Abstract: Background and Objective: Sweetpotato is an important crop widely cultivated in many developing countries and insect pests are major production constraint. This study aimed to examine the response of different varieties to sweetpotato weevils (SPW) infestation under different ecological and cropping conditions. Materials and Methods: Ten sweetpotato plants were randomly harvested from each of 125 farms in 3 ecological zones. Leaves were examined for characteristic punctures, base of vines bisected for presence of tunnel and larvae and storage roots assessed for burrows. Results: Four sweetpotato weevils, Cylas puncticollis, Cylas formicarius, Cylas brunneus and Alcidodes spp. were identified with C. puncticollis as the pre-dominant species. Population density of SPWs and damage to foliage, vine base and storage root yield loss were significantly higher in transitional than savanna and semi-deciduous ecological zones. Local sweetpotato varieties did not show superior tolerance to SPW over improved varieties. The influence of cropping system on C. puncticollis was also not explicit. Conclusion: Sweetpotato in the transitional zone was more prone to SPW and driving factors must be further investigated. Local sweetpotato varieties did not exhibit higher tolerance to SPW over improved ones and therefore; increased adoption will enhance productivity and improve nutrition.
INTRODUCTION
Sweetpotato, (Ipomoea batatas (L.) is one of the major staple crops recognized as vital commodity for food security, particularly in the developing world where hunger and malnutrition are high1-3. It has short planting to harvest period (4-5 months) and is rich in vitamins (B1, B2, B3, B6, C, E, beta-carotene and folate), dietary fiber, minerals (Calcium, Phosphorus, Magnesium, Iron, Potassium and Zinc). Varieties such as; orange and yellow-fleshed have very high beta-carotene content to address deficiencies, especially among children3-5. Its potential as source of income for poverty alleviation is immense due to its wide use in culinary, starch, sweetening, bakery, salad cream and ice cream industries6.
Africa accounts for 10.6%, (14.3 million t) of global production with sub-saharan Africa producing3 9.9 million t. In Ghana, the crop grows well in all agro ecological zones, but it is not widely cultivated in all regions6-8. Current total production is 143,111 t with 76,594 ha under cultivation9.
There are few key needs faced by farmers in major sweetpotato producing countries. These include availability of quality planting materials, management of sweetpotato weevil Cylas spp. and development of staple-type with low sugar and high β-carotene content in tubers to address vitamin A deficiency 4,10.
Although different pest species attack the crop, sweetpotato weevils (SPW) are the most important and diverse, particularly in Africa11,12 . Sweetpotato yield losses attributed to SPW range13 between 2 and 50%. Tanzubil8 found sweetpotato weevil Cylas spp. and sweetpotato butterfly Acrea acerata as the most serious pests of the crop in northern Ghana although termites, millipedes, aphids, white flies and white grubs were also encountered. Cylas spp. occurred in 90% of surveyed farms causing 30.8 and 41.4% damage to vines and storage roots, respectively. Some studies have been conducted on SPW to contribute to understanding the biology and ecology across the country to help develop appropriate management practices8,14,15. There were gaps that need to be filled to achieving development of desired protocols to effectively manage the pest.
The purpose of this study, therefore, was to evaluate response of different varieties to sweetpotato weevils infestation under different ecological and cropping conditions. Findings in this study were expected to contribute in filling the knowledge gap in developing integrated management strategies against the pest.
MATERIALS AND METHODS
Study sites: This study was conducted between February and May, 2017 in 3 ecological zones, covering 9 political districts in southern Ghana, where sweetpotato is widely cultivated as staple crop7 (Fig. 1). The ecological zones were semi-deciduous forest, coastal savanna and transitional. The respective rainfall, maximum temperature and relative humidity for semi-deciduous forest, transitional and coastal savanna16 are 1450 mm, 30.5°C and 66.00%; 1300 mm, 31.5°C and 57.0%; 950 mm, 30.2°C and 74.0%.
Sweetpotato weevil sampling: A total of 125 farms, comprising a minimum of 3 farms per community were randomly selected. Coordinates and farm sizes were recorded using Garmin eTrex® 20 GPS device. Coordinates were used to map out the research sites (Fig. 1). Data on variety cultivated and type of cropping systems were taken.
| Fig. 1: | Map of Ghana showing ecological zones and sampling sites |
| Source: Field survey, 2017 | |
Ten sweetpotato plants were randomly sampled for SPW infestation and associated damage. Damage levels of foliage, vine base and storage root loss were scored on a scale of 1-5 where, 1 = 0%, 2 = 1-25%, 3 = 26-50%, 4 = 51-75% and 5 = 76-100%. Foliage damage was assessed by inspecting the leaves of selected plants for characteristic punctures resulting from SPW feeding activities. Sweetpotato plant cuttings of 10-15 cm long were also collected from the basal portion and bisected to assess the depth of damage caused by the SPW larvae as vine basel damage. Storage root loss was scored by visual assessment for burrows created by SPW and overall appearance.
At harvest, storage roots were pooled and a random sample of ten storage roots were selected from the pool. Selected storage roots were inspected and scored for SPW infestation and damage.
Statistical analysis: Regression analysis was performed to establish the relationship between weevil population and damage to foliage, vine base, storage root loss and storage root infestation rate. Analysis of variance was done and difference between means separated using Tukey at probability level of 95%. All analysis involving the number of weevils were log (x+1) transformed. However, they were back transformed before interpretation. All the statistical analysis were done using Minitab 15.
RESULTS
Four species of weevils, SPW; Cylas puncticollis (Fabricius). C. brunneus (Fabricius) and C. formicarius (Coleoptera: Curculionidae) and sweetpotato striped weevil Alcidodes spp. (Coleoptera: Curculionidae) were recorded (Fig. 2). The C. puncticollis was the predominant species. Alcidodes spp., was the least and occurred in only Afram Plains south district (3 farms) and Fanteakwa district (1 farm) all within the transitional ecological zone.
Relationship between SPW incidence and sweetpotato damage: The relationship between SPW population and foliage and basal vine damage and storage root loss presented in Fig. 3 was positive. Vine basal damage exhibited the strongest relationship between SPW and level of sweetpotato damage. The proportional rate at which tubers were infested also corresponded to levels of foliage damage, vine basal damage and storage root loss (Fig. 4). The strongest correlation was between vine basal damage and storage root infestation rate.
| Fig. 2: | Proportional occurrence of sweetpotato weevils species in surveyed areas |
Varietal susceptibility: Based on farmer assertions, a total of 12 sweetpotato varieties were found to be cultivated by the farmers. Of these, 5 varieties, comprising Afram plains, Purple flesh, Nagochi, Anago and Blueblue were encountered only once in the farmer’s field and were therefore; excluded in the analysis of varietal response to SPW (Table 1). The 3 most frequently cultivated varieties were Local, Shanshango and Sauti. Varieties with the highest and least number of the pest were Shanshango (2.82) and yellow flesh/Ogyefo (1.00). In terms of damage, the highest on foliage and vine basal were observed in Nkani while orange flesh recorded highest storage root loss.
Influence of ecological zone on SPW incidence: The SPW numbers and all recorded damage parameters were highest at the transitional ecological zones compared to semi-deciduous forest and coastal savanna zones (Table 2, 3). Although the SPW population density for the semi-deciduous forest was significantly higher than that of coastal savanna, the corresponding damages in the two zones were similar (Table 3).
Effect of cropping system on SPW infestation: While SPW population and storage root infestation rate were similar under both cropping systems, the monocropping system recorded significantly higher foliage damage and basal vine damage (p = 0.000), but lower storage root loss (Table 3).
| Fig. 3: | Linear relationship between number of sweetpotato weevil (SPW) and damage on different parts of sweetpotato plant |
Vine basal: r2 = 0.90, y = 1.383x+2.034, Foliage: r2 = 0.71, y = 0.7220x+1.631, Tuber: r2 = 0.64, y = 0.7059x+1.233, Damage scale of 1-5: 1 = 0%, 2 = 1-25%, 3 = 26-50%, 4 = 51-75%, 5 = 76-100% |
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| Fig. 4: | Relationship between damage levels of different parts of sweetpotato plant and storage root infestation rate by sweetpotato weevils (SPW) |
Vine basal: r2 = 0.70, y = 0.0065x+1.741, Foliage: r2 = 0.67, y = 0.00515x+1.316, Tuber: r2 = 0.56, y = 0,00858x+0.6342, Damage scale of 1-5: 1 = 0%, 2 = 1- 25%, 3 = 26-50%, 4 = 51-75%, 5 = 76-100% |
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| Table 1: | Sweetpotato weevil (SPW) infestation and damage to different varieties of sweetpotato |
Means in a column that share a common letter are not significantly different, L: Local, I: Improved | |
| Table 2: | Sweetpotato weevil (SPW) infestation and damage across different ecological zones in Ghana |
Means in a column that share a common letter are not significantly different | |
| Table 3: | Sweetpotato weevil (SPW) infestation and damage to sweetpotato within different ecological zones under different cropping systems |
Means in a column that share a common letter are not significantly different | |
DISCUSSION
This study recorded 4 SPW species C. punticollis, C. brunneus, C. formicarius and Alcidodes spp. contrary to two recorded by other surveys in southern Ghana14,15. This suggested that two more species C. formicarius and Alcidodes spp. exist in this region as compare to the earlier reports. This give credence to Agbessenou et al.15 argument that C. formicarius population is sparse due to its inability to adapt well to conditions in Africa. It is also possible that the Alcidodes spp. population is sparse just as C. formicarius may not have been able to establish itself as a formidable pest of sweetpotato in Ghana. These argument seems to be validated by Tanzubil8, who recorded C. formicarius infestation in northern Ghana.
The ecological characteristics has influence on abundance of SPW and therefore damage caused. Climatic conditions at the transitional zones seem to favor the pest as highest population density was attained in these areas. The coastal savanna was however expected to record higher SPW abundance due to the more extensive cultivation of sweetpotato among communities in this zone. As to whether SPW population dynamics is linked to climatic variables such as; precipitation, relative humidity and temperature need to be further investigated. Several insect pests attack both the vegetative and storage roots of sweetpotato11,12,17,18.
The results indicated that 7 of the 12 sweetpotato varieties cultivated in the surveyed areas were widespread. Authenticity of the varieties through phenotypic characteristics could however not be ascertained as indicated by Essilfie and Motey19. A comprehensive genetic assessment and characterization of these varieties must be conducted. Although several improved sweetpotato varieties (Okumkom, Santompona, Sauti, CRI Apomuden, CRI-Ogyefo, CRI-Histarch, Faara, CRI-Otoo, CRI-Patron, CRI-Bohye, CRI-Dadanyui, CRI-Ligri, CRI-Gavana and CRI-Mbofara) have been released to farmers by the CSIR-Crops Research Institute6,20, local varieties are still popular among farmers. Farmer preferences as with most local crop varieties are usually due to taste and resistant to pest and diseases. Thus, with the exception of ‘Local’ (a local variety) that gave significantly lower storage root loss as against the four improved varieties, there were no clear-cut superiority against pest build up on the plant and damage to foliage and basal vine. In fact, the most popular local variety, Shanshango, recorded the highest SPW population and was the second highest in severity of damage to storage root. The comparable tolerance levels between the improved and local varieties concur with results obtained by Koussoube et al.21 in Burkina Faso.
CONCLUSION
There was no clear-cut influence of cropping system on SPW attack on the crop as both systems recorded SPW densities and storage root infestation rate. However, the significantly higher damage to foliage and basal vine and lower storage root loss which occurred in monocropped farms may be attributed to the feeding pattern of the SPW. Prior to storage root formation, the pest feeds on the leaves and reproduces in the vine basal, but mainly feeds and reproduces in the storage roots, once formed. The results suggest that substantial proportion of the pest possibly continued their activities in the basal vine even after storage roots were formed, under monocropping.
SIGNIFICANCE STATEMENT
This study discovered ecological zone influences population dynamics of SPW with the transitional zone being more prone to the pest. This will help researchers to uncover underlying climatic factors influence on distribution of the pest. Resistance of more nutritious and high yielding improved sweetpotato varieties is comparable to local varieties and this augurs well for their promotional drive.