Class
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Insecta
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Order
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Coleoptera
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Family
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Curculionidae
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Cylas formicarius is considered the single most important pest of sweetpotato. Weevil infestation ranges from 20 to 50% on many farms and can even reach to 100% depending on the season and variety. Higher infestation occurs during dry seasons.
Sweetpotato weevil is found throughout the tropical and subtropical regions worldwide except in Africa, where it has only been reported in South Africa and coastal Kenya. In areas where Cylas formicarius is not present, other weevils of the Cylas genus cause similar damage (see Other Sweetpotato Weevils). In Central and South America, the West Indian sweetpotato weevil is often more prevalent.
The adult weevils feed on the tender buds, leaves, vines and storage roots while the larvae, the most destructive stage, feed and tunnel into the mature stems and storage roots. The damage is characterized by small feeding and ovipositional punctures on the surface and larval tunnels filled with frass in the tissues. The damage leads to thickening, drying and cracking of the stems and to secondary infection by bacteria and fungi.
Infestation of the storage roots makes them unfit for human or animal consumption, even if only a small proportion of the flesh is damaged, as the damaged tissue produces terpines giving the flesh an unpleasant odour and bitter taste. Weevil damage increases during storage.
Egg. The ovoid eggs measure 0.65 mm long and 0.46 mm wide. The newly laid eggs are translucent white and soft with rugose surface. The older eggs, which are about to hatch, are creamy with small brown irregular specks.
Larva. The white larvae are legless with a brown subglobular head and reddish brown gut. The abdomen is subconical for the first and second instars and subcylindrical from third to fifth instars. Mature larvae measure 7-8 mm.
Pupa. The pupae are creamy white, exarate type, sub-ellipsoidal and measure 5-6 mm.
Adult. The adult weevil is 5-7 mm long, slender, smooth and hard-bodied ant-like insect with a distinct snout, metallic blue head, forewing and abdomen. The legs and thorax are reddish brown. The females differ from the males in antenna and body size. The terminal club of the female's antenna is ovoid while that of the male is cylindrical. Usually, the females are larger than the males.
All sweetpotato weevil species have a similar life history. The adult female lays eggs singly in cavities excavated in vines or in storage roots, preferring the latter. The egg cavity is sealed with a protective, grey fecal plug. The developing larvae tunnel in the vine base or storage root. Pupation takes place within the larval tunnels. A few days after exclusion, the adult emerges from the vine or storage root. Because the female cannot dig, she finds storage roots in which to lay her eggs by entering through soil cracks.
Hot, dry weather favours weevil development. At optimal temperatures of 27-30oC, C. formicarius completes development (from egg to adult) in about 33 days. Adult longevity is about 75-105 days and females lay between 100 and 250 eggs in this period. At suboptimal temperatures, development takes longer.
Detection of infestation is difficult as adults are most active by night. Early detection can be done by checking the base of the plant for feeding punctures and exit holes. When damage has been detected, several plants could be dug up to establish an idea of the damage level to the roots. This could lead to the decision for an early harvest. The species-specific pheromone that is released by females of C. formicarius attracting males has been identified. Pheromone lures for C. formicarius are commercially available. Traps are so sensitive that their failure to catch weevils is a good indication that the pest is not present. However, the relationship between numbers of males trapped and damage levels is not clear and the use of traps as damage detection tools might be limited.
Wild species of Ipomoea are important host plants.
When sweetpotato weevil populations are high, no single control method provides adequate protection. The integration of techniques - referred to as "Integrated Pest Management" (IPM), with emphasis on the prevention of infestation, provides sustainable protection.
Cultural control
Cultural control practices have proven to be effective against the sweetpotato weevil and should be the main basis of control. The local growing conditions and the use of the crop (commercial or for own consumption) will indicate which practices should be applied. Effective control measures include:
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Sanitation of the field, removing previous crop debris.
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Use of uninfested vine tips as planting material.
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Crop rotation.
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Timely planting and prompt harvesting to avoid a dry period.
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Hilling-up of soil around the base of plants and filling of soil cracks.
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Applying sufficient irrigation to prevent or reduce soil cracking.
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Use of sex pheromone traps.
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Sex pheromone trap in the field (E. Vasquez). |
Sex pheromones are chemicals emitted by adult female insects in order to attract males. The sweetpotato weevil pheromone can be synthesized artificially and is commercially available. It is used in traps, which are positioned at intervals throughout the crop, and has proven to be very effective in attracting male weevils in the field. The traps potentially provide a chemical control measure which does not harm beneficial insects, and leaves no residue on the crop. Several good and simple trap designs have been produced making use of locally available materials. However, experiences in farmers' fields have been mixed, with often no measurable impact on weevil damage despite large numbers of weevils trapped. The availability and price of pheromones will restrict the number of users. |
Chemical control
Dipping planting material in a solution of an insecticide (such as carbofuran or diazinon) for 10-15 minutes prior to planting can control sweetpotato weevils for the first few months of the growing season. Dipping longer than necessary can burn and kill the planting materials. Spraying insectides after planting is not (cost)-effective.
Host resistance
Cultivars with immunity or a high level of resistance are not available. Some cultivars have low levels of resistance. Early maturing and deeply rooted varieties expectedly get less weevil damage and can even escape weevil infestation.
Biological control
Promising biological control agents for sweetpotato appear to be the fungi Beauveria bassiana and Metarrhizium anisopliae , entomopathogenic bacterium, Bacillus thuringiensis and the nematodes Heterorhabditis spp. and Steinernema spp. The fungi attack and kill adult weevils, whereas the nematodes kill the larvae. A solution of B. bassiana could be used as a pre-planting dip.
Ants, spiders, carabids, and earwigs are important generalist predators that attack weevils. The ants Pheidole megacephala and Tetramorius guineese are used as biological control agents in Cuba.
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Beauveria bassiana on sweetpotato weevil (D. Amalin). |
Earwig is a predator of sweetpotato weevil (E. Vasquez). |
An ant, Pheidole sp., is a predator of sweetpotato weevil eggs (H. van den Berg). |
Larva of sweetpotato weevil infected with Bacillus thuringiensis (D. Amalin). |
Amalin, D.A. and Chujoy, E. 1995. Integrated management of sweetpotato weevil (Cylas formicarius Fabr.) Beauveria bassiana. In: The Potato and Sweet potato in Southeast Asia and the Pacific Region - Research Results Presented in a Series of Working Papers. 1993-1995. International Potato Center, Box 933, Manila, Philippines. pp.156-164.
Amalin, D.M. and Vasquez, E. A. 1993. A handbook on Philippine sweet potato pests and their natural enemies. International Potato Center (CIP), Los Baños, Philippines. 82 p.
Ames, T., Smit, N.E.J.M., Braun, A.R., O’Sullivan, J.N., and Skoglund, L.G. 1996. Sweetpotato: Major pests diseases, and nutritional disorders. International Potato Center (CIP). Lima, Perú. 152 p.
Braun, A.R. and van de Fliert, E. 1999. Evaluation of the impact of sweetpotato weevil (Cylas formicarius) and the effectiveness of Cylas sex pheromone traps at the farm level in Indonesia. International Journal of Pest Management, 45: 101-110.
Gonzales, S.S. 1925. The sweet potato weevil (Cylas formicarius Fabr.), Philippine Agriculturist 14:257-281.
PANS. 1978. Pest Control in Tropical Root Crop. Manual No. 4. Centre for Overseas Pest Research. London. 235 p.
Starr, C.K. and Wilson, D.D. 1997. Sexual dimorphism in the sweet potao weevil, Cylas formicarius (F.), (Coleoptera: Brentidae), The Canadian Entomol.129:61-69.
Sutherland, J.A. 1986. A review of the biology and control of sweet potato weevil Cylas formicarius (Fabr.) Tropical Pest Management 32:304-315.
Sutherland, J.A. 1986. A review of the biology and control of sweetpotato weevil Cylas formicarius (Fab.) Tropical Pest Management 32: 304-315.
Talekar, N.S. 1988. How to control sweetpotato weevil: A practical IPM approach. International Cooperator's Guide AVRDC 88-292. Asian Vegetable research and Development Center (AVRDC), Tainan, Taiwan, 6 p.
van de Fliert, E. and Braun, A.R. 1999. Farmer field school for integrated crop management of sweetpotato. International Potato Center. Lima, Peru.
Vasquez, E.A. and Sajise, C.E. 1990. Pests of sweet potato: Insects, mites and diseases. Philippine Root Crop Information Service, Philippine Root Crop Research & Training Center. 65 p.
Vasquez, E.A. and Gapasin, D.P. 1980. Stems and tubers for rearing sweet potato weevil. Ann. Trop. Res. 2(2):80-88.
Villacarlos, L.T., Vasquez, E.A. and Mandras, B.T. 1995. Field evaluation of pheromone and Metarhizium anisopliae for the control of sweet potato weevil Cylas formicarius Fabr. In: The Potato and Sweet potato in Southeast Asia and the Pacific Region - Research Results Presented in a Series of Working Papers. 1993-1995. International Potato Center, Box 933, Manila, Philippines. pp. 178-186.
Contributed by: Erlinda Vasquez and Nicole E.J.M. Smit
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