International Research Journal of Agricultural Science and Soil Science (ISSN: 2251-0044) Vol. 2(8) pp. 333-340, August 2012
Available online http://www.interesjournals.org/IRJAS
Copyright ©2012 International Research Journals
Full Length Research Paper
Simulating infestations and losses in storage from egg
and adult Callosobruchus maculatus
(Coleoptera: Bruchidae) sources in cowpea
(Vigna ungiculata L. Walp.)
Joy Mbaya Turaki
Department of Biological Sciences, Faculty of Science, University of Maiduguri, Nigeria
E-mail: drturaki@yahoo.com; Phone: +2348034589172
ABSTRACT
The experiment simulated field-to-store, cross- and re- infestations in storage by C. maculatus using 2 10 egg- and adult- infestation sources and effectively (91.9 - 94.5%) predicted storage losses. The study
lasted one generation of pest development cycle, spanning 42 days. Result expressed significant
(P<0.01) difference in both pest population build-up and cowpea weight loss, among the egg- and adult
source treatments. Thus, adult-source infestation treatments generated 177.8 - 426.8 eggs and 98.8 185.0 emerged beetles at success rate of 37.2 - 56.1% with concomitant seed weight loss of 28.2 52.9%, which significantly increased at higher infestation densities. In contrast, egg-source infestation
had relatively higher F1-progeny success rate (50.0 - 90.0%), however, the F1-beetle yield (1.8 - 7.3),
oviposited eggs (34.0 - 245.3) and seed weight loss (0.4 - 1.8%) were comparably low. Beetle population
build-up increased significantly (P<0.01) at the rate of 26.60 - 44.60 beetles per week among adultsource treatments compared to 0.35 - 1.75 beetles in the egg-source. In conclusion, C. maculatus
displayed the capacity to initiate infestation from both egg- and adult- sources; however, results clearly
portrayed higher pest population growth and seed weight loss from adult- than egg- source
infestations.
Keywords: Vigna unguiculata, Callosobruchus maculatus, adult-source infestation, egg-source infestation,
storage losses.
INTRODUCTION
Cowpea (Vigna unguiculata L. Walp.) is valued
universally for its rich protein (25%), hence its
significance in human nutrition. Nigeria accounts for
40.5% of the global production. However, both the
production and storage of cowpea are highly constrained
by insect pests, and reports further indicate that
infestation and losses by the bruchid (Callosobruchus
maculatus F.) is the most serious problem to the storage
of cowpea in Nigeria (Fatokun, 2002, AATF, 2003; Swella
and Mushobozy, 2007; Adam and Baidoo, 2008; Baidoo
et al., 2010; FAOSTAT, 2010). The bruchid, inflicts
serious damage to stored cowpea, in which the multiple
generations, larvae tunneling and adult exit holes result in
reduced seed weight, viability, aesthetic and market
value, as well as, nutritional qualities by denaturation of
protein (Umeozor, 2005; Bamaiyi et al., 2006; Deshpande
et al., 2011). C. maculatus initiates infestation in storage
10 - 15 % from residual eggs from the field, which
continues rapidly in storage, coupled with crossinfestation and re-infestation causing substantial losses
(Olubayo and Port, 1997).
Estimates of storage losses due to C. maculatus are
highly variable ranging widely from 4 - 90% (IITA, 1989;
Umeozor, 2005). Such discrepancies often occur when
loss estimates are made without reference to the pest
density, sufficient data points for reliable loss estimates
and prediction. The present study simulated field-to-store
(egg-source) and re- or cross-infestation (adult-source)
334 Int. Res. J. Agric. Sci. Soil Sci.
as tools for generating different infestation levels to infer
storage losses by C. maculatus.
MATERIALS AND METHODS
Culture Methods
Stock cultures of C. maculatus were maintained on
cowpea (cv: Borno brown) in University of Maiduguri,
Biological Sciences Laboratory. A thermohydrograph was
used to monitor temperature and relative humidity in the
laboratory. Ambient temperature and relative humidity in
the laboratory were 33.6±0.210C and 35.8±0.13% RH.
Culture method described by Buahin and Turaki (1999)
was used in which seeds with adults ready to emerge
were sorted into a kilner jar. Newly emerged adults were
then sieved (2 mm mesh), allowed to mate for 1 h, sexed
and the pre-mated females used for the adult-source test.
Same batch of pre-mated females were used to raise the
egg-source treatments, meant to harmonize the two
sources.
Experimental Design and Procedure
Randomized Complete Block Design was employed for
the experiment, in which egg- and adult- infestation
sources were meant to simulate field-to-store, cross- and
re-infestation in storage by eggs and adult C. maculatus,
respectively. Thus, treatments consisted of 2, 4, 6, 8 and
10 egg-laden seeds and similar batches of pre-mated
females for egg-source and adult-source, respectively.
Treatments were replicated four times and then randomly
assigned to kilner jars containing 100 g of clean seeds,
screened with muslin cloth and secured rubber band to
prevent escape.
Data Collection and Analysis
Egg count and adult census were taken at bi-weekly
interval spanning one generation of pest development
cycle of 42 days. Actual loss (%) was calculated as
described by Deshpande et al. (2011), thus:
U(Nd) – D(Nu)
Actual weight loss (%) = _____________
U (Nu + Nd)
RESULTS
Results of the study both expressed significant (P<0.01)
difference in infestation levels among adult-source and
egg-source treatments (Figures 1 and 2). Figure 1
compares oviposition and adult emergence among the
adult-source infestation treatments, in which both egg
and beetle counts were invariably significantly (P<0.01)
higher at higher infestation densities of 8 and 10 than
lower infestation with 2 - 6 that were statistically at par.
Oviposition ranged from 119.3 - 299.8, 149.0 - 402.3, and
177.8 - 426.8 eggs, at 7, 14 and 21 DAI, respectively
(Figure 1a), while emerged adults ranged from 44.8 95.8, 80 - 143.3 and 98.8 - 185.0 beetles at 28, 35 and
42 DAI, respectively (Figure 1b). Similarly, both
population of F1-adult that emerged in the egg-source
treatments and eggs laid increased significantly with
increase in egg-density, thus the highest emergence
consistently occurred at 10-, followed by 8- then 6-egg
density than at lower egg-densities of 2 and 4 that were
at par. Mean number of F1-progeny ranged from 1.0 - 3.8,
1.5 - 7.3 and 1.8 - 7.3 at 7, 14 and 21 DAI, respectively
(Figure 2b) and the mean number of eggs laid ranged
from 19.3 - 186.0, 25.5 - 218.0 and 34.0 - 245.3 at 28, 35
and 42 DAI, respectively (Figure 2b).
Result of the regression for adult-source infestation
indicated highly significant (P<0.01) rate of increase in
oviposition (r2 = 0.8883 – 0.9999) and beetle population
2
(r = 0.9423 - 0.9986) over time (Figure 3). The
regression coefficient showed oviposition rate of 27.50 63.50 eggs and beetle population growth rate of 26.60 44.60 beetles per week. In the egg-source infestation, the
weekly rate of beetle population growth increased
significantly (P<0.01) at 0.35 – 1.75 beetles and
oviposition at 7.00 - 50.65 eggs per week (Figure 4).
Actual loss in grain weight varied significantly from
28.2 - 52.9% in the adult-source infestation, in contrasts
to 0.4 - 1.8% in the egg-source treatments that were
statistically at par (Figure 5a). These losses occurred at
the rate of 5.69% and 0.13% in the adult source and eggsource, respectively from each successive increase in
infestation level (Figure 5b). Figure 6 expressed that the
egg- and adult- sources had effectively simulated field-tostore, cross- and re- infestations in storage by C.
maculatus in predicting losses by 91.9% and 94.5%,
respectively.
x 100%
Where,
U - weight of undamaged seeds,
Nd - number of damaged seeds,
D - weight of damaged,
Nu - Number of undamaged seeds.
Data obtained were analyzed using the analytical
software, Statistix 8.0, while charts were drawn using
Microsoft Excel, 2003.
DISCUSSION
C. maculatus is a serious constraint to cowpea storage
that has displayed the capacity to initiate infestation from
as low as 2 - 10 egg- and adult- sources. Both egg and
beetle count significantly increased with infestation level,
irrespective of the source of infestation. However, results
clearly portrayed higher population build-up from adult-
Turaki et al. 335
Figure 1. Comparison of a) oviposition and b) F1-adult emergence among the adult-source infestation
treatments
source than egg-source; F1-beetle yield from one
generation of the adult-source infestation ranged from
98.8 - 185.0, in contrast, to lower beetle yield (1.8 - 7.3)
from egg-source. This clearly suggests that cross- or reinfestations by adults might trigger higher population
growth and consequently losses than field-to-store
infestation from eggs. However, in a related study on
egg-source infestations, Baidoo et al. (2010) reported
higher population build-up range (1.7 - 136.7 beetles), as
harvest was delayed by 20 days. The difference is attri-
336 Int. Res. J. Agric. Sci. Soil Sci.
Figure 2. Comparison of a) F1-adult progeny and b) oviposition among the egg-source
infestation treatments
butable to the number of generations, which lasted one
generation (42 days) in the present study compared to
two (80 days) in the earlier report. However, the results
indicted 177.8 - 426.8 and 34.0 - 245.3 eggs, oviposited
by the emerged F1-beetles with success rates of 37.2 56.1% and 50.0 - 90.0% for adult-source and egg-source
treatments, respectively, indicative that such pest buildup could be attained over two generations.
The present study also revealed variation in weight
losses stemming from different densities of adult- and
egg- source infestations. The adult-source inflicted higher
losses of 28.2 - 52.9% compared to the egg-source that
Turaki et al. 337
Figure 3. Relative rates of a) oviposition and b) F1-adult emergence among adultsource treatments over three weeks period
registered losses of 0.4 - 1.8%. However, Deshpande et
al. (2011) recorded lower weight losses (3.14 - 27.02%),
where as Baidoo et al. (2010) obtained higher losses (8.7
-12.5%) from egg infestation, than was obtained in the
present study, probably due to differences in genotype
susceptibility and the test period, which according to
Adam and Baidoo (2008) are important loss indices. The
cowpea variety used in the present study has suscep-
338 Int. Res. J. Agric. Sci. Soil Sci.
Figure 4. Relative rates of a) F1-adult emergence and b) oviposition among egg-source
treatments over three weeks period
tibility index of 4.0 (Turaki and Buahin, 1999) while pest
activity lasted 42 days. Although, Umeozor (2005)
construed higher losses in actual storage (0.026g per
beetle) than in laboratory experiments (0.016 g per
beetle), the present result of the regression expressed
significant increase in beetle population build-up of 26.60
- 44.60 beetles per week among adult-source treatments
compared to 0.35 - 1.75 beetles in the egg-source. The
implication of the present findings is that these weight
losses would further impose additional economic burden
Turaki et al. 339
Figure 5. Simulated of the a) fecundity of C. maculatus and b) weight loss in cowpea
from different sources and levels of infestation
to production cost on farmers. In conclusion, the efficacy
of the present experiment in predicting storage losses
due to the simulated field-to-store, cross- and reinfestations was 91.9% and 94.5%, respectively; attesting
to higher infestation and losses from adult source (crossand re- infestation) than egg-source (field-to-store
infestation).
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