African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 9, Num. 1, 2001, pp. 165-174

African Crop Science Journal, Vol. 1, No. 9, March 2001, pp. 165-174

Code Number: CS01045

ABSTRACT

Studies were carried out in the field at Serere Agricultural and Animal Production Research Institute (SAARI), Eastern Uganda, to establish whether the existing sweetpotato germplasm in Uganda has cultivars resistant to the sweetpotato weevils, Cylas spp. The trials were conducted during the two growing seasons of 1997. Root size was the only sweetpotato rooting characteristic that significantly influenced tuber infestation (P=0.009) and damage (P=0.049). Root size was positively correlated to tuber infestation by Cylas spp. (Y=0.0456x + 21.206; R2 = 0.2731) and negatively correlated to damage by Cylas weevils (Y=-0.027x + 29.684, R2=0.1647). A laboratory study demonstrated the effect of temperature on oviposition, survival and developement of Cylas puncticollis (Boheman). The variety 'Tanzania' that was used as the susceptible check registered the highest weevil emergence under both wet and dry weather conditions while varieties 'Kasira' and 'Akere-Ikokolak' registered the least emergence of adult C. puncticollis. These results indicate that the latter two varieties posses considerable levels of resistance to sweetpotato weevils.

Key Words: Germplasm, resistance, sweetpotato weevils, Uganda

RÉSUMÉ

Des études ont été conduites en champ et au laboratoire à l'Institut de Recherche de Production Animale et Agricole de Serere (SAARI), dans l'Est de l'Uganda, pour déterminer si le germoplasme de la patate douce en Uganda possède des cultivars résistants aux charançons, Cylas spp. Des essais ont été conduits pendant les saisons culturales de 1997. La taille du tubercule fut la seule caractéristique du tubercule qui a influencé l'infestation du tubercule (P=0.009) et les dommages (P=0.049). La taille du tubercule était positivement corrélée avec l'infetation du tubercule par Cylas spp. (Y=0.456X+ 21.206, R2=0.2731) et négativement correlée avec les dommages causés par les charançons du Cylas (Y=-0.027X+29.684, r2=0.1470). L'étude au laboratoire a démontré l'effet de la température sur l'ovoposition, la survie et le développement du Cylas punticollis (Boheman). La variété Tanzanie qui était utilisée comme témoin a enregistré l'émergence la plus élevée des charançons dans les conditions sèches et pluvieuses alors que les variétés Kasira et Akere-Ikokolak ont enregistré les plus faibles émergence de charançons d'adultes de C.punticollis. Ces résultats ont montré que les deux dernières variétés possèdent des niveaux de résistance considérables aux charançons de la patate douce.

Mots Clés: Germoplasme, résistance, les charançons de la patate douce, Uganda

INTRODUCTION

Sweetpotato, Ipomoea batatus (L) Lam., is an important food security crop in many of the poorest regions of the world. In East Africa, sweetpotato is grown both for home consumption and to supplement household income (Sengooba, 1994). In Uganda, many communities have become increasingly dependant on sweetpotato, especially where the major staple food cassava has been decimated by the Cassava Mosaic Virus (CMV). The root tubers are usually harvested piece-meal over a period of 2 - 7 months, providing a flexible source of food for the household. In areas with long and hot dry seasons, farmers harvest, chip, and dry the sweetpotato roots in order to preserve and store the crop.

Sweetpotato weevils, Cylas spp. (Coleoptera: Curculionidae) are a major constraint to sweetpotato production and utilisation worldwide. In Uganda, crop losses of up to 73% due to sweetpotato weevils and plant diseases that often follow weevil attack have been reported (Smit, 1997). Because of the crop losses they cause, there is urgent need to institute interactive measures to reduce and control sweetpotato damage by Cylas spp. One such measure could be the use of resistant varieties.

A number of field trials suggest physical traits that allow sweetpotato to avoid damage such as rooting depth, arrangement, and root size and shape, as playing important roles in conferring resistance to Cylas spp. in the field (Cockerham and Dean, 1947; Sing et al., 1987; Talekar, 1987). Laboratory investigation at the International Institute of Tropical Agriculature (IITA) also suggested that resistance in several Nigerian cultivars was a result of non-preference or antibiosis (Hahn et al., 1989), relating to root chemical composition rather than morphology. However, no detailed studies have been conducted to examine the influence of rooting characteristics on the level of tuber infestation and damage by sweetpotato weevils, Cylas spp., among the many sweetpotato varieties available in Uganda.

Given the potential of rooting characteristics and other resistance mechanisms in the management of sweetpotato tuber infestation and damage by these pests, the present study was instituted in order to assess the infestation and damage levels of the sweetpotato weevils (Cylas spp.) in relation to sweetpotato rooting pattern. A laboratory experiment was also set up in September and December 1997 to identify the existence of any antibiosis type of resistance to Cylas puncticollis in the 24 sweetpotato lines and varieties, and to relate such resistance to the variation in infestaion and damage expressed by the lines and varieties in the field.

MATERIALS AND METHODS

The studies were carried out at Serere Agricultural and Animal Production Research Institute (SAARI), located in eastern Uganda (33027'E, 1032'N and 1,140 m above sea level). The area receives an annual rainfall ranging from 1110 to 1450 mm, which is bi-modally distributed, with peak periods occurring from the months of April to May and August to September, and a long dry spell from November to March. The maximum and minimum temperatures of the area range from 27 to 33oC and 17 to 18°C, respectively.

Twenty-four sweetpotato accessions used in this study were selected from the Sweetpotato Programme of Namulonge Agricultural and Animal Production Research Institute (NAARI) and from farmers in Soroti district. The eleven lines and three varieties obtained from NAARI were #69, #148, #178, #192, #202, #218, #271, #277, #282, #316, #324, 'Sowola' (#389A), 'Bwanjule' (#29), 'Tanzania' (SPN/O in Tanzania). Ten local sweetpotato varieties were selected from farmers' fields in Soroti district. The varieties selected were: 'Odopelap', 'Opejo', 'Esamiat', 'Epuramonjong', 'Kasira', 'Anyara', 'Emadirait', 'Okunguru-Dere', 'Akere-Ikokolak' and 'Ecuru'. These varieties were described by farmers as either less susceptible to weevils or better for in-ground storage than the other varieties grown in the region.

The trials were set up in May (first rainy season) and July (second rainy season) of 1997. The experimental plots measured 6 x 3 m, and were arranged in a randomised complete block design (RCBD) with four replications. Planting was done on 3 m long and 0.5 m wide ridges, made one metre apart. The vine cuttings were planted singly 30 cm apart along each ridge, with each plot having six ridges. The outer ridges in each experimental plot were used as guard rows.

Re-mounding of the ridges was done at 6 and 12 weeks after planting, and weeding was carried out whenever necessary until harvest. No irrigation, fertilisers or insecticides were applied, and this ensured that the research was carried out under conditions similar to farmers' fields in Uganda.

Data were taken at 4 and 6 months after planting (MAP) since maturity period of sweetpotato lines/varieties vary within that period. The four inner ridges of each plot were used for data taking, i.e., two at each time of data collecting.

To determine varietal rooting characteristics and weevil damage, five plants per plot were selected at random along the two ridges to be harvested and all the tubers on each of the plants were carefully exposed. Measurements of rooting depth, root size, root length and root neck length were taken using a tailor's tape.

Given that Cylas spp. does not burrow far through soil (Jayaramaiah, 1975), it was hypothesised that the depth of storage roots within the soil, and therefore the distance that an adult Cylas spp. weevil would need to burrow through the soil to reach a root, would be critical for infestation. For five selected plants per plot, the actual vertical and horizontal positions of roots in relation to surface of the ridge (Fig. 1) were measured and the shortest distance from the soil surface to the root was calculated. This was used to obtain the 'shortest weevil distance' (RD).

To determine the shortest weevil distance (RD) (the parameter that reflects how accessible the roots were to infesting Cylas spp. weevils), the vertical (V) and horizontal (H) distances were measured. The vertical distance was that from top of the ridge to top-most part of each root while the horizontal distance was that from the side surface of the ridge to closest part of the root (Fig. 1). Using Pythagoras' Theorem, the shortest distance between the top of the ridge and the point where the horizontal line intersects the side surface of the ridge (R) was calculated using the measured V and H values as R2 = V2 + H2, and hence R=(V2+H2)1/2.

The shortest weevil distance (RD) was determined from the values of V, H and R, and was calculated using the equation RD = (V * H)/R. Using Figure 1, the shortest weevil distance RD was calculated as follows:

Area of triangle AOB = 1/2 AB * OB = 1/2 H * V
Also area of triangle AOB = 1/2 AO * CB = 1/2 R * RD
Therefore, 1/2 R x RD = 1/2 H * V, and RD
= (H * V)/R.

Root neck length, which was the distance between the top of the root tuber and the point of attachment to the mother vine, was measured by stretching the root neck along a tape measure. Root tuber length was also measured from one end of the root to the other. Root tuber girth was derived from measurement of the circumference at three different points along the root tuber, namely the middle point and then the middle of each half. From this, an average root girth measurement was calculated. The root tuber size index was then obtained by multiplying the average root girth by the root length.

After measuring the rooting characteristics, all plants on the two sample ridges were removed and counted. The root tubers on these plants were then dug out, counted and weighed. The total tuber weight was expressed in t ha^-1.

The tubers were later separated into weevil infested and non-infested groups and counted. The weevil-infested tubers were expressed as percentage of total number of tubers obtained from an area of 6 m2.

Yield losses due to the weevils were determined by measuring the total weight of all the tubers, the weight of the Cylas damaged tubers, and then the weight of the remaining edible portion of the Cylas infested tubers after the damaged parts were cut off. Yield losses due to Cylas weevil damage were then expressed as percentages using the equation:

Yield loss =	Total tuber weight - clean weight x 100
	Total tuber weight

Statistical Analytical Systems (SAS) and Microsoft Excel computer packages were used to analyse data obtained and for graphical presentation, respectively. Microsoft Excel computer package was also used to carry out regression analysis and lines of best fit were drawn to establish the relationship between sweetpotato rooting characteristics and infestation and yield loss.

Given the variable correlations between rooting characteristics and damage in the present study, a study was conducted in the laboratory at Serere where the 24 sweetpotato accessions studied in the field were subjected to infestation in the laboratory to evaluate their relative resistance to C. puncticollis. The procedure used by Magenya and Smit (1994) at the International Centre for Insect Physiology and Ecology (ICIPE), Kenya, was followed, with the variety Tanzania being used as the susceptible check.

Five clean and weevil-free root tubers for each of the 24 sweetpotato lines and varieties were selected and each put in a plastic container (30 x 15 x 15 cm) with perforated lid immediately after harvesting to avoid prior weevil infestation and oviposition. Root tubers of uniform sizes were used for all the 24 accessions. The sweetpotato tubers were infested with C. puncticollis adults and these were monitored for survival and population development. Two sets of experiments were established, one in the hot, dry period (September to October 1997) and the other in the wet cooler period (December 1997 to January 1998).

The C. puncticollis used in the study were raised from colonies kept at Namulonge Sweetpotato Programme Entomology Laboratory under ambient conditions. To do so, non-infested fresh sweetpotato root tubers, of 'New Kawogo' variety of average size, were washed with cold water and dried in the sun for two hours. Adult weevils of both sexes were placed on the sweetpotato root tubers in a plexi-glass rearing container (45 x 30 x 50 cm) for three days. After infestation, the root tubers were transferred to other plexi-glass containes for incubation. Two weeks after start of exclusion, C. puncticollis adults were expected to be sexually mature and mated (Smit, 1997) and they were sexed.

Ten newly mated adult C. puncticollis females, were introduced at the same time in each of five replicate test containers used for each line/variety. The weevils were left in the containers for 5 days to lay eggs on the tubers, after which they were removed. The containers were then kept under ambient conditions for 45 days. It was assumed that because of similarity in age, all the weevils used had equal potential for laying uniform number of eggs in all the tubers. The differences that were observed in weevil emergence were therefore due to either non-preference and/or antibiotic factors existing in the sweetpotato lines and varieties tested.

For the five containers set up for each variety, monitoring for emerging weevils started after the 18th day of incubation. This was because studies done at ICIPE revealed that the development of C. puncticollis from egg to adult under laboratory conditions (27±1°C, 45±5% RH and 12 hour photophase) took 20-28 days (Skoglund and Smit, 1994; Smit, 1997). After the first weevil emerged, jars were checked daily for 18 days, and the weevils that emerged were counted and removed. Total and mean daily weevil emergence from tubers of each variety was used to assess their relative resistance to Cylas weevils. This approach has been used in previous studies (Smit, 1997). Using the individual and pooled results of both seasons, the 24 sweetpotato accessions were classified for relative susceptibility to C. puncticollis based on a 1 - 3 scale where, 1 = highly susceptible, 2 = moderately susceptible, and 3 = least susceptible. Mean C. puncticollis emergence for each variety were separated using Least Significant Differene (LSD) test at P<0.05 level of significance.

RESULTS

Relationship between rooting characteristics and tuber infestation. The relationships between sweetpotato rooting characteristics and tuber infestation and yield loss are given in Tables 1 and 2. Graphical presentation of the different relationships and influence are shown in Figures 2 and 3. The correlation data presented in Table 1 revealed that the size of the root tubers influenced their infestation by Cylas spp. Root size was positively correlated with tuber infestation, and the relationship was significant (P=0.009). This indicated that ovipositing Cylas weevils preferred or could easily access bigger tubers.

Root length and neck length were positively correlated to tuber infestation but the relationships were not significant (P>0.05). The shortest weevil distance to roots (RD) was negatively correlated to tuber infestation by Cylas weevils, but the influence was not significant (P>0.05). The relationships between different rooting characteristics and tuber infestation by Cylas weevils are presented in Figures 2a - d. Tuber infestation by Cylas weevils and root neck length, root length and root size were negatively correlated, although the correlation were weak and non-significant (P>0.05). Correlation between the shortest weevil distance to roots (thinnest root cover) and tuber infestation were negative (Y = -1.8803x + 50.158; R2=0.0375). The only significant relationship was that between root size and tuber infestation by Cylas (Y=0.0456x 21.206; R2 = 0.2731) at 1% probability level (Fig. 2C).

Rooting characteristics and yield loss due to Cylas spp. weevils. Relationships between yield loss due to Cylas weevils and rooting characteristics are presented in Table 2. Size of root tubers influenced their damage and hence yield loss due to Cylas spp. Yield loss was negatively correlated with root size (Y = -0.027x + 29.684; R2 = 0.1647) and the correlation was significant (P=0.049). This indicated that the bigger tubers infested by Cylas weevils were less damaged than the smaller ones. Many of the bigger tubers suffered only partial damage, with the proportion of damaged area being small.

Correlation of rooting characteristics and yield loss revealed that there was no significant (P>0.05) relationship between yield loss due to Cylas weevils and root and neck lengths. Nevertheless, root and neck lengths were negatively correlated to yield loss (Y = -0.5159x + 28.231; R2 = 0.0329 and Y = -1.1083x + 29.098; R2 = 0.1094, respectively). Yield loss due to Cylas weevils was also negatively correlated to the shortest weevil distance to roots (RD) (Y = -1.1083x + 25.199; R2 = 0.0255), but the influence was not significant (P>0.05).

The relationships between the different rooting characteristics and yield loss due to Cylas weevils are presented in Figures 3a - d. Yield loss due to Cylas weevils and root neck length, root length, shortest weevil distance to roots and root size were negatively correlated, although the relationships were weak and not significant (P>0.05). The only significant relationship was that between yield loss due to Cylas and root size (Y=-0.027x+29.684, R2=0.1647) (P<0.05) (Fig. 3C).

Relative resistance to C. puncticollis in the laboratory. The effect of various sweetpotato lines and varieties on the number of weevils that emerged is shown in Table 3. Significant differences (P<0.05) were observed in the total number of adult C. puncticollis weevils that emerged from the various sweetpotato lines/varieties. A relatively higher weevil emergence was observed in the experiment set in the dry hot months of January - February 1998 than in the one set in the wet cool months of September - November 1997. The variety 'Tanzania' gave the highest weevil emergence in both periods.

The varieties 'Akere-Ikokolak' and 'Kasira' registered the lowest levels of emergence of C. puncticollis adults, and were therefore considered to be somewhat resistant to the pest. Similarly, on the three red-skinned varieties, namely 'Emadirait', 'Bwanjule (#29) and 'Odopelap', the weevils exhibited slightly delayed emergence which suggested the presence of biochemical resistance mechanisms.

Comparison of data on field infestation by Cylas weevils and weevil emergence in the laboratory revealed a negative correlation between the two (Y = -0.0099x + 40.697; R2=0.0004), but the correlation was not significant (P>0.05).

DISCUSSION

Results obtained in the present study demonstrated the importance of rooting characteristics in influencing sweetpotato tuber infestation and yield loss by Cylas spp. Root size was the only characteristics of sweetpotatoes that significantly influenced both tuber infestation and yield loss due to Cylas spp.

Root size was positively correlated with tuber infestation but negatively correlated with yield loss due to Cylas weevils. Bigger root tubers could easily cause soil cracking, thus exposing them to weevil attack. They also had localised damage compared to small ones which were completely damaged. The localised damage was attributed to relatively larger area provided by large tubers. The extent of damage influenced the volume of tuber sliced off.

The positive and negative relationships of root length to infestation and yield loss due to Cylas weevils, respectively, were attributed to the tendency of tubers of some varieties to protrude out of the soil, which are then exposed to weevil feeding and oviposition. However, the results in this study contrasted with those reported by Singh et al. (1987) who found a negative correlation between root length and tuber infestation. The discrepancy is probably due to difference in the methods used to assess infestation in the two studies. Singh et al. (1987) in their assessment of infestation counted the number of plants with infested tubers, unlike in the present study where infestation was based on the exact number of tubers infested.

The negative relationships of the shortest weevil distance to roots (RD) (i.e., depth of root coverage) to infestation and yield loss due to Cylas weevils was attributed to the fact that Cylas weevils are not able to dig through the soil (Jayaramaiah, 1975; Smit, 1997). The deep-rooted tubers were not easily accessed by the weevils for feeding and oviposition.

It was also observed that cultivars with long root neck had a tendency to push their tubers towards the sides of the ridge hence exposing them for weevil infestation. These results contrast with those of Singh et al. (1987) who reported a negative correlation between root neck length and weevil damage. However, a flat plant bed was used in the study by Singh et al. (1987) in contrast to the ridges used in the present study. Methods of planting may have to be considered for sweetpotato cultivars with different rooting characteristics.

The results of this study demonstrated that the differential varietal resistance to weevil attack among sweetpotato varieties tested was not due to variation in rooting characteristics but was probably due to other factors.

On the basis of the laboratory study, it is possible that a few of the varieties tested posses antibiosis resistance mechanisms to Cylas weevils. Several studies have identified cultivars that adversely affected the development of C. formicarius (Cockerham and Dean, 1947; Sing et al., 1987) and C. puncticollis (Anota and Odebiyi, 1984; Ngeve, 1994).

From this study, it is clear that further investigations should be carried out on the factors that determine variation in relative infestation of sweetpotato cultivars in the field. Systematic evaluation of the varieties that have shown some degree of antibiotic resistance to C. puncticollis should also be undertaken.

ACKNOWLEDGEMENT

We acknowledge the financial and technical assistance given by Natural Resources Institute (NRI) and International Potato Centre (CIP) during the study.

REFERENCES

Anota, T. and Odebiyi, J.A. 1984. Resistnce in sweetpotato to Cylas puncticollis Boh. (Coleoptera: Curculionidae). Biologia-Africana 1:21-30.

Cockerham, K.L. and Deen, O.T. 1947. Resistance of new sweetpotato seedlings and varieties to attack by the sweetpotato weevil. Journal of Economic Entomology 40:439-441.

Jayaramaiah, M. 1975. Bionomics of sweetpotato weevil Cylas formicarius (Fabrics) (Coleoptera: Curculionidae). Mysore Journal of Agricultural Sciences 9:99-109.

Ngeve, J.M. 1994. Response of sweetpotato clones to wdeevils and environments in Cameroon. Journal of Horticultural Science 69:963-968.

Sengooba, T. 1994. Root crops for food security in Africa. In: Proceedings of the 5th Triennial Symposuim of International Society for Tropical Root Crops- Africa Branch held at Kampala, Uganda 22-28 November 1992. pp. 22-25.

Singh, B., Yazdani, S.S. and Hameed, S.F.1987. Sources of resistance to Cylas formicarius Fab. in sweetpotato. 1. Morphological characters. Indian Journal of Entomology 49:414-419.

Smit, N. E. J. M. 1997. Integrated pest management for sweetpotato in Eastern Africa. Ph.D. Thesis, Agricultural University, Wageningen. 151 pp.

TABLE 1. Rooting characteristics, corresponding correlation coefficients (r) and probability levels (P) as related to sweetpotato tuber infestation by weevils

Rooting characteristic	Correlation coefficients (r)	Probability level (P)	Regression equations and coefficient (R2)
Root size	0.523	0.009	Y = 0.0456X + 21.206; R2 = 0.2731
Root length	0.150	0.483	Y = 0.5601X + 29.417; R2 = 0.0225
Root neck length	0.270	0.202	Y = 0.1.1868X + 28.63; R2 = 0.073
Shortest weevil distance to root (RD)	-0.194	0.364	Y = -1.8803X + 50.158; R2 = 0.0375

TABLE 2. Rooting characteristics, corresponding correlation coefficients (r) and probability levels (P) as related to yield loss due to Cylas weevils

Rooting characteristic	Correlation coefficients (r)	Probability level (P)	Regression equations and coefficient (R2)
Root size	0.406	0.049	Y = 0.027X + 29.684; R2 = 0.1647
Root length	0.181	0.397	Y = 0.5159X + 28.231; R2 = 0.0329
Root neck length	0.331	0.115	Y = 1.1083X + 29.098; R2 = 0.1094
Shortest weevil distance to root (RD)	0.160	0.456	Y = 1.8833X + 25.199; R2 = 0.0255

TABLE 3. Average number of C. puncticollis that emerged within 44 days after exposure from 24 sweetpotato accessions in a laboratory assessment for relative resistance to the pest, carried out at Serere Agricultural and Animal Production Research Institute

Variety	Total emergence pf Cylas puncticollis			Susceptibility ranking
	Sept. 1997 (n=5)	Dec. 1997 (n=5)	Pooled results (n=5)
Tanzania	173.0 a	97.8 a	145.4 a	1
218	155.0 ab	62.6 bcdefghi	108.8 bc	2
Anyara	153.4 ab	86.2 ab	119.8 ab	2
282	153.4 ab	48.8 efghi	101.1 bcde	2
69	143.4 bc	72.8 abcdefg	108.1 bc	2
271	140.6 bc	84.8 abc	112.7 bc	2
316	140.0 bc	82.4 abc	111.2 bc	2
Okunguru-Dere	139.0 bc	53.4 defghij	96.3 bcdef	2
277	138.4 bc	81.2 abcd	109.8 bc	2
192	136.6 bcd	75.6 abcde	106.1 bcd	2
324	136.6 bcd	63.8 bcdefgh	100.2 bcde	2
178	132.0 bcd	75.2 abcdef	103.6 bcd	2
Sowola	129.2 bcd	63.6 bcdefgh	96.4 bcdef	2
Odopelap	118.2 cde	76.6 abcde	97.4 bcdef	2
Epuramojong	117.2 cde	47.8 fghi	82.5 cdefg	2
202	109.4 cde	61.0 bcdefghij	85.2 cdefg	2
148	101.2 ef	33.8 j	67.5 fgh	2
Opejo	100.6 efg	50.0 efghij	75.3 defgh	2
Bwanjule (#29)	96.6 efg	53.4 defghij	75.0 defgh	2
Esamiat	88.6 fg	46.4 ghij	67.5 fgh	2
Ecuru	85.0 fgh	56.6 cdefghij	70.8 efgh	2
Emadirait	73.2 gh	37.6 hij	55.4 gh	2
Akere-Ikokolak	59.2 h	33.2 j	46.2 h	3
Kasira	59.2 h	34.3 ij	46.2 h	3
C.V (%)	40.53	36.02	40.26
LSD (P=0.05)	27.9	28.7	32.7

Figure 1. A cross section of a sweetpotato ridge showing how the vertical and horizontal (V and H) positions of roots in relation to the surface of the ridge to the root, were measured.

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