Screening chemical pesticides for the management of sweet potato weevil, Cylas puncticollis (Bohemann)

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1 IJAAR 1 (2013) ISSN Screening chemical pesticides for the management of sweet potato weevil, Cylas puncticollis (Bohemann) Alehegne Taye 1 and Eyob Tadesse 2 1 Culture and Tourism Bureau, Wolaita Zone, South Nation, Nationalities and People s Region State, Ethiopia. 2 Wondo Genet College of Forestry and Natural Resources, Hawassa University, P. O. Box 128, Shashemene, Ethiopia. Article History Received 22 May, 2013 Received in revised form 19 June, 2013 Accepted 24 June, 2013 Key words: Chemical pesticides, Cylas puncticollis, Sweet potato, Marketable yield. Article Type: Full Length Research Article ABSTRACT Field study was conducted to screen the efficacy of some selected chemical pesticides and to evaluate the economic benefit of the respective chemical control, in South Ethiopia. Experimental design of Randomized Complete Block Design (RCBD) with 4 replications were laid out. The treatments were Chlorpyrifos 20 Ec (Dipping), Diazinon 60 Ec (Dipping), Endosulfan 35 Ec [Foliaire (FA)], Carbofuran 3G [Soil Application (SA)], Malathion 5D [Dusting (D)], Chlorpyrifos (Dipping) + Endosulfan (FA), Chlorpyrifos (Dipping) + Malathion (D), Carbofuran (SA) + Endosulfan (FA), Carbofuran (SA) + Malathion (D) and Control. Diazinon and Chlorpyrifos dipping were significantly superior in reducing weevil population and level of infestation on both stem and tuber than other insecticides tested singly (P=0.05). The two insecticides increased the marketable tuber yield by 247 and 232% over the Control with a net profit of 4,622 and 4,458 birr/ha, respectively. Stem treated with Chlorpyrifos + Endosulfan spray after 45 days of planting was the best insecticide combination in controlling Cylas puncticollis population and damage level throughout the cropping season with increased yield of 399% over the Control and highest profit of 7,434 birr/ha. Diazinon or Chlorpyrifos treatment with possible spray of Endosulfan after 45 days of planting was found to be the best chemical pesticides for the management of sweet potato weevil in Southern Ethiopia BluePen Journals Ltd. All rights reserved INTRODUCTION Sweet potato, Ipomoea batatas (Lam) is an advantageous crop that has high yielding potential and wide range of adaptability (Yen, 1982; Hill and Waller, 1988). It has a broad ecological and agro ecological adaptation (Woolfe, 1992). In addition, it resists a wide range of environmental conditions including considerable periods of drought (Anonymous, 1987). It is a staple crop and marketable vegetable in many part of the world (William et al., 1991; Woolfe, 1992). In Ethiopia, sweet potato covers hectares of *Corresponding author. eyobt78@gmail.com. land with annual production of 389,612 tones (Anonymous, 2002). Out of the total land devoted for sweet potato production in Ethiopia, about 95% is situated in Southern Ethiopia and it covers 44,688 ha of land with estimated annual production of 104,543 tons (Anonymous, 2004). Currently, its production is expanding to new drought prone areas of Northern Ethiopia as food security crop against drought (Emana, 1990). These days however, several problems limit sweet potato production. These include insects, diseases, rodents and other biotic and abiotic factors. Among insect pests, sweet potato butterfly and sweet potato weevil are found to be the most limiting factors in Southern Ethiopia

2 Int. J. Adv. Agric. Res. 49 (Abate, 1995; Ferdu, 1999). There are more than 270 insect pests and 17 mite pests listed out that limit its production in the world (Jansson and Raman, 1991). Sweet potato weevil, Cylas sp. are more important since the insect causes damage both in the field and in storage (Schmutterer, 1969); and as such even in low infestations render the tubers unfit for human or livestock consumption (Proshold, 1986; Sutherland, 1986; Ruben, 1989). In Ethiopia, losses attributable to Cylas puncticollis range from 20-75% and considered higher in areas where harvesting is delayed (Emana, 1987; Emana, 1990; Amanuel, 1994). What makes the situation worsen in Southern Ethiopia is that, subsistence farmers usually plant sweet potato as security and fall back when the food from cereals dwindle. During such time, farmers in this area mainly depend on sweet potato for two to three months every year. Therefore, the end of the rainy season (September and October) is the main planting period during which time, relatively larger plots are devoted to sweet potato production. Unfortunately, late planting (planting at the end of the rainy season) has been observed to be an insecure planting time because of very high weevil infestation and subsequently, yield losses (Alehegne, 2007). Several insecticides have been tested worldwide for the control of C. puncticollis; Sutherland (1986) listed 59 different chemical insecticides including botanicals of unknown chemical composition that were tested against the sweet potato weevil. These chemicals, which were applied as post plant foliar sprays, resulted in varying levels of control. The life stages of sweet potato weevils take place underground (but within the plants). Therefore, post-plant application of insecticide requires frequent applications to kill newly emerged adults. This is not cost effective for subsistence farmers. Pre-planting insecticide application by dipping cuttings in systemic insecticide kill weevils within the stem and can protect it for at least one month after planting (Talekar, 1991; Smith and Odongo, 2002). Attempts to screen chemical pesticides for the management of sweet potato weevil in Ethiopia indicate primiphos methyl and cypermethrin to be effective at Areka and Awassa (Emana, 1990; IAR, 1991). Resource poor farmers in Ethiopia stagger to manage the pest with some cultural practices that includes earthling up around the tubers (Anonymous, 1975; Emana, 1990). However, recent reports revealed that the pest aided by the present climate variability, its severity and subsequent yield loss has escalated in Southern Ethiopia where the crop is listed as the number staple food. Consequently, if there is need to plant sweet potato crop late to serve as a food security crop, it need to be protected to achieve better production and also to realize the purpose for which it is planted. Therefore this experiment was conducted to: 1.) Screen the efficacy of some selected insecticides against C. puncticollis; 2.) Recommend the time and methods of their application and; 3.) Evaluate the economic benefit of the respective chemical pesticides tested. MATERIALS AND METHODS Description of the study area Field experiment was conducted at Humbo Woreda, Wolaita zone, Southern Ethiopia, as it was reported to be the host spot area. Humbo is located at 6 47ˈ N latitude and 37 43ˈ E longitude. It is found 188 km South West of Hawassa (the capital of Southern Nation Nationalities People s Regional State). The dominant soil type of the area is deep reddish brown, fine to medium textured Dystric Nitosols. The altitude of the Woreda, where the study site is located ranges between m.a.s.l. and is categorized under both Kola (dry hot tropical climate) and Woinadega (moist to sub-humid warm sub-tropical climate) agro ecological zones. The annual average rainfall ranges between mm, and the annual mean temperature range from 21 to 27.5 C. The total area covered by sweet potato plant in this particular district (Woreda) is estimated to be 2866 ha (Anonymous, 2006). Experimental design Field experiment was laid out in randomized complete block design (RCBD) with 10 treatments and 3 replications using susceptible variety locally called Guntute. Plot size was 6 m 6 m with 60 cm 30 cm spacing between plants and rows, respectively; and treatments were tested in different mode of applications according to the type of pesticides. It included: vine/stem dipping, soil applications, foliar applications and dusting. The sets (dipping) were treated with Chlorpyrifos 20% Ec and Diazinon 60% Ec at the time of planting, at dilution rate of 1 ml per ml of distilled water and 1 ml per 200 ml of distilled water, respectively. The cuttings of sweet potato vine were immersed in the chemicals for 30 min at a rate of 1 L per 500 cuttings (Tesfaye, 2003). Soil application was done for the granular pesticide (Carbofuran 3G) on the 21st day after planting, at rate of 1.2 kg a.i./ha. applied manually all around the base of the plant. Foliar application with Endosulfan 35 Ec and dusting with Malathion 5D were carried out at 45 DAP (one day after planting). Endosulfan 35 Ec was diluted at the rate of 1 ml per 100 ml of distilled water and sprayed using manually operating knapsack. Malathion 5D was dusted manually on the foliar using a nylon mesh at the rate of 10 kg/ha. Details of insecticide treatments are

3 Taye and Tadesse 50 Table 1. Insecticide treatments as applied to experimental plots. T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 Insecticides tested Chlorpyrifos 20 Ec Set treatment (ST) alone Carbofuran 3G Soil Application (SA) alone Endosulfan 35 Foliaire Application (FA) Ec alone Malathion 5D Dusting (D) alone Chlorpyrifos (ST) + Endosulfan (FA) Chlorpyrifos (ST) + Malathion (D) Carbofuran (SA) + Endosulfan (FA) Carbofuran (SA) + Malathion (D) Diazinon 60 Ec (ST) alone Control shown in Table 1. The economic benefits of using insecticides were calculated as; Profit = [(yield from treated plot yield from untreated plot) x price of sweet potato] cost of insecticides and their application. Data collection C. puncticollis sampling Sampling of adult weevils using sweeping net was carried out six times at 30 days interval starting from 30 days after planting. At each sampling, ten sweeps were taken from each plot crossing diagonally. The mean population density of adult C. puncticollis captured was recorded and analyzed to verify for treatments significance of differences in weevil population. C. puncticollis density, extent of damage and percent of infestation on vines and root tubers Weevil population density (adult, pupae and larvae) in the tuber and vine of the respective treatments was recorded by taking three samples at one month interval, beginning from three months after planting. At each sampling, 15 plants from each plot at the inner rows were randomly selected and uprooted. Both the stem and roots of every sampled plant from the respective plots were dissected and the number of adult weevils, pupae and larvae were counted and recorded. The extent of damage on root tubers and stem (vines) was rated according to Rangi et al. (1994) and the percent of infestation on stem (vine) and tuber were calculated. C. puncticollis population at harvest At harvest ten medium-sized roots (~150 gm each) were collected from the central rows of each plot of the respective treatments and kept in mush bags at room temperature in the laboratory for 30 days. The emerging adult weevils were counted every day and removed from the bags to prevent oviposition. The total number of adult weevils that emerged from the respective treatments were summarized and the significant differences with respect to treatments were calculated using LSD, at P=0.05. Yield losses At harvest, root tubers from 1 m 2 area of central rows of the respective treatments plots were collected and separated into healthy and damaged ones. Then the weight of healthy and damaged tubers was recorded and expressed in tons/ha. The percent root tuber yield loss from the respective treatments was calculated. Data analyses The number of C. puncticollis, percentage of vine and root tuber infestation, extents of vine and tuber damages and yield losses were subjected to analysis of variance and post hoc analysis using LSD test at P=0.05 (SAS, 2000). All count and percentile data were transformed using log x+1, and arcsine transformations, respectively, before they were subjected to analysis of variance. RESULTS C. puncticollis count from ten sweeps during cropping season and ten medium sized tubers reserved at harvest C. puncticollis density was significantly reduced due to insecticides application as highest number of adult weevils were captured from untreated plot (4.00), (P=0.05). Next to the untreated control, the highest

4 Int. J. Adv. Agric. Res. 51 Table 2. Effect of different chemical pesticides on C. puncticollis population. C. puncticollis /10 Sweeps C. puncticollis /10 medium sized tubers (~ 150 gm each) Chlorpyrifos 1.67 ed ed Carbofuran 1.89 cd d Endosulfan 2.33 cb c Malathion 2.89 b b Chlorpyrifos + Endosulfan 1.00 gf 1.67 g Chlorpyrifos + Malathion 1.55 edf e Carbofuran + Endosulfan 1.11 edf ef Carbofuran + Malathion 1.78 cd ed Diazinon 0.89 g 5.00 gf Untreated Control 4.00 a a Standard error ( Coefficient of variation LSD (0.05) Means followed by the same letters within the columns are not significantly different according to LSD at P=0.05. weevils number was recorded from Malathion dusting (2.89) and Endosulfan foliar (2.33). The rest treatments were equal in total number of adult weevils captured during all sweep samplings (Table 2). Similarly, the maximum number of adult weevils emerged from the untreated control (38.33). On the contrary, the lowest weevil population emerged from root tubers collected from Chlorpyrifos + Endosulfan (1.67) followed by single application of Diazinon dipping (5.00). Among all treatments, root tubers collected from Malathion dusting harbored the highest adult weevil population (29.67) followed by Endosulfan foliar (23.33), (Table 2), (P=0.05). Effect of selected insecticides on weevil population in vine and root tubers Significantly lower number of C. puncticollis population in vine was recorded from insecticide treated plots when compared to the untreated control in all sampling periods. However, there was significant difference in weevil population count among treatments (P=0.05). For the purpose of comprehension, if we observe the mean total weevil population count/vine at six months after planting (6MAP), the highest weevil population was recorded from Malathion dusting plots (20.47) followed by Endosulfan (12.89). Among all single applications, Diazinon dipping (4.29), Chlorpyrifos dipping (5.31) and Carbofuran (6.11) showed similar performances; recording the least weevil population in the vine with respect to the combined applications: Chlorpyrifos + Endosulfan (3.56), Chlorpyrifos + Malathion (4.8), Carbofuran + Endosulfan (4.11) and Carbofuran + Malathion (5.78) (Table 3). Like that of vines, weevil population count in root tubers was found to be increasing throughout the sampling periods; however, significant difference among treatments was observed. The highest number of C. puncticollis population in root tuber was recorded from untreated plots (1.93, 7.3, and 30.22) for the four consecutive samplings (Table 4). On the other hand, root tubers sampled from all insecticide treated plots were free from weevils except for Malathion dusting (0.99, 2.5) and Endosulfan (0.59, 2.31), during the 1st and 2nd sampling, respectively. Root tubers collected from single applications of Chlorpyrifos, Diazinon and combined application of Chlorpyrifos + Endosulfan, Carbofuran + Endosulfan and Chlorpyrifos + Malathion were also free from weevil population up to five months after planting (5MAP) (Table 4). Diazinon dipping was the best in reducing weevil population in all samplings compared to the other single applications. Chlorpyrifos dipping also performed similar to diazinon in the last sampling, (5.31) and (4.29), respectively. At harvest, combined applications, Chlorpyrifos + Endosulfan and Carbofuran + Endosulfan were also similar to single application of Diazinon (P = 0.05). Malathion treated plots recorded the highest number of weevil population in root tubers (20.47) followed by Endosulfan (12.89) among all insecticidal treatments (Table 4). Effect of selected insecticides on damage extent in vines and root tubers All insecticide treatments resulted in lower vine and root tuber damage level when compared to the untreated check throughout all sampling periods (Tables 5 and 6). Significantly, the least vine damage level was observed in Chlorpyrifos + Endosulfan and Carbofuran + Endosulfan (0.22), (0.25), respectively.

5 Taye and Tadesse 52 Table 3. Effect of selected chemical pesticides on population of sweet potato weevil on vines (stem). Mean number of C. puncticollis per stem 3MAP 4MAP 5MAP 6MAP Larva Pupa Adult Total Larva Pupa Adult Total Larva Pupa Adult Total Larva Pupa Adult Total Chlorpyrifos 0.14 e 0.04 d 0.00 d 0.18 e 0.81 bc 0.42 c 0.22 cd 1.46 c 1.93 d 0.98 de 0.44 de 3.36 def 2.69 d 1.49 d 1.13 d 5.31 d Carbofuran 0.29 d 0.05 d 0.06 c 0.40 d 1.02 b 0.44 c 0.27 bc 1.73 c 2.10 d 1.18 d 0.78 d 4.06 d 2.91 d 1.62 d 1.58 d 6.11 d Endosulfan 0.36 c 0.11 c 0.11 b 0.59 c 1.03 b 0.99 b 0.29 b 2.31 b 3.29 c 1.73 c 1.22 c 6.24 c 5.51 c 4.43 c 2.96 c c Malathion 0.67 b 0.19 b 0.13 b 0.99 b 1.15 b 1.09 b 0.31 b 2.55 b 4.40 b 2.44 b 2.19 b 9.03 b 9.09 b 6.42 b 4.96 b b Chlorpyrifos + Endosulfan 0.02 f 0.00 f 0.00 d 0.02 g 0.18 d 0.07 d 0.02 e 0.27 d 0.64 f 0.20 g 0.18 e 1.02 h 2.11 d 0.80 d 0.64 d 3.56 d Chlorpyrifos + Malathion 0.12 e 0.02 e 0.00 d 0.14 ef 0.80 bc 0.41 c 0.19 d 1.39 c 1.64 de 0.82 def 0.38 de 2.84 ef 2.49 d 1.27 d 1.11 d 4.87 d Carbofuran + Endosulfan 0.04 f 0.00 f 0.03 cd 0.07 fg 0.36 bc 0.11 c 0.08 e 0.55 d 1.40 def 0.65 efg 0.29 e 2.34 fg 2.22 d 0.89 d 1.00 d 4.11 d Carbofuran + Malathion 0.27 d 0.05 d 0.05 c 0.37 d 0.82 bc 0.44 c 0.27 bc 1.53 c 2.09 d 1.11 de 0.73 d 3.94 d 2.82 d 1.58 d 1.38 d 5.78 d Diazinon 0.06 f 0.02 e 0.02 cd 0.11 ef 0.60 bcd 0.11 d 0.06 e 0.78 d 0.98 ef 0.40 fg 0.22 e 1.60 gh 2.3 d 1.02 d 0.93 d 4.29 d Untreated control 1.00 a 0.29 a 0.64 a 1.93 a 4.42 a 2.04 a 0.89 a 7.35 a 7.67 a 3.31 a 2.71 a a a a 6.84 a a Standard error ( ) Coefficient of variation LSD (0.05) Means followed by same letters within columns are not significantly different according to LSD at P=0.05. MAP, Months after planting. On the contrary, the highest vine damage was observed in untreated check (2.53) followed by Malathion dusting (1.18) and Endosulfan (0.84), at 1st sampling (Table 5). Extents of vine damage continue to increase with similar trend in all treated and untreated plots in the 2nd and 3rd sampling. At harvest, vine damage level was significantly the lowest for Chlorpyrifos + Endosulfan (1.18) and it was equal with single application of Diazinon (1.78) and Chlorpyrifos (2.20) (Table 5). On the other hand, the highest vine damage was recorded from Malathion dusting (3.89) next to the control (4.62), P=0.05 (Table 5). Similarly, highest level of root tuber damage was observed for untreated plots in all the sampling periods (Table 6). Root tubers collected from all insecticide treated plots were free from damage except for Malathion dusting and Endosulfan at 1st, 2nd and 3rd samplings. Observation at last sampling reveals that Malathion dusting recorded the highest root tuber damage (2.31) next to the untreated control (3.11) followed by Endosulfan (1.79) at harvest and the rest insecticides recorded similarly lower root tuber damage levels, P=0.05 (Table 6). Effect of selected insecticides on percent infestation of vines and root tubers Vine percent infestation was observed to be reduced in all insecticides treated plots in all samplings compared to untreated control (Table 7). The highest level of infestations was recorded from control plot (33%) at 1st sampling time. The level of vine infestation percentage was relatively lower in all insecticide treated plots except Malathion (18%), followed by Endosulfan (13%). Infestation percentage was seen to increased at harvest and Malathion dusting recorded the highest (70%) followed by Endosulfan (56%); but infestation in the control was significantly the highest (85%) (Table 7) (P=0.05). Combined treatments, Chlorpyrifos + Endosulfan showed the least vine percent infestation (17%) while the rest showed relatively similar performance among each other (Table 7). Likewise, percent root tuber infestation was seen to be lower in all insecticidal treated plots than control in all the sampling periods (Table 6). Observation at 1st sampling showed highest root tuber infestation percentage on control (14%) (P=0.05). Root tubers sampled from all insecticide treated plots were free of infestation except for Malathion dusting (9%)

6 Int. J. Adv. Agric. Res. 53 Table 4. Effect of selected chemical pesticides on C. puncticollis population of root tubers. Number of C. puncticollis per plant 3MAP 4MAP 5MAP 6MAP Larva Pupa Adult Total Larva Pupa Adult Total Larva Pupa Adult Total Larva Pupa Adult Total Chlorpyrifos 0.00 d 0.00 c d 0.00 d 0.00 d 0.00 d 0.00 d 0.00 e 0.00 e 0.00 d 0.00 f 0.27 de 0.13 ef 0.73 d 1.13 ef Carbofuran 0.00 d 0.00 c d 0.00 d 0.00 d 0.00 d 0.00 d 0.24 d 0.10 d 0.13 d 0.47 d 0.44 d 0.49 d 0.84 d 1.76 d Endosulfan 0.07 c 0.00 c c 0.20 c 0.13 c 0.31 c 0.64 c 1.13 c 0.29 c 0.47 c 1.89 c 1.87 c 1.41 c 1.31 c 4.59 c Malathion 0.13 b 0.16 b b 0.38 b 0.24 b 0.38 b 1.00 b 1.82 b 0.42 b 0.64 b 2.89 b 2.57 b 1.71 b 1.72 b 6.00 b Chlorpyrifos + Endosulfan 0.00 d 0.00 c d 0.00 d 0.00 d 0.00 d 0.00 d 0.00 e 0.00 e 0.00 d 0.00 f 0.01 e 0.09 f 0.13 f 0.23 h Chlorpyrifos + Malathion 0.00 d 0.00 c d 0.00 d 0.00 d 0.00 d 0.00 d 0.00 e 0.00 e 0.00 d 0.00 f 0.20 de 0.10 f 0.53 de 0.83 fg Carbofuran + Endosulfan 0.00 d 0.00 c d 0.00 d 0.00 d 0.00 d 0.00 d 0.00 e 0.00 e 0.00 d 0.00 f 0.04 e 0.09 f 0.26 ef 0.39 gh Carbofuran + Malathion 0.00 d 0.00 c d 0.00 d 0.00 d 0.00 d 0.00 d 0.20 de 0.09 d 0.02 d 0.31 e 0.30 de 0.34 de 0.78 d 1.43 de Diazinon 0.00 d 0.00 c d 0.00 d 0.00 d 0.00 d 0.00 d 0.07 de 0.00 e 0.00 d 0.07 f 0.04 e 0.02 f 0.20 f 0.27 h Untreated control 0.31 a a a 0.87 a 0.38 a 0.64 a 1.89 a 2.42 a 1.50 a 1.77 a 5.69 a 3.24 a 2.79 a 2.80 a 8.82 a Standard error ( ) Coefficient of variation LSD (0.05) Means followed by same letters within columns are not significantly different according to LSD at P=0.05. MAP, Months after planting. and Endosulfan (5%). Single application of Diazinon and Chlorpyrifos and combined application of Chlorpyrifos + Endosulfan, Chlorpyrifos + Malathion performed best and were free of root tuber infestation up to the 3rd sampling. At harvest, single application of Diazinon performed best with the lowest percentage of tuber infestation (10%). The highest percentage of root tuber infestation was recorded from Malathion dusting (48%) followed by Endosulfan (40%) in all insecticides; on the other hand, Chlorpyrifos (12%) and Diazinon (10%) dipping and the combination of Chlorpyrifos + Endosulfan (7%) and Chlorpyrifos + Malathion (12%) exhibited the lowest root tuber percent infestation at harvest (Table 7), (P = 0.05). Yield loss and economic benefit of chemical pesticides tested Root tuber yield was seen to increase in pesticide treated plots than control (Table 8). The level of yield ranged from 11.7 to t/ha for control and combined treatments of Chlorpyrifos + Endosulfan, respectively. Among singly applied pesticides, Chlorpyrifos and Diazinon dipping showed 247 and 232% yield increments respectively, the over control (Table 8). Least root tuber yield was observed in Malathion dusting; nevertheless, increased yield of 24% was obtained compared to the control. The highest root tuber yield was obtained from combined treatments of Chlorpyrifos + Endosulfan (26.85 t/ha) which improved the yield by 399% (Table 7). The current price of sweet potato is assumed to be approximately 40 Birr/quintal. Accordingly, the highest economic benefit (profits) was obtained from combined application of Chlorpyrifos + Endosulfan (7434 birr/ha). Among single applications, Chlorpyrifos (4622 birr/ha) and Diazinon (4458 birr/ha) recorded the highest profit. The lowest profit was obtained from Malathion dusting (6 birr/ha). Considering cost of insecticides, Diazinon was found to be the cheapest while Malathion dusting + Carbofuran

7 Taye and Tadesse 54 Table 5. Extent of damage on vines (stem) and root tubers treated with different chemical pesticides. Stem Tuber 3MAP 4MAP 5MAP 6MAP 3MAP 4MAP 5MAP 6MAP Chlorpyrifos 0.40 d 1.11 c 1.39 d e 2.20 de 0.00 d 0.00 d 0.00 d 1.14 d Carbofuran 0.47 d 1.29 c 1.51 d 2.40 d 0.00 d 0.00 d 0.27 c 1.37 d Endosulfan 0.84 c 1.69 b 2.41 c 3.13 c 0.18 c 0.47 c 0.89 b 1.79 c Malathion 1.18 b 1.64 b 3.31 b 3.89 b 0.29 b 0.69 b 0.98 b 2.31 b Chlorpyrifos + Endosulfan 0.22 e 0.40 d 0.52 e 1.18 f 0.00 d 0.00 d 0.00 d 0.96 d Chlorpyrifos + Malathion 0.40 d 0.62 d 1.37 de 2.20 de 0.00 d 0.00 d 0.00 d 1.04 d Carbofuran + Endosulfan 0.25 e 0.58 d 1.30 de 1.78 ef 0.00 d 0.00 d 0.02 d 0.96 d Carbofuran + Malathion 0.42 d 1.26 c 1.41 d 2.37 de 0.00 d 0.00 d 0.24 c 1.15 d Diazinon 0.40 d 0.56 d 0.83 de 1.78 ef 0.00 d 0.00 d 0.04 d 0.50 e Untreated control 2.53 a 2.93 a 4.20 a 4.62 a 0.36 a 0.84 a 1.67 a 3.11 a Standard error ( ) Coefficient of variation LSD (0.05) Means followed by the same letters within columns are not significantly different according to LSD at P=0.05. MAP, Months after planting. Table 6. Percentage infestation on vines and root tubers treated with different insecticides. Percentage infestation Stem Tuber 3MAP 4MAP 5MAP 6MAP 3MAP 4MAP 5MAP 6MAP Chlorpyrifos 6 de 17 cde 18 d 31 d 0.00 d 0.00 d 0.00 e 12 f Carbofuran 7 d 20 bcd 21 d 42 d 0.00 d 0.00 d 7 d 31 d Endosulfan 13 c 26 bc 37 c 56 c 5 c 9 c 18 c 40 c Malathion 18 b 27 b 52 b 70 b 9 b 15 b 21 b 48 b Chlorpyrifos+ Endosulfan 3 e 6 f 9 d 17 e 0.00 d 0.00 d 0.00 e 7 f Chlorpyrifos + Malathion 6 de 15 def 17 d 30 d 0.00 d 0.00 d 0.00 e 12 f Carbofuran + Endosulfan 4 de 8ef 17 d 30 de 0.00 d 0.00 d 1 e 21 e Carbofuran +Malathion 6 de 18 bcde 19 d 41 d 0.00 d 0.00 d 6 d 28 de Diazinon 6 de 9 ef 15 d 304 de 0.00 d 0.00 d 2 e 10 f Untreated control 33 a 51 a 69 a 85 a 14 a 27 a 32 a 57 a Standard error ( ) Coefficient of variation LSD (0.05) Means followed by same letters within the columns are not significantly different according to LSD at P=0.05. MAP, Months after Planting. the most expensive (Table 7). Occurrence of other insect pests in various insecticides treated plots While conducting the research, striped weevils Alcidodes dentipes (Olivier) and clear winged moths Synanthedon dasysceles (Bradley) were retrieved from the stem. Among singly applied insecticide, Malathion dusting exhibits the highest populations of stripped weevils (5.00/plant) while Diazinon had the lowest. From the combined treatments, Chlorpyrifos + Endosulfan had lowest population at 4, 5 and 6 MAP, which was similar to single Diazinon application (Table 8). DISCUSSION For individually applied treatments, Diazinon and

8 Int. J. Adv. Agric. Res. 55 Table 7. Yield loss and the respective economic benefit of selected chemical pesticides tested against sweet potato weevil. Total tuber yield (t/ha) Yield of healthy tubers (t/ha.) Damaged tuber (t/ha.) Increased yield over control (t/ha.) Increased yield over control (%) Cost of insecticides and their application (birr) Market price (birr) Chlorpyrifos ab b 2.24 c Carbofuran b bc 5.43 ab Endosulfan b 7.17 c 4.96 ab Malathion b 6.20 c 5.79 ab Chlorpyrifos+ Endosulfan a a 1.83 c Chlorpyrifos+ Malathion b b 2.11 c Carbofuran + Endosulfan ab b 4.44 b Carbofuran + Malathion b bc 4.77 b Diazinon ab b 1.91 c Untreated control b 5.01 c 6.70 a Standard error ( ) Coefficient of variation LSD (0.05) Means followed by same letters with in columns are not significantly different according to LSD at p=0.05. MAP, Months after planting. Net Profit (birr/ha) Chlorpyrifos stem dip were found to be the best insecticide in bringing down weevil population, vine and root tuber damage extent and percentage infestations, which was similarly reported by Tesfaye (2003). On the contrary, single spraying of Endosulfan and Malathion dusting failed to reduce weevil population; percentage root tuber and vine infestation; and extent of damage on vine and tubers, which is in line with the results of Teli and Salunke (1994) and Mistric (1955). Frequent heavy rains following spraying and dusting activities might be the reasons for the insecticides failure as reported by Dalvi et al. (1992). The combined treatments: stem dipping with Chlorpyrifos + Endosulfan spraying was the best in controlling weevil population and the subsequent vine and root tuber damage, followed by Carbofuran soil application + Endosulfan spraying. Teli and Salunke (1994) and Dalvi et al. (1992) had also reported similar result. Similarly, Carbofuran applied alone or along with neem cake had revealed a satisfactory results in reducing percentage root tubers damage (Pillai et al., 1981). Maximum healthy tuber yield was recorded on Stem dipping with Chlorpyrifos followed by Endosulfan foliar application (Dalvi et al., 1992). On the other hand, maximum healthy root tuber yield was reported from plots treated with Carbofuran 3G, applied at the rate of 1 kg a.i./ha. Highest yield increment of 399% over control was recorded in stem dipping with Chlorpyrifos + foliar application of Endosulfan, and least yield only 23.75% over the control was recorded in Malathion dusting alone. In view of this, highest profit was recorded in stem dipping with Chlorpyrifos + Endosulfan foliar application (7434 birr/ha) followed by single application of Chlorpyrifos (4622 birr/ha) and Diazinon (4458 birr/ha). Based on the price of sweet potato and cost of the insecticides, managing sweet potato weevil with these pesticides at the recommended rate and mode of application has been revealed to be economical and profitable. Therefore, if sweet potato planting is a must during late rainy seasons when weevil infestation is found to be aggressive, stem dipping with Chlorpyrifos + Endosulfan foliar, or single application of Chlorpyrifos or Diazinon appears to be the best chemical pesticide recommended for sweet potato weevil management in South Ethiopia. Moreover, based on the criteria of cost, effectiveness and availability, Diazinon 60% Ec (effective concentration) is recommended to be

9 Taye and Tadesse 56 Table 8. Occurrence of other insect pests of sweet potato treated with different insecticides. Striped weevils/15 stems Clear wing moth/15 stems 4MAP 5MAP 6MAP 4MAP Chlorpyrifos 0.33 f 2.00 de 2.00 de 0.00 c Carbofuran 1.33 d 6.67 b 3.67 bc 0.33 b Endosulfan 1.67 c 6.67 b 4.33 ab 0.33 b Malathion 2.00 b 8.00 a 5.00 ab 0.00 c Chlorpyrifos + Endosulfan 0.00 g 1.00 e 1.00 e 0.00 c Chlorpyrifos + Malathion 0.00 g 1.67 de 2.00 de 0.00 c Carbofuran + Endosulfan 0.00 g 2.33 d 1.33 de 0.00 c Carbofuran + Malathion 1.00 e 4.00 c 3.6 bc 0.00 c Diazinon 1.00 e 1.67 de 1.00 e 0.00 c Untreated control 3.00 a 8.33 a 5.33 a 0.67 a Standard error ( ) Coefficient of variation Lsd (0.05) Means followed by same letters in the columns are not significantly different according to LSD at P=0.05. 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