Field Efficacy of Neem Seed Oil and Diazinon in the Management of Sweetpotato Weevil , Cylas puncticollis ( Boh . ) in South-eastern Nigeria

The efficacy of the preand post-planting application of neem seed oil (NSO) or/and diazinon and time of harvest for the control of sweetpotato weevil, Cylas puncticollis (Boh.) was evaluated during the 2009 and 2010 cropping seasons. Forty-eight plots of 3 x 4 m each were demarcated and 240 sweetpotato (var. TIS2532.op.1.13) vine cuttings were dipped into 0, 30 and 50 mls NSO and 30 mls of diazinon (60 EC) mixed in 2 and 10 litre soapy water, respectively and kept for 30 mins before planting. Similar treatment was also basally applied post-planting at 1MAP. In 2009, results showed that treatments (NSO or diazinon or their combinations) did not significantly influence percentage plant stand at harvest in both years, but lower percentage plant stands were obtained at 6 MAP (47.72%) than at 5MAP (56.87%). Significantly higher mean total weights of 34.44 t/ha was obtained in 2009 and 8.99 t/ha in 2010 respectively. Similarly, yield ranged from as high as 53.69 t/ha at 5MAP in 2009 to as low as 5.51t/ha at 6MAP in 2010. Significantly lower attributes of C. puncticollis as a pest were obtained in 2009 than in 2010. Treatments significantly suppressed root damage from 68.59% (Pr0Pt0) to 44.38% (Pr3Pt3) at 5 to 6MAP. Time of application was significant as 50 mls of NSO applied preand post-planting gave highest (35.30%) control of the pest at harvest.


Introduction
Sweetpotato, Ipomoea batatas L. (Lamarck) is a low labour, low cost and low risk crop (Kennedy, 2002), with the potential for yield improvement and year-round availability (Edison et al., 2009).It can be eaten boiled, fried or roasted and converted to canned or pureed form to enhance the shelf life.Its vines and leaves are used for human and animal consumption (Kimber, 1972).Recently, the crop has come to be appreciated as high fibre food and as green vegetable because of high levels of vitamin A, vitamin C, iron and potassium (Loebensein, 2009).The clamour for orange-fleshed sweetpotato (OFSP) is due mainly to the nutritional and health advantages in its high vitamin A content.
Despite the importance of sweetpotato to the subsistence economy, growth in output in the last three decades was accounted for by increase in land area (Ojiako, 2008) than by increase in yield.This could be attributed to a major biotic constraint to its production and utilization worldwide identified by farmers as Cylas species of which Cylas puncticollis (Boh.)(Coleoptera: Brentidae) and C. brunneus (Fab.) are confined to Africa (Chalfant et al., 1990;Lenne, 1991;Wolfe, 1991).Studies by Nwana (1979) and Ehisianya et al. (2011) have shown that they are a major production constraint in Nigeria, with losses due to sweetpotato weevil damage range from 1-100%.Thematic survey of sweetpotato production, utilization and marketing in the South Eastern Nigeria shows that farmers grow the crop twice a year and practice the so-called in-ground storage and piece-meal harvesting (Ehisianya et al., 2011).This enables the presence of the crop in the field throughout the year, thereby providing a suitable environment for the weevil population growth.The pests infest all plant parts; roots, stems, foliage, and flowers seeds (Edison et al., 2009).Larval feeding in the stems at times can be substantial; the role of this feeding in yield reduction has produced contradictory results (Lema& Hahn, 1987;AVRDC, 1991), but extensive damage to root (Stathers et al., 2003) can lead to a complete loss.
Progress in breeding weevil resistant cultivars has been slow because the heritability of the trait (Kreuze et al., 2009) is extremely low.Consequently, varieties with immunity or a high level of resistance are not yet available (Collins et al., 1991) despite the progress in finding weevil resistant components in some varieties (Stevenson et al., 2009).
Reports show that chemical control in the field can reduce weevil populations (Muruyanda et al., 1986: Edmunds et al., 2008), although chemical insecticides may not adequately control weevils because of the cryptic nature of immature developing within vines and roots.Several other factors such as their limited flying activity which implies that the insect is carried from place to place via movement of plant materials; their host specificity of the genus Ipomoea, weevil preference for oviposition in the absence of root, in older vines and their characteristic mode of entry and damage to plant, make sweetpotato weevils vulnerable to effective control by integration of simple cultural practices such as clean cultivation, the use of tender vine cuttings for planting a new crop and timely planting and prompt harvesting to avoid dry period.According to Smit (1997), the practice of piecemeal harvesting has a positive effect on the control of weevil infestation.
Diazinon (Diazol 60 EC) is one of the world widely used insecticide for agricultural pest control (U.S.G.S, 2001) and it is classified as moderately toxic, depending on the formulation (Extension Toxicology Network, 1996).Neem-based pesticides are easy to prepare, cheap and highly effective, providing a long term protection (Emosairue & Ukeh, 1996;Umoetok et al., 2009) to plants against pest and constitute an important source of pesticide (Brahmachari, 2004) for economically poor third world country farmers.
Only in the last decade has the crop been the focus of an intense, coordinated, global effort to realize its full potential as a source of food, feed, processed products, and income for millions of small farmers and low-income consumers in Africa, Asia, and Latin America.The objectives of this study were to determine the efficacy of integrating neem seed oil and Diazinon, and time of application in the management of C. puncticollis.

Materials and Methods
Field trial was conducted to determine the efficacy of diazinon and neem seed oil for the control of C. puncticollis at the research farm of National Root Crops Research Institute (NRCRI), Umudike (Latitude 05° 29°N, Longitude 07° 33°E and Altitude 122m above sea level and altitude 1231.6mabove sea level) during the 2009 and 2010 cropping seasons.The trial was laid out in a randomized complete block design with sweetpotato cv.TIS2532.op.1.13,neen seed oil (0, 30 and 50 mls), and diazinon (30 mls).Treatments were mixed in 2 and 10 litres of soapy water, respectively and replicated three times.Preparation of soap emulsifier was done by weighing and dissolving 20g of toilet soap (Ajayi et al., 1999) in 5 litres of water.Ten vines were planted on a ridge of 3 m with two ridges in a plot at a space of 0.3m between plants, 2 m between plots and 2 m between blocks.Weeding was done manually at 4WAP and rouging at 8 and 10 WAP after which Fertilizer (NPK,15:15:15) was applied at the rate of 400 kg/ha.Banded application method was used for post-treatments because Chalfant et al. (1987) reported that it provide a better control.
At harvest, five and six months after planting MAP, roots were sorted and graded.The numbers and weights of total and marketable (≥100g) (Levett, 1993) roots (yield) at harvest were determined (ha -1 ).Count data such as C. puncticollis adults, immature and total progeny numbers were transformed to square root values, whereas data in percentages were transformed to arcsine values.This was done to improve the normality of variable (variance stability) after which data obtained were subjected to analysis of variance using SAS GLM procedure.Treatment means were separated using Sudentized Newman Keul's (SNK) test (P=0.05).
Mean number of roots/hectare = (X 2 divided by X 1 ) multiplied 33333,333 Mean weight of roots/hectare (tonnes) = [(W 1 divided by X1) multiplied 33333, 333] divided by 1000 Where, 0.3x1m = 33333,333 plant per hectare, X 1 = number of plant per plot at harvest, X 2 = number of roots per plot at harvest and W 1 = weight of root per plot at harvest.Percentage infestation = (number of infested roots per plot divided by the number of total roots harvested per plot multiplied by 100), while percentage damage = (weight of infested roots per plot divided by weight of total roots harvested per plot multiplied by 100).
Severity of root (n=5) damage per plot was assessed using a five-point score, where 1 = 0%; 2 = 1-25%; 3 = 26-50%; 4 = 51-75%; 5 = ≥75% (Stathers et al. 2003).3).Analysis of variance showed no significant (P ≤ 0.05) treatment differences in the mean C. puncticollis severity score of roots at 5MAP in both years.At 6MAP, high score values were obtained from Pr0Pt0, Pr0Pt1, Pr0Pt3, Pr2Pt3, although not significantly different from Pr1Pt0, Pr2Pt1, Pr2Pt2, Pr1Pt1, Pr1Pt3 and Pr2Pt0, but significantly (P < 0.05) different from Pr0Pt2, Pr3Pt2, Pr3Pt1 and Pr2Pt3 (1.67).However, there were no significant (P > 0.5) differences in treated root scores in 2010 (Table 4).In all treated sweetpotato plots, there were no significant differences in number of C. puncticollis total progeny in roots harvested 5 and 6MAP in both years, excerpt in 6MAP of 2009.The control (Pr0Pt0) gave significantly (P < 0.05) higher number of total progeny than others, excerpt for Pr3Pt0, Pr3Pt2, Pr3Pt1 and Pr3Pt3 (Table 4).Table 5 presents the mean number of immature and adult C. puncticollis in treated roots harvested 5 and 6 MAP in 2009 and 2010.Roots harvested at 5MAP in both years showed no significant differences (P > 0.05) in the number of weevils.However at 6MAP in 2009, significant treatment differences (P < 0.05) were observed in the numbers of immature and adults in dissected roots.The control plots (Pr0Pt0) did not significantly differ from the others, but was significantly lower than plots treated pre-planting with 50 mls of NSO.The results based on combined data of two years showed significant differences (P < 0.05) in the percentage of root and damaged by C. puncticollis.Sweetpotato roots treated with Pr0Pt0 had 68.59%, followed by Pr1Pt0 (59.56%), while Pr3Pt3 (44.38%) was the least damaged, respectively.The trend in percentage control by treatments was similar to damage as Pr3Pt3 gave highest control of 35.30%, but was not significantly different from other, excerpt the Pr0Pt0 (0.00%) (Table 6).Data also indicated that there were significantly (P ≤ 0.05) higher numbers and weights of total (34.44 and 8.99t/ha) and marketable (33.66 and 8.74t/ha) sweetpotato roots harvested in 2009 than in 2010 (  Means within a column followed by the same letter do not differ significantly from each other (P > 0.05; SAS, PROC GLM, SNK).

Meteorological data obtained during the field trial is presented in
Data were transformed to arc sine √x prior to analysis.Means within a column followed by the same letter do not differ significantly from each other (P > 0.05; SAS, PROC GLM, SNK).
Data were transformed to arc sine √x prior to analysis.

Discussion
The results indicated that the year of planting, neem seed oil and diazinon applied as pre and post planting or their combinations had no significant impact on percentage plant stands at harvest of sweetpotato roots from a delayed (or minor) cropping season in Umudike, in 2009 and 2010.This is an indication that NSO and Diazinon or their combinations are not phyto-toxic at these dosages.However, sweetpotato yields (root number and weight) from treated roots were significantly higher in 2009 than in 2010.Lowe and Wilson (1974) reported that wide variability in sweetpotato root yield among individual plants of the same cultivar has been attributed to propagation material, environmental and soil factors and not the treatments.Yields ranged from 34.44t/ha to 8.99t/ha in 2009 and 2010 and 24.84t/ha to 18.60t/ha at 5 and 6MAP respectively.This result is in agreement with Akoroda (2009) who reported that sweetpotato yield ranged from 1-40t/ha in 3-7 months after planting.This may be attributed to rainfall, temperature and soil condition of the experimental sites in both years.Earlier study carried out by Gruneberg et al. (2005) showed that the highest mean root yields were obtained at a location with 600 mm rainfall in the growing season.On the other hand, lowest root yields were obtained at locations with very high rainfall and very low rainfall in the growing season.Consequently, Cylas puncticollis attributes (infestation and damage) of roots also increased significantly in 2010 when compared with 2009.This could be attributed to the residual effect of azadirachtin and may have been hastened by climatic factors (Jacobson, 1986).
Earlier study by Isman et al. (1990) concluded that the bioactivity of neem oil was dependent on its azadirachtin content.Furthermore, the high incidence of the weevil at various stages of development on the control plot indicated the high susceptibility of the sweetpotato variety (TIS2532.op.1.13)to weevil attack as reported by Anioke and Ogbalu (2003).However, the application of NSO and Diazinon treatments had a significant reduction in the sweetpotao weevil infestation and damage of roots.The precise effect of plant oils (Don-Pedro 1989) on gravid female beetles is still unclear.Nevertheless, oils (Singh et al., 1978;Credland, 1992) have been reported to cause the death of developing embryo through asphyxiation.However, the significantly lower number of the immature stages, adults (total progenies) despite the higher scores in 2010 could be attributed to exit of the weevil following it emergence from the roots.
Harvesting of the treated sweetpotato roots at 5MAP significantly increased yield when compared with 6MAP.This agrees with Onunka (2009) report that elite variety (TIS87/0087) matures at 5MAP when compared with the land races.Furthermore, the reduction in yield at 6MAP may be attributed to root dehydration caused by increased weevil tunneling, soil cracking, higher temperature and lower rainfall.Delayed harvesting to 6MAP also increased infestation especially as the weevils were more abundant and injurious during the drier growing season.
Fifty mls of NSO applied pre-and post-planting gave highest control (35.30%) of the pest at harvest, this corroborates the findings that neem products (Ogunwolu & Oddunlami, 1996;Lale & Mustapha, 2000;Ukeh et al., 2007) performed equally or sometimes better than some synthetics.It appears that neem seed oil could be a potential source of natural and low-cost insecticide to control sweetpotato pests.

Conclusion
The study revealed that the integration of neem seed oil and diazinon, applied singly or in combination was not phytotoxic to the crop and it significantly lowered C. puncticollis infestation and damage of sweetpotato roots in delayed production and thus, provided adequate protection of roots.Under minor rain-fed sweetpotato cropping, harvesting should be done at 5MAP to minimize damage by C. puncticollis in field.Furthermore, sequential harvesting may have reduced roots fresh weight, but may not reduce the root dry matter which is of major important to processors.
Percentage control = Percentage damage of check (Pr0Pt0) minus percentage damage of treatment divided by percentage damage of control) multiplied by 100 Treatments were: Pr0 =Pre-Planting Application of Control Pr1 = Pre-Planting Application of Diazinon EC (30 mls) Pr2 = Pre-Planting Application of Neem Seed oil EC (30 mls) Pr3= Pre-Planting Application of Neem Seed oil EC (50 mls) Pt0 = Post-Planting Application of Control Pt1= Post-Planting Application of Diazinon EC (30 mls) Pt2 = Post-Planting Application of Neem Seed oil (30 mls) Pt3= Post-planting application of Neem Seed oil EC (50 mls)

Table 1 .
Table 1 and showed that monthly rainfall in Umudike had a characteristic bi-modal rainfall distribution with peaks in major and minor farming seasons: May and October in 2009, June and August in 2010.The mean monthly temperature ranged from 26.0-29.0ºC in 2009 and 26.15-29.40ºC in 2010.Mean rainfall (mm), temperature (ºC) and relative humidity (%) of Umudike, Abia State during the field trial Pr3Pt3, but not with others.The trend for percentage damage was similar to that of infestation.As significant differences among treatments were only observed at 6MAP in 2009.High percentage damage to sweetpotato roots recorded in roots treated withPr0Pt0 (75.28),Pr0Pt1 (72.57),Pr1Pt0 (66.19) and Pr1Pt2 (61.85).Moderate damage were recorded on the other roots and they differ significantly from Pr3Pt3 (12.29) (Table

Table 2 .
Table7).In 2010, C. puncticollis attributes namely: damage (76.10 and 30.45) and severity score (3.75 and 2.55) values were significantly (P ≤ 0.05) higher than 2009, except total progeny numbers (137 to 86.2).The time of harvest showed no significant (P > 0.05) treatment differences in the mean number of total and marketable sweetpotato roots harvested at 5 MAP (47726.00 and 39208.00)and at 6MAP (46676.00and 38722.00),whereas, significantly lower weights of total and marketable sweetpotato roots were harvested at 6MAP (18.60 and 18.08t/ha) than at 5MAP (24.84 and 24.32t/ha).Sweetpotato roots harvested 5MAP was significantly lower when compared with roots harvested 6MAP in respect of root percentage infestation, severity score, adult and immature numbers respectively.Effect of treatment combinations and time of harvest on mean percentage of sweetpotato (var.TIS2532.op.1.13)plant stand at harvest in 2009 and 2010

Table 3 .
Effect of treatment combinations and time of harvest on mean percentage Cylas puncticollis infestation and damage to sweetpotato (var.TIS2532.op.1.13)roots in 2009 and 2010

Table 4 .
Effect of treatment combinations and time of harvest on mean Cylas puncticollis severity score ( a Data (Progeny) were transformed to square root (x+c) **0.5 prior to analysis.

Table 5 .
Effect of treatment combinations and time of harvest on mean number of immature and adult Cylas puncticollis in sweetpotato (var.TIS2532.op.1.13)roots in 2009 and 2010

Table 6 .
Effect of treatment combinations and time of harvest on mean percentage damage to sweetpotato (var.TIS2532.op.1.13)roots by Cylas puncticollis and control based on combined data from two years

Table 7 .
Effect of year and time of harvest on yield and Cylas puncticollis attributes of treated sweetpotato (var.TIS2532.op.1.13)roots based on combined data from two years Mean percentage infestation and damage are arcsine values, whereas mean C. puncticollis (Boh.) total progeny, adult and immature stages are square root values.