Integrated Management of Sweetpotato Weevil, Cylas puncticollis (Boheman) (Coleoptera: Curculionidae) in Eastern Ethiopia
Sweetpotato weevil, Cylas puncticollis, is the most destructive insect pest that ranks as the number one constraints for the production of sweetpotato in Eastern Ethiopia. A field experiment with the aim to develop compatible integrated management methods for sweetpotato weevil was conducted in 2011 cropping season at Haramaya University Horticultural Research Field (Rare) in Eastern Ethiopia. The experiment consisted of three factors: Cropping system at three levels (sole cropping of sweetpotato, sweetpotato intercropped with maize and sweetpotato intercropped with haricot bean), earthing-up at three levels (1,2 and 3 times earthing-up) and harvesting time at two levels (prompt and delayed harvesting) making up 18 treatment combinations. The treatments were replicated thrice and laid out in Randomized Complete Block Design (RCBD). Results of the studies revealed that the interaction effect of cropping systems, earthing-up and harvesting periods significantly (p<0.05) reduced sweetpotato percentage infestation and storage root damaged, number of unmarketable roots, number of weevils/kg of damaged roots. On the other hand, the interaction effect increased the number of marketable roots/plant and the total yield. Hence, from the current study it can be concluded that the integration of three times earthing-up at monthly interval starting from one month from planting; prompt harvesting, harvesting exactly at the physiological maturity of the roots; and mixed cropping of haricot bean at the ratio of three rows of sweetpotato to one row of haricot bean can sufficiently off set the risk of sweetpotato weevil (C. puncticollis) in Eastern Ethiopia.
December 28, 2013; Accepted: March 07, 2014;
Published: May 09, 2014
Sweetpotato (Ipomoea batatas (L) Lam.) is one of the worlds most
widely grown crops and it is an important crop in East Africa where it is grown
as a staple food (Stevenson et al., 2009). Globally,
the crop ranks the seventh among the most important food crops after wheat,
rice, maize, potato, barley and cassava. It serves as animal feed and raw material
for industries in the world (Ray and Tomlins, 2009).
In Ethiopia sweetpotato has been cultivated for many years and is important
in diet where population growth is highest, land holding is least and threat
of large-scale starvation is ever present (Habtu, 1995).
The crop is most important root and tuber crops as one of the major traditional
food crops of the country (Endale et al., 1994),
where it is a major source of sustenance and food security (CIP,
2004). In eastern Ethiopia sweetpotato is mainly produced for human consumption,
as income generator and to feed livestock. However, in Eastern Ethiopia sweetpotato
weevil (SPW) (C. puncticollis) ranks the number one constraint for sweetpotato
production. C. puncticollis limit sweetpotato production by damaging
vines, tubers and occasionally the foliage, thereby reducing both the yield
and quality of the crop. The cryptic feeding nature of the pest made some control
practices like chemical control and biological control ineffective (Smit
et al., 2001). The crop is also considered as a poor man crop which
does not call for expensive investment like the use of chemical control for
pest management. C. puncticollis causes 60-70% yield loss in East Africa
(Kabi et al., 2001) and 21-78% in Ethiopia (Emana,
1990). Often producers relay on chemical control for C. puncticollis
management, however, it caused frequent pest outbreaks, pesticide resistance,
contamination of the environment, pest resurgence and less pest suppression
Some cultural practices like earthing-up, prompt harvesting and intercropping
were found to be more effective in the management of C. puncticollis (Emana,
1990). Nevertheless, each component of these cultural practices only suppresses
the population and the effect of C. puncticollis to certain extent which
is well above the economic injury level. The hypothesis now is if these cultural
practices are combined in the form of integrated pest management there is a
possibility that effective management of C. puncticollis can be attained.
Integrated Pest Management (IPM) has emerged as an important approach of pest
control strategy, which encourages applying measures that causes least disruption
of agro-ecosystem. IPM is effective, economical, environmentally benign alternative
to chemical pest control (Schalk et al., 1991;
Lawrence et al., 1997). Among the integrated
pest management methods, cultural pest management tools are very useful and
effective; economically and environmentally friendly.
Pest damage is lower in diverse cropping systems suggesting that crop mixtures
provide a greater diversity of habitat for arthropods offering a greater abundance
and variety of prey and hosts for predators and parasitoids (natural enemies
release hypothesis) or through affecting their ability to find and utilize its
host plants. Thus, the non-host plants may mask the herbivores host-finding
stimuli (visual and chemical) so that colonization of the host plant is minimum
(resource concentration hypothesis). A number of studies have indicated that
the provisioning of floral resources and habitat can increase the density and
diversity of natural enemies (Jervis and Heimpel, 2005).
Earthing-up is another important C. puncticollis management. It prevents
exposure of storage roots to weevil infestation by thickening the soils around
the storage roots and filling up the soil cracks, so that the adult C. puncticollis
can not reach storage root to cause damage (Emana, 1990;
Palaniswami and Mohandas, 1994). Timely harvesting
is another cultural practice that has been used for sweetpotato weevil management.
It has been reported that weevils populations build up when harvesting
is delayed because it allows continuous reproduction on available food (Kabi
et al., 2001). Delay in harvesting increases infestation by SPW (Emana,
1990) and damage to sweetpotato roots (Ebregt et
al., 2004). Therefore, the general objective of this study was to investigate
and develop integrated sweetpotato weevils management (IPM) methods for
MATERIALS AND METHODS
Description of the study area: A field experiment was conducted during
the rainy season of 2011 (June to November) in Eastern Ethiopia (East Hararge)
at Haramaya University field experimental station (Rare). Haramaya University
field experimental station is located at an altitude of 1197m above sea level
and lies at coordinates of 9°6N and 41°8E. The station lies
in the semi-arid belt of the eastern rift valley escarpment with a long-term
average rainfall of 612 mm. The soil is classified as Eutric Regosol with a
gentle slope (3 to 8%). The texture and structure of the topsoil (0 to 30 cm)
are sandy loam and sub angular blocky, respectively. The soil has an average
pH of 8.54 and organic matter content of 1.94% (0 to 15 cm) and 1.84 (15 to
30 cm). The mean annual rainfall is 520 mm and means maximum and minimum temperatures
range from 28.1-34.6°C and 14.5-21.6°C, respectively (Belay,
Planting material: Three planting materials namely: A sweetpotato variety,
Barkume; an early maturing variety of maize, Katumani and haricot bean variety,
Kufanzihki were used for this experiment. Top or middle vine parts of Barkume
variety of sweet potato at 30 cm length with 3-4 nodes was used as planting
materials. The intercrops (maize and haricot bean) were planted on the two sides
within sweetpotato rows (as mixed intercropping) on the same day after plowing,
disking and ridging of the experimental field.
Land preparation and its managements: The experimental land was prepared
twice. First and second land preparations were done at two weeks
interval to facilitate organic matter decomposition of the soil. Land was prepared
by ploughing using tractor and any residuals of the pervious remains were collected
from the plowed land and buried, seed bed were also prepared as per the recommendation
for the crops. After planting, hand and light hoeing was used to remove weeds,
diseased plants and off types.
Treatments and experimental design: The combined treatments (Table
1) consisted of three levels of cropping systems (sole sweetpotato (as a
control), sweetpotato with maize and sweetpotato with haricot bean intercropping),
three levels of earthing-up (1, 2 and 3 times earthing-up) and two levels of
harvesting periods (prompt and one month delayed harvesting). The treatment
combinations were laid-out using Randomized Complete Block Design (RCBD) in
factorial arrangement with three replications. Each plot size was 3 m width
and 3.6 m length. The spacing was 60 cm between rows and 30 cm between plants
for sweetpotato, 10 cm between plants and 60 cm between rows for haricot bean
and 15 cm between plants and 60cm between rows for maize. The spacing between
blocks and plots were 2x1 m, respectively. Except the treatment combinations
all the other agronomic and management practices were applied to each experimental
plot in the same way as per the recommendations for the area.
|| Lists of the different factors, their levels and treatment
|*Treatment combinations with their respective codes given
in parenthesis for each level of a factor, **DAP: Days after planting
|| Scale used to differentiate the tubers harvested in to damaged
and normal tubers based on external damage
Number of adult C. puncticollis and percentage infestation:
The infestation level (colonization) of sweetpotato by C. puncticollis
was determined by visual counting of the adult C. puncticollis from 12
randomly selected plants. The same sample plants were considered as infested
when damage was evident on the stem base starting 30 Days After Planting (DAP).
Record was made at about 2:00 pm when the insects were active. Total percentage
infestation was calculated as follows:
where, I is percentage infestation of the sweetpotato plant by C. puncticollis
in a plot, N is number of sample plants infested per plot and T is total number
of sample plants per plot.
Number of tubers with C. puncticollis damage: Weevils damaged/infested
roots of each sample was separated based on the external damage symptoms as
indicated in Table 2 on scale basis (Mtunda
et al., 2001), then the percentage of infested tubers was calculated
using the formula:
where, I is percentage damaged tubers, a is number of infested tubers, b is
number of healthy tubers.
Number of marketable and unmarketable tubers: Harvested tubers were
separated depending on the size and healthiness of the tuber into those of marketable
if the weight of tuber is greater or equal to 100 g and it is healthy and unmarketable
(if the weight of tuber is less than 100 g and unhealthy) tubers visually.
Data analysis: Data were checked for normality before analysis. Those
which violate normality were transformed (Gomez and Gomez,
1984) using arcsine transformation prior to analysis. Those data which assume
normality were subjected to two-way ANOVA using SAS version 9.2 software (SAS,
2008) packages. Significant means (p<0.05) was separated using Least
Significant Differences (LSD). The cause-effect relationship of sweetpotato
weevils population density and the percent damaged roots was analyzed
using regression analysis. The associations between percentage of infestation,
damaged sweepotato roots, weight and yield loss, number of weevils per kilogram
of storage roots, number of marketable roots and total yield of sweepotato roots
were analyzed using simple correlation analysis.
Percentage infestation of sweetpotato by C. puncticollis: The three
way interaction effect of the factors on the percentage infestation of sweetpotato
by C. puncticollis was significant (p<0.05) (Table 3).
The percentage infestation was significantly reduced by the interaction effect
of cropping systems, earthing-up and harvesting period. The lowest (14.33%)
C. puncticollis infestation on sweetpotato was recorded from the interaction
effect of sweetpotato intercropping with maize, prompt harvesting of sweetpotato
and three times earthing-up, while the highest (92.33%) percent infestation
was recorded from the interaction effect of sole cropping of sweetpotato, delayed
harvesting and two times earthing-up, but this treatment combination was statistically
on par with the interaction effect of sole sweetpotato cropping, delayed harvesting
and one time earthing-up (90.66%).
Percentage damaged tubers: The percentage damaged storage tubers by
C. puncticollis was significant (p<0.05) due to the three way interaction
effect of cropping systems, earthing-up and harvesting periods (Table
4). Minimum (5.52%) percentage damaged storage tubers of sweetpotato due
to C. puncticollis was recorded from the interaction effect of sweetpotato
intercropped with maize, prompt harvesting and two times earthing-up but this
treatment combination was statistically non-significantly different from sweetpotato
intercropping with maize, prompt harvesting and one or two times earthing-up;
sweetpotato intercropping with haricot bean, prompt harvesting and two or three
times earthing-up. On the other hand, maximum (59.45%) percentage damaged storage
tubers was recorded from sole sweetpotato cropping, delayed harvesting and one
time earthing-up and this treatment combination was followed by the combined
effect of sole sweetpotato cropping, delayed harvesting and two times earthing-up
||Interaction effects of cropping systems,
earthing-up and harvesting periods on the percentage infestation of sweetpotato
by C. puncticollis
|The No. inside parentheses are the transformed data (arcsine
transformation). Means with the same letter within the table is not
significantly different at 5% significant level, Small letters within the
table indicates mean separation for higher order interaction effects,
Capital letters indicates mean separation for main effects, *Means of the
main effect of cropping systems
||Effects of intercropping, earthing-up and harvesting periods
on the percentage damaged storage tubers of sweetpotato by C. puncticollis
|Means with the same letter within the table is not significantly
different at 5% significance level, Small letters within the table indicates
mean separation for interaction effects, Capital letters indicates mean
separation for main effects, *Means of the main effect of cropping
Population density of C. puncticollis under the different cultural
practices: When the main effect of cropping systems were considered, the
result indicated that the population density of C. puncticollis was found
to be significantly correlated with the percentage of storage tubers damaged.
More population density of C. puncticollis on sole cropping of
sweetpotato was positively correlated with more damaged storage tubers and less
percentage of damaged storage tubers on intercropped sweetpotato was associated
with less number of weevils per kilogram of damaged sweetpotato tubers. The
relationship is depicted by the regression equation, y = 0.624x+6 (r2
= 0.94) (Fig. 1), which indicate the high dependency of percentage
of damaged sweetpotato storage tubers on the population density of C. puncticollis.
Number of marketable tubers/plant: The interaction effects of cropping
systems, earthing-up and harvesting period on the number of marketable tubers
were significant (p<0.05) (Table 5). Maximum (9.67 roots
plant-1) marketable roots were recorded from the interaction effect
of sweetpotato intercropping with maize, prompt harvesting and three times earthing-up
which was followed by intercropping sweetpotato with haricot bean, prompt harvesting
and three times earthing-up (8.27 tubers plant-1) which is not statistically
different from the combined effect of intercropping sweetpotato with maize,
prompt harvesting and two times earthing-up (7.25 tubers plant-1).
On the other hand, minimum (1.8 roots plant-1) marketable healthy
tubers were recorded from sole cropping of sweetpotato, delayed harvesting and
one time earthing-up but the difference was statistically non-significant from
sole cropping of sweetpotato, delayed harvesting and two times earthing-up (2.89
tubers plant-1); intercropping sweetpotato with maize, delayed harvesting
and one times earthing up (2.37 tubers plant-1) and intercropping
sweetpotato with haricot bean, delayed harvesting and one/two times earthing
up (2.7 and 2.0 tubers plant-1, respectively).
||Effects of cropping systems, earthing-up
and harvesting periods on the number of marketable storage tubers of sweetpotato
as affected by C. pucticollis
|Means with the same letter within the table is not significantly
different at p<0.05 Fisher’s least significant difference test,
small letters within the table indicates mean separation for interaction
effects, capital letters indicates mean separation for main effects, *Means
of the main effect of cropping systems
||Cause-effect relationship of population density of C. puncticollis
per kilogram damaged storage tubers, S0: Sole sweetpotato, S1:
Sweetpotato+maize, S2: Sweetpotato+haricot bean
Number of unmarketable tubers/plant: The two way effects (cropping systems
and harvesting periods (Fig. 2), cropping systems and earthing-up
(Fig. 3) showed a significant (p<0.05) difference for number
of unmarketable sweetpotato tubers per plant.
||Interaction effects of cropping systems and harvesting periods
on the number of unmarketable tubers/plant
||Interaction effects of cropping systems and earthing-up on
the number of unmarketable sweetpotato tubers/plant
Considering cropping systems and harvesting periods, the highest number of
unmarketable tubers/plant was obtained from sole sweetpotato cropping integrated
with delayed harvesting but it was non-significantly different with sole sweetpotato
cropping integrated with prompt harvesting where as the lowest number of unmarketable
tubers per plant were obtained when sweetpotato was intercropped and harvested
promptly which was on par with intercropping of sweetpotato with both intercrops
and delayed harvesting. Among the two intercrops tested, maize intercropping
reduced the number of unmarketable storage tubers in comparison to haricot bean
but statistically non-significant difference was observed among the two crops.
Simple pearson correlation coefficient of some variables measured
|ns: Non-significant at 5% level of significance, *Significant
at 5% level of significance and **highly significant at 0.01 level of significant,
PI: Percentage infestation, DR: Percentage damaged tubers, PWL: Percentage
weight loss, PYL: Percentage yield lose, WprKG: Number of weevils per kilogram
of sweetpotato storage tubers, MHT: Number of marketable healthy roots,
Yield t ha-1: Total yield of storage tubers of sweetpotato per
The result also revealed that delay in harvesting time resulted in more number
of unmarketable storage tubers but statistically non-significant from prompt
harvesting indicating the significant effect of cropping system than harvesting
periods. Besides, the interaction effect of cropping systems and earthing-up
was significant on the number of unmarketable storage tubers of sweetpotato.
Maximum number of unmarketable tubers/plant was harvested from sole sweetpotato
cropping system with one-time earthing-up. However, statistically no significant
different were observed among the two crops intercropping with sweetpotato and
two or three times earthing-up. It is the cropping systems which is more influential
to the number of unmarketable storage tubers than the earthing up frequencies
similar to the harvesting periods.
Simple correlation analysis of the variables: The simple linear association
between the variables (Table 6) indicated that, percentage
infestation was significantly and positively-correlated with percent weight
loss (r = 0.6**), percent yield loss (r = 0.38*), number of weevils per damaged
sweetpotato tubers (r = 0.56*) and inversely non-significantly correlated with
number of marketable healthy tubers (r = -0.18ns) and total yield
of sweetpotato (r = -0.002ns). Percentage damaged tubers was positively
and significantly correlated with percentage weight loss (r = 0.68**), yield
loss (r = 0.61**) and number of weevils (r = 0.60**) but negatively and significantly
correlated with marketable healthy tubers (r = -0.61) and yield (r = -0.30).
In the present study, variation was observed on the infestation of sweetpotato
by C. Puncticollis under the different cultural practices considered
for integrated management of sweetpotato weevil. Reduced infestation of sweetpotato
by C. puncticollis, damaged storage tubers, number of unmarketable tubers,
etc and on the contrary an increased number of marketable and healthy tubers
in the combined effect of intercropping sweetpotato with maize/haricot bean,
prompt harvesting and three times earthing-up was observed when compared with
sole cropping of sweetpotato, delayed harvesting and less frequent earthing-up.
This result were may be due to the confusing olfactory and visual cues received
from intercropped crops (maize and haricot bean) act as physical barriers against
C. puncticollis movement than sole cropping of sweetpotato. Also, the
non-host plants (maize/haricot bean) may increases the number of natural enemies
in the field and may be responsible for the lower number of C. puncticollis
in the intercropped sweetpotato in comparison to sole sweetpotato resulting
in positive responses for the growers (less number of weevils, more yield, less
loss, less infestation and less damage). The population density of C. puncticollis
and percentage of damaged storage tubers were lower in the intercropping systems
than in the sole sweetpotato cropping. Frank and Liburd
(2005) found reduced number of Bemisia tabaci and aphid in more diverse
cropping systems involving squash and a living mulch, buckwheat indicating the
importance of cropping system for the management of crop pests. Funderburk
et al. (2011) reported that planting of sunflower on the perimeter
of pepper fields increase the density of minute pirate bugs (predatory
bug) in the pepper, helping to suppress western flower thrip. Suris
et al. (1995) and Alexander (1992) also reported
intercropping of sweetpotato with corn resulted in lower percentage of sweetpotato
weevil population and damaged storage tubers than sweetpotato pure stand. Similarly,
Rao (2005) and Rao et al.
(2006) reported low incidence of Cylas formicarius in the multiple
cropping systems. Christerson (1995) observed a decrease
in the number of Aphis fabae in an intercropping of beet with phacelia.
Hassanali et al. (2008) also reported that a
low incidence of Striga (Striga hermontheca) and maize stem borer (Chilo
partellu) in maize/desmodium, legume and napier grass intercropping.
Lower number of C. puncticollis was recorded in sweetpotato/maize intercropping
system (9.31 weevils kg-1 of damaged tubers) and sweetpotato/haricot
bean cropping system (11 weevils kg-1 of damaged tubers). On the
other hand sole sweetpotato cropping gave the maximum number of C. puncticollis
per damaged tubers (33.36 weevil kg-1 of damaged tubers) resulting
in significant damage (internally/externally) to storage tubers. Less percentage
storage tubers damage was recorded from integrated effect of intercropping sweetpotato
with maize, prompt harvesting and three times earthing-up. The low percentage
damaged sweetpotato tubers from this treatment combination may be due to the
low infestation of C. puncticollis in plots that received this treatment
combination. The report of Alexander (1992) also indicated
that high relationship of population density of weevils and damaged storage
tubers which confirm the present study.
Besides, lower number of unmarketable tubers in the intercropped cropping systems
may be due to efficient use of nutrients and low vine damage resulting in increasing
the marketable storage tuberous roots, while the high number of unmarketable
storage roots harvested from sole sweetpotato was due to the high competition
among the same species (sweetpotato) for the same resources and high infestation
on the veins which may result in infested vascular tissues. Low incidence of
C. puncticollis in the intercropped sweetpotato resulted in high marketable
tuberous roots. This finding is in consistency with the finding of Suris
et al. (1995) who observed lower percentage of sweetpotato weevil
damage in sweetpotato intercropped with maize than sweetpotato pure stand in
The reduction of soil cracking through earthing-up and prompt harvesting may
also caused reduced reproduction/multiplication of sweetpotato weevils and escape
of the main crop, respectively from C. puncticollis infestation in the
field leading to better management of weevils and greater yield responses from
the crop. Damage to sweetpotato roots was significantly decreased as the frequencies
of earthing-up increased, when sweetpotato was harvested on time and intercropped
with maize/haricot bean. Earthing-up the plant three times starting from the
first month after planting resulted in high marketable healthy tubers. The role
of sweetpotato tuberous root hilling up as a management tools were reported
by Emana (1990) and Macfarlane (1987)
who reported that earthing-up offers protection against sweetpotato weevils.
Rashid (1999) mentioned the advantages of earthing-up
and suggested that 2-3 times earthing-up were helpful for Mukhi Kachu production.
The result of Qadir (1997) and Qadir
et al. (1999) also examined that earthing-up at 15 days resulted
in better plant and yield performance in potato crop.
Promptly harvesting when sweetpotato reached physiological maturity plays a
vital role in managing the infestation of sweet potato weevil resulting in reduced
damage to the storage tubers of sweetpotato. On an average, delayed harvesting
resulted in significantly higher infestation (59.66%) and tuber damage (22.59%)
by C. puncticollis and lower number of marketable tubers (2.99 tubers/plant)
in the three cropping systems and three frequencies of earthing up. On the other
hand, prompt harvesting of sweetpotato gave lower percentage of damaged tubers
(13.00%) and infestation (47.53%) of sweetpotato by C. puncticollis and
more number of marketable tubers (6.14 tubers/plant). The result also revealed
that delay in harvesting period resulted in high unmarketable storage tubers
may be due to the presence of their host in the field which provide suitable
environment for the growth and development of C. puncticollis. Similar
to this finding Ebregt et al. (2007) reported
piecemeal harvesting of sweetpotato caused more storage root damage by weevil
(Blosyrus spp.) than with one-time harvesting.
The results of the present study revealed that the interaction effect of intercropping,
earthing-up and harvesting period of sweetpotato would be effective in the management
of C. puncticollis reducing the infestations and other associated damage.
Therefore, intercropping sweetpotato with maize/haricot bean, earthing-up two
or three times and harvesting of sweetpotato on time at its physiological maturity
can be a promising integrated management tools to control C. Puncticollis
in sweetpotato production systems in Eastern Ethiopia.
This study was supported by Potato and Sweetpotao Bio-innovate Consortium Project
(Addis Ababa University) and JUCAVM (Jimma University College of Agriculture
and Veterinary Medicine). The authors greatly acknowledge these two institutions
for their financial support and Haramaya University for providing research site
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