciation for broken rice (discount), on the other hand. The
first type of loss was calculated by multiplying the price of paddy for each
period by the corresponding weight loss incurred. This is the economic loss
to paddy rice producers and sellers.
The second type of loss was calculated by subtracting the price (value) of
broken rice due to insects from the price of whole grain. This is the additional
economic loss that comes after hulling. The prices entered into the different
calculations were based on producer prices set when the samples were bought
or those declared by producers during the agronomy surveys. Thus, one kilogram
of paddy was supplied at 150 FCFA (Franc of the African Financial Community)
after 2-3 months of storage, at 200 F after 4 months and at 250 F after 5-6
months. As for the broken rice, it was sold at 210, 280 and 350 F per kilo after
storage periods of 2, 3, 4 and 5-6 months, respectively. Finally, whole grain
costs 300, 400 and 500 F, respectively over the same duration. Producer prices
were slightly above market price because the producers saw us as project officers.
Complementary agronomy survey: During the prospection, complementary
information was collected from local stakeholders to assess storage states (i.e.,
treated or not with chemicals), rice variety, total quantity and destination
of the stock, initial date of storage and finally, product price and fluctuations
over time. Producer perceptions about insect pest damage to stored grain were
Statistical data analysis: Means of the different losses were first
calculated using samples from the various farmer storage units. They were then
converted into tons per farmer stock in order to ensure that all measurements
were harmonized and make it easier to compare the means.
A Variance Analysis (ANOVA) was then carried out on the mean percentages of
infestation, weight and economic losses using statistical analysis software
(SAS, 9.1). The Student Newman Keuls test was used to separate means according
to different storage periods and/or regions where samples were collected.
Results showed that rice is usually stored in rural Benin for more or less
long periods. From 65 storage units visited, observations were made for some
160 tons of paddy rice. Stored rice is customarily destined for trade (153,400
kg), consumption (4,950 kg) or for use as seed (2,550 kg) (Table
1). In terms of volume, nearly all of the stored paddy stock was destined
for sale: 95.34% of total amounts inspected. The other stocks- especially for
consumption or seed-were in the minority, both in terms of quantity and volume
On the basis of climate measurements taken at the different sites visited,
certain regional variations were observed. Temperatures dropped slightly in
the southern regions (30.4 °C) and in the center (31.2 °C), compared
to the northern region where they were relatively high (32.3°C). Conversely,
the relative humidity gradient regressed from 85.2 to 58.4% from the south to
the north (Fig. 2).
Quantitative loss from storage insect pests: There were considerable
variations in stored rice losses depending on study sites, geographical regions
in the country and storage duration. In the southern and central regions, they
were higher than in the North. In the South, infested grain percentage ranged
from 3.29 to 10.94% between 2 and 6 months of storage (Fig. 3).
||Mean relative humidity and temperatures recorded during the
2011 and 2012 seasons
This caused weight loss varying from 1.64 to 5.47% (Table 2).
In the Center, there was just as much damage but slightly lower than in the
South. Recorded percentages of infestation went from 1.8 to 8.13% between 2
and 6 months of storage (Fig. 3). As for weight losses, they
ranged from 0.9 to 4.07% over the same periods (Table 2).
Finally, in the North, very little damage was recorded irrespective of storage
duration namely, 1.32-3.27% between 2 and 6 months of storage for infestation
percentages (Fig. 3) and 0.66-1.64% for weight losses over
the same periods (Table 2).
Thus, in the South and Center regions, weight loss recorded from 4 and 6 months
of storage was significantly more important than that recorded for durations
of less than 4 months. In the North, on the other hand, no significant variations
were associated with storage duration (Table 2).
Economic losses from storage insect pests: As it was the case for quantitative
losses, levels of economic loss varied according to geographical regions in
the country and product storage periods.
For producers who sell paddy rice, economic losses were limited to weight loss
values in amounts ranging from 2,471 FCFA (Franc of the African Financial Community)
to 13,675 FCFA t-1 of rice stored between 2 and 6 months in the South
||Variation in insect caused damage of farm stored paddy in
|| Distribution of farmer stocks according to use and source
Cost estimates are put at 1,350 FCFA and 10,169 FCFA for the central region
and 990 FCFA and 4,096 FCFA in the North.
Of note is the fact that the economic incidence of these losses was felt more
severely as from the fourth month in the South and from the fifth month in the
central region. On the other hand, in the North, the incidence was insignificant
irrespective of storage duration (Table 3).
Based on the total losses summing up the weight loss values and the depreciation
in broken rice post-hulling prices, producers in the southern region lost, on
average, between 4,197 F and 21,315 FCFA per ton of rice stored rom 2-6 months.
|| Losses from insect infestation in stored paddy grains in
|Values followed by the same letter in the column do not differ
significantly according to SNK test (p<0.05)
As for the central region, overall losses amounted to 2,628 and 14 170 FCFA
over the same periods. These figures dropped to 1,832 and 8,088 FCFA in the
northern region. The results show a highly significant difference (p<0.01)
between losses recorded over the different periods in the southern region. In
the central region, economic losses were significant after 5 and 6 months of
storage, compared to the other periods. Yet, in this region, there were no significant
differences in losses recorded from 2- 3 months and those at 4 months. Finally,
in the northern region, no significant variation in economic loss was related
to different storage periods (Table 3).
These results showed that the economic incidence of insect damage on stored
rice is only perceived as significant in the south and center of the country.
In both regions, a regression analysis of the economic loss of paddy and total
economic loss showed rising and linear trends depending on storage duration,
with regression coefficients R2 = 0,9796 for the first parameter
and R2 = 0,986 for the second (Fig. 4).
|| Estimated economic losses due to insect damage to stored
paddy grains in Benin
|Values in the same column not followed by the same letter
are significantly different according to SNK test (p<0.05)
|| Loss trends during rice storage in the southern and central
regions of Benin
|| Farmer perception of insect pest damage on stored rice in
Farmer perception of pest infestation and storage damage: According
to the survey findings, 36.92% of the farmers considered insects as pests with
a major economic impact on rice during the post-harvest period but only 12%
of them ranked them at the top. In fact, more than half the farmers surveyed
found that rodents (especially mice) were of greater economic impact (Table
Mention should be made of the fact that the insect pests identified on stored
rice were primary insect pest species, namely rice weevil (S. oryzae),
the Angoumois grain moth (S. cerealella) and the lesser grain borer (R.
dominica). They caused most of the recorded damage to farmer storage units.
Rice weevil and Angoumois grain moth featured predominantly in the South and
Center regions whilst the lesser grain borer was found in all three regions,
although in greater numbers in the North. Of all storage insect pests, producers
only vaguely recognized Coleoptera.
Results show that paddy rice stored for long time on-farm in Benin is, to a
large extent, destined for sale and to a lesser extent, for consumption and
seed. In fact, producers and traders prefer to store rice destined for sale
until off-season when they can sell at higher prices. This may be because Benin
rice growers depend on an attractive market-at the local level as well as in
Nigeria- for their paddy rice (Bauer et al., 2011).
However, these rice distribution results may depend on sampling periods to the
extent that each of the quantities in storage would depend on time of use. For
example, contrary to stocks for sale, reserves for consumption are drawn on
almost every day after harvesting. In some cases, they are hulled before storage
as polished grains. When these factors are added to low levels of local production
compared to household consumption needs, they may explain why such low quantities
were recorded as stocks for consumption. Finally, it should be stated that a
large quantity of stocks for sale are purchased locally or diverted to meet
subsistence needs. This often makes it difficult to distinguish between stocks
for sale and grain for consumption. As for seed, producers store small quantities
for use during the next cropping season. In some cases, farmers only have seed
during the sowing period or if they have bought some from the agricultural services
that are the only large storage holders of such seed.
The scale of insect pest damage to the different farmer storage units was correlated
with storage duration and regions where samples were collected. Concerning storage
duration, infestation caused more damage to paddy stored between 4- 6 months
than stocks less than 4 months, especially in the southern and central regions
of the country. In fact, long storage periods and especially when the reserves
are not treated, give the insects enough time to colonize the stocks and multiply
extensively, causing considerable damage. The scale of damage caused by post-harvest
insect pests correlated to storage duration was studied by Tefera
et al. (2011). As for the effect of geographical regions on the amount
of damage, it may be linked to the ecology of the insect pests. Thus, in the
South and the Center of the country where temperatures are constantly low, relative
humidity levels high and exposure to sunlight much less, climate conditions
are particularly favourable for the reproduction and multiplication of most
principal orders of insect species such as S. cerealella (Hansen
et al., 2004; Perez-Mendoza et al.,
2004; Togola et al., 2010) and S. oryzae
for which favourable temperature ranges are from 15 to 32.5°C and relative
humidity is between 50 and 90% (Logstaff and Evans, 1983).
Similarly, since R. dominica is an insect pest capable of adapting to
different climate conditions (Beckett et al., 1994;
Walker and Farrell, 2003), its presence in these regions
may contribute to aggravating the damage. What is more, stable levels of relative
humidity in the southern and central regions may increase water content of the
grains during storage. This is a risk factor in relation to insect infestation
(Arbogast and Throne, 1997; Hagstrum
et al., 2008). Contrary to the South and Center of the country; damage
to farmer reserves was minimal in the northern region where climate conditions
are less conducive to storage insect multiplication for many of the species.
Regarding economic losses, the study showed that paddy storage in Benin may
lead to average economic losses of 2 FCFA (Franc of the African Financial Community)
for each kilo of rice stored over 2- 3 months. This loss may reach 6 F, 9 F
and 11 F per kilo of paddy for 4, 5 and 6 months of storage, respectively. In
addition, on examination of losses after processing paddy, economic damage may
reach 4 F, 9 F, 13 F and 17 FCFA kg-1 of rice after 2, 4, 5 and 6
months of storage, respectively. As is the case for damage, economic losses
to paddy caused by storage insect pests in Benin are higher for producers in
the south than in the north. With the re-emergence of primary pest populations
in many rice-producing zones (Togola et al., 2010)
and risks of dispersal to other, previously uninfested zones, losses may increase
in the years ahead with a corresponding drop in producer income. For the time
being, economic losses from quantitative loss caused by insect pest damage have
not attained critical proportions amongst producers in the north. Similarly,
where producers store for relatively short periods (2-3 months), the economic
incidence of insect infestation is lower. However, when storage is for longer
periods (4 months and more), there is a high risk of economic loss, especially
for producers in the southern and central regions of the country. In these areas,
major rice producers, farmer associations, processors and traders who store
large stocks over long periods run the risk of non-negligible economic losses.
The linear trend of losses in relation to storage duration proves without a
doubt that storage duration is a major risk factor for post-harvest insect infestation,
in general (Ngamo and Hance, 2007; Hagstrum
et al., 2008) and for rice stored in Benin, in particular. Among
survey respondents, 36.92% are aware of the economic impact of storage insect
pests whilst more than half fear the effect of rodents. Indeed, contrary to
rodents, storage insects often go unnoticed (Onolemhemhen
et al., 2011) especially when they are at an immature stage. Under
these conditions, damage may remain hidden much longer and thus escape the vigilance
of the producers. Although rodents generally have a considerable economic impact
on stored grains (Singleton, 2010), the role played by
insects in weight and economic losses to stored rice in Benin remains a reality.
This scale of insect pest damage and the concomitant economic losses could
mean that, as already observed, traditional varieties are increasingly abandoned
for improved varieties which, in many cases, offer less natural protection against
storage insects than does the former (Smale et al.,
1995; Adda et al., 2002; Jones,
Apart from the substantive quantitative losses to stored grains that generally
stem from insect damage (Subramanyam and Hagstrum, 1996),
qualitative losses also take various forms notably, loss of seed germination
capacity, product discoloration, change in taste, etc. These types of losses
may lower the nutritional and organoleptic value of the rice (Jood
et al., 1996), leading to a drop in market value and therefore, economic
loss to producers, processors, traders and consumers (Onolemhemhen
et al., 2011). All these harmful effects were to be taken into consideration,
the economic impact could attain even higher proportions.
This study produced an update on knowledge about quantitative and economic
losses to stored paddy rice in Benin caused by insect pests. It also showed
farmer perceptions of the economic impact of post-harvest insect pests. Among
factors cited for increased quantitative and economic loss, mention should be
made on climate conditions in the regions, insect behavior and anthropic causes
(storage duration, abandonment of traditional varieties, neglect of damage caused
by insects, etc.).
Judging from present losses, control measures should be adopted right away
to limit an upsurge in post-harvest predators and protect stocks against multiple
harmful manifestations. However, the cost of control measures should take into
consideration the value of losses forestalled.