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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 5, Num. 1, 1997, pp. 39-45
African Crop Science Journal, 1997,Vol. 5. No.1, pp. 39-45.

Adult longevity, fecundity and oviposition characteristics of Cylas puncticollis Boheman on sweet potatoes

R.A. Sathula, J.M. Logan, D.C. Munthali and G.K.C. Nyirenda

Soil Pest Project, Chancellor College, P.O. Box 280, Zomba, Malawi

(Received 2 May, 1995; accepted 12 December, 1996)


Code Number: CS97006
Sizes of Files:
    Text: 16.7K
    Graphics: Tables and line drawings (gif) - 34.7K
ABSTRACT

The biological characteristics of Cylas puncticollis Boh. were studied on the aerial parts of an exotic sweet potato (Ipomea batatas L.) variety Kenya (SPNO) in the screenhouse at Chancellor College, Malawi from 1 July 1991 to 20 January 1992. The lifespan of males ranged from 97 to 194 days and that of females ranged from 53 to 213 days. The difference in the lifespan of adults between sexes was not significant. Fecundity of females ranged from 6 to 209 eggs. Oviposition occurred in vines and leaf petioles. There was no significant difference in oviposition between vines and leaf petioles, but it differed significantly (P<0.05) between assessment weeks. Eggs were laid singly in cavities excavated by females. Seventy five percent of the sealed cavities and 26% of the unsealed cavities contained eggs. Egg incubation period was not more than 7 days, larval period took about 21 days and the pupal period was about 7 days.

Key Words: Cylas biology, Ipomea batatas, Malawi

RESUME

Les caracteristiques biologiques de Cylas puncticollis Boheman ont ete etudiees sur la partie aerienne d'une variete exotique de patate douce (Ipomea batatas L.), Kenya (SPNO) dans une serre de selection a Chancellor College, Malawi depuis le premier juillet 1991 jusqu au 20 janvier 1992. La duree de vie d'insectes males etait entre 97 et 194 jours et celle de femelles allait de 53 a 213 jours. La difference dans la duree de vie des adultes entre les sexes etait insignifiante. La fecondite de femelles allait de 6 a 209 oeufs. L'oviparite a eu lieu dans les tiges et les petioles. Il n'y avait pas une difference importante en ce qui concerne l'oviparite sur les tiges ou les petioles, meme s'il y avait une grande disparite entre les semaines d'intervalle de determination (P<0.05). Les oeufs etaient pondus uniquement dans des trous creuses par les femelles. Soixante-quinze pour cent de trous scelles et vingt-six pour cent de trous non-scelles ont contenu des oeufs. La periode de larves a dure environ 21 jours alors que la duree de pupes etait de 7 jours.

Mots Cles: Biologie du Cylas, Ipomea batatas, Malawi

INTRODUCTION

In Malawi, sweet potatoes (Ipomea batatas L.) are grown by subsistence farmers throughout the country but the most important production areas are Lilongwe plains and the Shire valley. Sauti (1985) estimated about 36,000 ha to be under the crop. The contribution of the crop to national economy is limited but it is an important income source in localised areas. There is no doubt that the crop plays an important role in the diet of many Malawians, especially in seasons of maize failure.

Cylas spp. weevils are the most damaging insect pests of sweet potato worldwide. Three species, namely; C. puncticollis, C. brunneus and C. formicarius are found in East Africa (Lenne, 1990) but, so far, only C. puncticollis has been confirmed to occur in Malawi (Munthali, 1988). Although there are no data from Malawi, reports from elsewhere indicate that yield losses due to the pest may reach 50 to 100% under low input subsistence agricultural systems (Chalfant et al., 1990; Lenne, 1991).

The biology and ecology of the C. formicarius group has been extensively reviewed (Sutherland, 1986; Chalfant et al., 1990), but only limited studies have been done on the biology of C. puncticollis. Under laboratory conditions (25 - 30 C, 79% RH), the development from egg to adult of C. puncticollis averaged 20.2 days, pre-oviposition period averaged 3.6 days and the oviposition period was 71.4 days. The incubation period averaged 3.3 days, larval and pupal periods averaged 11.1 and 6.2 days, respectively (IITA, 1982).

This paper presents some of the findings of a recent study on the biology of C. puncticollis. The study aimed at determining the oviposition characteristics of the insect on aerial parts of sweet potatoes under semi-natural conditions. Adult longevity, fecundity and oviposition characteristics, such as the relationship between sealed/unsealed cavities and the number of eggs laid in these cavities, are highlighted in the paper.

MATERIALS AND METHODS

The experiment was carried out in a screenhouse at Chancellor College, Malawi. The screenhouse was constructed from a timber frame with all sides covered with fine mesh. The roof, 2.9 m high at the gable and 2.5 m high at the eaves was covered with reinforced fibreglass sheets to allow light into the room for plant growth. The room measured 4 m by 5 m. The temperature in the screenhouse ranged between 7.63 - 21.05 C minimum and 23.68 - 36.58 C maximum.

Sweet potato planting material was obtained as vines from Chancellor College Biology Department research field. The vines were cut into 30 cm sections, each with three internodes. During planting, two internodes were buried in the soil. Two vines were planted 25 cm apart in a single wooden box measuring 40 cm long, 25 cm wide and 26 cm deep. When the vines were about 60 cm long, they were trained on stakes. The supply of fresh plant material was assured by growing new plants every 5 to 6 weeks.

Cultures of the weevil were established on sweet potato tubers in the screenhouse. To obtain adults of the same age, tubers were artificially infested with ten females and ten males collected from a sweet potato field. The tubers and weevils were placed in a plastic basin of 30 cm diameter and 15 cm deep and covered with a netting. The adults were removed after 20 days but the tubers were kept in the laboratory until F1 adults started emerging 12 days later. The F1 adults were collected daily, sexed and stored in jars.

Ten pairs of weevils of known age were used. A pair of weevils was confined to a 20 cm long vine section using a piece of netting cloth measuring 35 cm by 35 cm sewn into a sleeve shaped cage. The sleeve cages were located 25 to 30 cm from the base of plants. Each cage enclosed a vine section with 3 to 5 leaves depending on the internodal length of the vine. Tanglefoot was smeared around the vine at the upper and lower sections of each cage to prevent predacious mites and ants from getting into the cages. The weevils were in cages for 7 days and then transferred to new vines. The used vine sections were cut from the plant and the number of sealed and unsealed cavities were recorded for each oviposition site. Thereafter, the sections were dissected and fecundity (the total number of eggs and larvae found in each vine) recorded. The number of eggs and larvae found in each of the oviposition sites was used to determine oviposition site preference. The percentage of sealed and unsealed cavities with eggs in each of the oviposition sites were transformed to arcsines before analysis. Death of a female marked the end of study for the pair. In case a male died first, another was paired with the remaining female to continue the study.

RESULTS

Adult lifespan and fecundity. The lifespan of adult male and female C. puncticollis averaged 137.9+/-10.2(s.d.) and 143.2+/-20.6(s.d.) days, respectively; and the mean fecundity was 115.4+/-21.6 eggs (Table 1). Four distinct phases of oviposition were observed during weeks 4 to 10 with an average of 4 eggs, 12 to 15 with an average of 11 eggs, 18 to 24 with an average of 7 eggs and 25 to 30 with an average of 1 egg (Fig. 1). Fecundity varied significantly (P<0.05, Tukey test on transformed data, log10 (x + 1)) within assessment weeks but not between oviposition sites. The lowest mean of 1.17 and largest of 6.17 occurred in weeks 8 and 14, respectively. The overall mean fecundity in vines and leaf petioles were 3.28 and 3.51, respectively, and were not significantly different from each other at P>0.05.

Sealed and unsealed cavities. The sealed cavities (Table 2) varied significantly (P<0.01) between asssessment weeks but not between oviposition sites. The largest overall mean of sealed cavities (6.71) occurred during week 11 of oviposition and was significantly different from weeks 5,6,8 and 16. The overall means of sealed cavities for the vines and leaf petioles were 3.11 and 3.44, respectively. There were no significant differences in unsealed cavities within assessment weeks and between oviposition sites.

Sealed and unsealed cavities with eggs. The percentage of sealed cavities with eggs ranged between 31.0 to 64.8, whereas that of unsealed cavities with eggs ranged between 7.5 to 45.8 (Table 3). Analysis of variance on transformed data showed that sealed cavities with eggs did not vary significantly between assessment weeks and oviposition sites. However, data on eggs in unsealed cavities varied significantly (P<0.01) between assessment weeks. About 84% of sealed cavities in leaf petioles and 67% of sealed cavities in vines had eggs in them compared with only 25% of unsealed cavities in leaf petioles and 48% of unsealed cavities in vines (Table 4). Figure 2 shows a strong correlation [r2 = 0.89] between the total number of cavities and the number of sealed cavities with eggs in them. There was a weak correlation (r = 0.2, P>0.05) between the total number of cavities and the number of unsealed cavities with eggs in them.

DISCUSSION

The mean lifespan of C. puncticollis adults observed under this study is much longer (143.2 days for females, 137.9 days for males) than that reported in studies conducted under controlled conditions at the International Institute of Tropical Agriculture, 80.5 days for females, 54.8 days for males (IITA, 1982). Eggs were laid singly in cavities drilled through the epidermis and vascular bundle as reported for C. formicarius (Sutherland, 1986). In leaf petioles, the cavities were also drilled to the centre of the petiole. The overall mean number of eggs of 115.4 laid on the vegetative parts is much lower than 329.8 eggs laid on sweet potato tubers as reported by other researchers (IITA, 1982). This suggests that oviposition stimulants reported to occur in sweet potato tubers (Nottingham et al., 1987) are either not present in the vegetative parts or are present in small amounts.

Although it has been widely reported that Cylas spp. lay eggs in vines and tuberous roots (Jayaramaiah, 1975) this study has revealed that when vines and leaves are offered, eggs will equally be laid in either part. Since previous studies on fecundity of Cylas spp. were exclusively done on tubers (Jayaramaiah, 1975; IITA,1982; Nottingham et al., 1987) it was difficult to realise that leaf petioles also constitute sites for oviposition. The non-significant differences in the number of sealed and unsealed cavities and the number of eggs found in the vines and leaf petioles, suggest that C. puncticollis did not prefer one site to the other for oviposition. The oviposition behaviour of C. puncticollis and C. formicarius are similar in that, after oviposition, the cavity is sealed by the female (Sherman and Tamashiro, 1954). In order to estimate oviposition, Nottingham et al. (1987) scored the distinctive plugged (sealed) holes. This method assummed that all plugged holes had eggs in them; and all unplugged holes had no eggs in them. However, this study has shown that 30.3% of cavities without plugs contained eggs, and that about 50.2% of plugged cavities did not have eggs in them. Although there is a good correlation between the number of sealed cavities and the number of eggs in them, the use of sealed holes consistently underestimated the actual number of eggs laid by 10 to 15%. Thus, counting sealed cavities only will not give a reliable estimate of the total number of eggs laid. However, with more data from a greater number of females, it may be possible to devise a correction factor which could be used to accurately convert sealed holes to total eggs.

CONCLUSION

Cylas puncticollis lives longer under protected semi-natural conditions than under controlled laboratory conditions. A female lays more eggs (over 200) in the aerial parts than in the tubers. The eggs are laid singly in cavities excavated in both the vines and leaf petioles. The cavities are sealed off after oviposition, and not all sealed cavities have eggs in them. Conversely, not all the unsealed cavities have no eggs in them. The adult longevity and fecundity of C.puncticollis has important implications on field infestation and control of the weevil in sweet potato fields. Since eggs are laid in the vines and leaf petioles, and sweet potato is often planted as cuttings from the vines, eggs can be transferred from one site to another in the planting material.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial support from the Overseas Development Administration (UK) through the Natural Resources Institute (UK) and Chancellor College, University of Malawi for accomodating the work as part of its Soil Pest Project.

REFERENCES

Cobley, L.S. 1976. An Introduction to the Botany of Tropical Crops. Second Ed.(rev.by W.M.Steele) Longman, New York, U.S.A. pp. 111-113.

Chalfant, R.B., Jansson, R.K., Seal, D.R. and Schalk, J.M. 1990. Ecology and management of sweet potato insects. Annual Review of Entomology 35:157-180.

FAO. 1984. FAO Production Yearbook, 1983. Food and Agriculture Organisation, Rome.

IITA, 1982. IITA Annual Report 1981. International Institute of Tropical Agriculture, Ibadan, Nigeria. p.72.

Jayaramaiah, M. 1975. Bionomics of sweet potato weevil Cylas formicarius F. at Coimbatore. Madras Agricultural Journal 146:293-297.

Lenne, J.M. 1991. Diseases and Pests of Sweet Potato: Southeast Asia, the Pacific and East Africa. Natural Resources Institute Bulletin No.46 viii+116 pp.

Munthali, D.C. 1988. Susceptibility of five locally grown sweet potato (Ipomoea batatas) cultivars to Cylas puncticollis infestation in the field in Zomba (Malawi). Luso: Journal of Science and Technology (Malawi) 9 :1-10.

Nottingham, S.F., Son, K.-C., Severson, R.F., Arrendale, R.F. and Kays, S.J. 1989. Attraction of adult sweet potato weevil, Cylas formicarius elegantulus (Summers) (Coleoptera: Curculionidae), to sweet potato leaf and root volatiles. Journal of Chemical Ecology 15:1095-1107.

Sauti, R.F.N. 1985. Cassava and sweet potato improvement programme: 1984/85-1985/86 work programme. Min. of Agric. Makoka Res. Sta. 40pp.

Sutherland, J.A.1986. A review of the biology and control of the sweet potato weevil Cylas formicarius Fab. Tropical Pest Management 32:304-315.

Sherman, M. and Tamashiro, M. 1954. The sweet potato weevils in Hawaii: their biology and control. Hawaii Agricultural Experiment Station Technical Bulletin 23:7-36.

Copyright 1997 The African Crop Science Society


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