Cylas formicarius (sweet potato weevil)
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Plant Trade
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Cylas formicarius Fabricius
Preferred Common Name
- sweet potato weevil
Other Scientific Names
- Brentus formicarius Fabricius, 1798
- Cylas elegantulus (Summers, 1875)
- Cylas formicarius elegantulus Summers
- Cylas formicarius formicarius
- Cylas turcipennis Boheman
- Otidocephalus elegantulus Summers, 1895
International Common Names
- English: sweet potato root borer
- Spanish: gorgojo del camote; piogán de la batata (Dominican Republic); tetuán del boniato (Cuba)
- French: charançon de la patate douce; charançon faux-fourmi
Local Common Names
- Germany: zweifarbiger kaefer
- Japan: arimodoki-zomusi
- Netherlands: batatensnuitkever
- CYLAFO (Cylas formicarius)
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Arthropoda
- Subphylum: Uniramia
- Class: Insecta
- Order: Coleoptera
- Family: Apionidae
- Genus: Cylas
- Species: Cylas formicarius
Notes on Taxonomy and NomenclatureTop of page
DescriptionTop of page
The adult insect is ant-like. The length of the adult female is between 4.80 and 6.70 mm; the males are slightly larger, between 5.00 and 6.75 mm. The basic colour of the insect is red (Kemner, 1924), but this colour is usually masked, because the head is black, the elytra blue or bluish-green, sometimes black, and shining, the back of the thorax and sternites are usually dark bluish-green. The legs are red with a broad dark ring around the tibiae which sometimes may not be very distinct in teneral specimens. The head extends into a long snout which is either uniformly wide or slightly wider at the front. (The snout or rostrum is that part of the head which is anterior to the eyes.) Antennae have 10 segments. In males the distal segment of antenna is a narrow club, uniformly wide, sausage-shaped, densely pubescent and more than twice as long as the flagellum. The distal antennal segment in the female is egg-shaped, only two-thirds the length of the flagellum.
The full-grown larva turns into a pupa in an enlarged area of the feeding tunnel. The pupa is whitish, 6.0-6.5 mm long. The long snout is bent towards the ventral side.
The newly hatched larva is somewhat larger than the egg. The full-grown apodous larva is 7.5-8.0 mm long and 1.8-2.0 mm wide. The head is comparatively large, measuring approximately one third of the body length and half the width. The colour is white or pale-yellow. The head and mandibles are chitinized yellow to brown; mandibles are almost black. The body is slightly curved.
Creamy-white, oval, narrow at attached end, 1.0-1.5 mm. Shell is thin and fragile.
DistributionTop of page
A record of C. formicarius in Barbados (CABI/EPPO, 2004; EPPO, 2013) published in previous versions of the Compendium has been removed as it was based on a source which erroneously recorded C. formicarius as a pest of sweet potatoes in Barbados (Review of Applied Entomology, Ser. A, 1917, V, p. 479).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 30 Jun 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Congo, Democratic Republic of the||Present|
|South Africa||Present, Localized|
|British Indian Ocean Territory||Present|
|-Andaman and Nicobar Islands||Present|
|-Lesser Sunda Islands||Present|
|South Korea||Absent, Intercepted only|
|Netherlands||Absent, Intercepted only|
|Antigua and Barbuda||Present|
|Barbados||Absent, Invalid presence record(s)|
|Saint Kitts and Nevis||Present, Localized|
|Trinidad and Tobago||Present|
|U.S. Virgin Islands||Present|
|United States||Present, Localized|
|-New South Wales||Present|
|Federated States of Micronesia||Present|
|New Zealand||Absent, Intercepted only|
|Northern Mariana Islands||Present|
|Papua New Guinea||Present|
|Wallis and Futuna||Present|
Hosts/Species AffectedTop of page
C. formicarius feeds and breeds on Ipomoea spp. and other Convolvulaceae. In addition to those in the list of hosts, the following wild species have been recorded as attacked by C. formicarius: Calystegia soldanella, Dichondra carolinensis, I. alba, I. barlerioides, I. cordato-triloba, I. hederacea, I. hederifolia, I. horsfalliae, I. imperati, I. indica, I. lacunosa, I. macrorhiza, I. obscura, I. pandurata, I. sagittata, I. separia, I. setosa, I. sinensis, I. triloba, I. tubinata, I. turbinata, I. wrightii, Jacquemontia curtissii, Merremia dissecta and Stictocardia tiliifolia (Cockerham, 1940; Cockerham et al., 1954; Austin, 1991; Austin et al., 1991).
Host Plants and Other Plants AffectedTop of page
|Calystegia sepium (great bindweed)||Convolvulaceae||Wild host|
|Colocasia esculenta (taro)||Araceae||Other|
|Cuscuta (dodder)||Cuscutaceae||Wild host|
|Ipomoea (morning glory)||Convolvulaceae||Wild host|
|Ipomoea aquatica (swamp morning-glory)||Convolvulaceae||Other|
|Ipomoea batatas (sweet potato)||Convolvulaceae||Main|
|Ipomoea cairica (five-fingered morning glory)||Convolvulaceae||Wild host|
|Ipomoea pes-caprae (beach morning glory)||Convolvulaceae||Wild host|
|Ipomoea purpurea (tall morning glory)||Convolvulaceae||Wild host|
|Ipomoea quamoclit (cypress vine)||Convolvulaceae||Wild host|
|Jacquemontia tamnifolia (Smallflower morningglory)||Wild host|
|Pharbitis nil (Japanese morning glory)||Convolvulaceae||Wild host|
Growth StagesTop of page
SymptomsTop of page
List of Symptoms/SignsTop of page
|Leaves / external feeding|
|Roots / external feeding|
|Roots / internal feeding|
|Stems / distortion|
|Stems / external feeding|
|Stems / internal feeding|
Biology and EcologyTop of page
Eggs are laid singly in cavities in the root or stems. Following egg deposition the egg hole is covered with a greyish mass which hardens to form a protective cap over the developing egg (Reinhard, 1923; Gonzales, 1925). Egg incubation period ranges from 4 days at 30°C to 7.9 days at 20°C (Mullen, 1981). Cockerham et al. (1954) reported an incubation period of 4 to 56 days at mean temperatures of 20 and 10.5°C, respectively.
Larvae feed inside roots or stems where oviposition occurs for 25-35 days during which they complete three larval instars (Sherman and Tamashiro, 1954). Mullen (1981) found a larval development period of 16.2 days at 30°C and 58.2 days at 20°C. Gonzales (1925) found a larval period of 25 days under Philippines field conditions while Cockerham et al. (1954) reported a range of 12-154 days under field conditions in the USA.
Pupation takes place within the sweet potato roots or stems where larvae feed. The mature larva excavates a cell 2 to 3 times the size of its body in which pupation occurs. The pupal period lasts 4-8 days (Franssen, 1935; Sherman and Tamashiro, 1954). In a laboratory study, Mullen (1981) found pupal period duration from 5 to 10.7 days at 25 and 20°C, respectively. Cockerham et al. (1954) also reported a mean pupal period of 7.5 days at 25.6-27.7°C.
Soon after emergence from the pupa, the adult stays in the pupal chamber and then cuts its way through the plant tissue. Adults mate soon after emergence but oviposition does not occur for a minimum of 4.5 days at 30°C or 7.7 days at 20°C (Mullen, 1981). Reinhard (1923), Subramanian (1959) and Jayaramaiah (1975) reported similar preoviposition periods.
Males have a greater flight ability than females of all ages and are more active in the field. No distinct migratory behaviour of adults has been detected. Flight activity of males peaked just after the onset of darkness in laboratory studies, and gradually decreased toward the onset of light. More than 90% of male adults flew at least once within 2 weeks after emerging from sweet potato tubers, but female flight activity was low with only 25% flying at least once (Shimizu and Moriya, 1996a). Females disperse mainly by walking because of their low flight ability (Moriya and Hiroyoshi, 1998). Walking activity in both sexes is also greatest in the dark (Shimizu and Moriya, 1996b). Adults feign death as a predator avoidance strategy.
Males are attracted to a sex pheromone released by females (Coffelt et al., 1978). Females in the laboratory can mate repeatedly for 40 days after eclosion, but in the field females rarely remate because secretion of the sex pheromone is inhibited after the first mating (Sugimoto et al., 1996).
Adult weevils are attracted to sweet potato foliage and tubers by plant odour (Nottingham et al., 1989). An oviposition stimulant, identified as boehmeryl acetate (a pentacyclic triterpenoid), has been identified from the periderm of tubers. This compound, which increased oviposition in root core bioassays, was found in greater quantities in susceptible lines of sweet potato. Lower levels of boehmeryl acetate may therefore be a factor int he moderate resistance of certain sweet potato cultivars, for example, Regal and Resisto (Wilson et al., 1989; Wilson et al., 1990).
Fecundity varies greatly. Gonzales (1925) reported 90-340 eggs per female, Franssen (1935) 185 eggs, Cockerham et al. (1954) 1-319 eggs with an average of 119 and Subramanian (1959) 97-216 eggs. Mullen (1981) found a single female to lay up to 179 eggs; fecundity varied depending upon the number of matings and crowding.
Mullen (1981) reported the mean generation duration from egg to egg of 84.7, 33, and 33.7 days at 20, 27 and 30°C, respectively. Subramanian (1959) recorded life cycle duration of 36-43 days under undefined conditions. The sex ratio in Mullen's study was 50.3% female to 49.7% male, which is almost identical to the findings of Subramanian's study.
Adult survival varies greatly. Franssen (1935) in Indonesia reported females to survive a maximum of 113 days. Gonzales (1925) in the Philippines reported that males survived 63-120 days and females 81-107 days. Subramanian (1959) found males and females surviving for 94 and 109 days, respectively, in India. Under laboratory conditions survival was an average of 238 days at 15°C (Mullen, 1981).
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
Notes on Natural EnemiesTop of page
Among the 15 wasp parasitoids of C. formicarius reported from India, Philippines and USA, none is effective in controlling the pest. As the centre of C. formicarius origin is India, parasitoids found in that country could be useful in controlling the pest. The Central Tuber Crop Research Institute in Kerala (India) is engaged in biological control projects against this weevil (NS Talekar, AVRDC, personal communication, 1996).
Amongst entomopathogenic fungi, Beauveria bassiana is one of the most frequently recorded pathogens. High levels of mortality (80-90%) were obtained in laboratory tests when spores of B. bassiana were applied to sterile soil (Diaz Sanchez and Grillo Ravello, 1986; Su et al., 1988). Despite the presence of sufficient densities of inoculum, however, epizootics of this pathogen are rare.
Under simulated field conditions, nematodes belonging to genera Steinernema and Heterorhabditis have been pathogenic to C. formicarius (Jansson et al., 1990). However, lack of their persistence in open field soil for sufficiently long periods limits their utility in the control of sweet potato weevil.
Means of Movement and DispersalTop of page
Adults can disperse by flight. Males fly more frequently than females. Results from mark-release-capture experiments on Okinawa Island, Japan, using pheromone traps, suggested that for C. formicarius eradication projects, a buffer zone with a minimum radius of 2-4 km should surround the area targeted for control (Miyatake et al., 1997).
However, flight is relatively infrequent and it is spread over long distances mostly via infected sweet potato tubers. Immature stages (eggs, larvae and pupae) can be dispersed within tubers. Oviposition punctures may be visible, which can provide a guide as to whether a tuber is likely to be infested.
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Bulbs/Tubers/Corms/Rhizomes||eggs; larvae; pupae||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Plant parts not known to carry the pest in trade/transport|
|Fruits (inc. pods)|
|Growing medium accompanying plants|
|Stems (above ground)/Shoots/Trunks/Branches|
|True seeds (inc. grain)|
ImpactTop of page
C. formicarius is a destructive pest of sweet potato throughout most of the tropical and subtropical regions of Asia, the Pacific, the Caribbean, the USA, Central and South America, and several African countries. Few areas in the above regions where sweet potato is grown are free from its destruction. Crop losses from weevil damage range from 5 to 80%, with damage increasing the longer the crop remains unharvested (Kemner, 1924). In Hawaii, Sherman and Tamashiro (1954) showed that damage increased sharply between 24 and 30 weeks after planting. C. formicarius is an important pest both in the field and postharvest. Infestation damage causes yield reductions in terms of tuber weight, but also a marked reduction in tuber quality. A bitterness (due to sesquiterpenes) develops in infested tubers, which makes them unmarketable and inedible (Padmaja and Rajamma, 1982).
In experiment station trials, losses of 3-80% were recorded in Indonesia, depending on location and season (Bahagiawati, 1989) and damage from weevils was highest during the dry season (Braun and van de Fliert, 1999). In Guangdong province, China, sweet potato weevil reduces yield by 5-20% and in some cases it can reach 80% (Anon., 1984). In Penghu Island, Taiwan, Talekar et al. (1989) mentions losses of 40-75% in the absence of coordinated IPM efforts. In Vietnam, Dinh et al. (1995) documented farm-level losses as high as 30-40%. The use of pheromone traps in Kerala, India, was shown to be highly effective at mass trapping male weevils leading to a significant decline in population build-up and consequent yield increases. Mean tuber damage was 7% with pheromone traps compared with 45.7% damage in the control. The marketable yield was 9 t/ha in the treated production compared with 4.7 t/ha in the control (Pillai et al., 1996). Field-plot tests in Tamil Nadu, India, showed that the application of insecticides lead to increased yields. Applications of fenthion, fenitrothion and carbaryl reduced the % infestation by C. formicarius and increased the yield of good tubers to 18.87, 12.85 and 16.49 t/ha, respectively, compared with 6.7 t in the untreated control (Subramaniam et al., 1973). In Kerala, India weevils can cause a yield loss of 19-54% (Palaniswami, 1987). In 1991, Palaniswamy et al. reported that C. formicarius was a major limiting factor in upland production and yield losses were estimated at Rs 96.04 lakhs annually (Paniswamy et al., 1991). In Malaysia, Ho (1970) reported a yield loss of about 4 tons/acre or 80%. In the Philippines, C. formicarius reduces sweet potato yield by 50% (Gapasin, 1989). In the Amami Islands of Japan, losses of 15% have been documented (Suenaga et al., 1987). Similar losses are found in other Asian and Pacific countries, for example, in Papua New Guinea (Hartemink et al., 2000).
In Kenya, where farmers practice piecemeal harvesting, losses are in the order of 10% (Smit and Matengo, 1995). In Uganda, Smit (1997) showed that when the crop was harvested all at once, the percentage of damaged roots increased linearly the longer the harvest was delayed. Losses ranged between 3% at a harvest 3.5 months after planting (MAP) and 73% at 9.5 MAP. When the crop was managed according to the traditional method of harvesting piecemeal, total yield and undamaged yield for the piecemeal harvesting treatments were comparable to the yields at the optimum harvest times for once-over harvesting at 6-7.5 MAP.
North and Central America
Yield losses of up to 80% have been reported for southern Florida, USA (Jansson et al., 1987). In Georgia, USA, the effect of infestation by C. formicarius on the yield of 12 sweet potato cultivars was studied. Significant reductions in yield were demonstrated by comparing uninfested fields with infested ones. The average yield reduction was 69% and was thought to be caused by a number of factors, the most important of which was the death of infested plants (Mullen, 1984). In the Dominican Republic, Swindale (1992) estimated losses averaging 39%. In Cuba, Perez et al. (1987) reported damage to roots between 14-40% depending on the season and the variety.
Detection and InspectionTop of page
Size: 4.8-6.75 mm long
Behaviour: Adult feigns death, active day or night
Trap: Attracted to various light sources. Strongly attracted to sex pheromone: (Z)-3-dodecen-1-ol (E)-2-butenoate
Food: Associated with sweet potato and other Ipomoea species.
Damage: Sweet potato storage roots contain surface holes and deep tunnels with sawdust-like excrement and have a bitter taste.
C. formicarius weevils are not usually seen on the crop, on the soil surface under the vines or in the soil around the base of plant. Infestation is determined by uprooting the storage tubers and cutting them open to expose galleries or tunnels containing different stages of the weevil.
Similarities to Other Species/ConditionsTop of page
C. formicarius is also similar to the Afrotropical Cylas brunneus which has the femora not obviously bicolorous and the antennal club much shorter.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Because of its concealed feeding habits, C. formicarius can be difficult to control with conventional insecticide applications. However, because of its limited or almost non-existent flying activity, which implies that the insect is carried from place to place via movement of the plant material, host specificity to the genus Ipomoea, and characteristic mode of entry and damage to the plant, this pest is amenable to suppression by crop rotation, clean cultivation, mulching and similar simple cultural practices. Among various control measures attempted, modification of cultural practices has the greatest potential in combating the sweet potato weevil at very little cost.
Cultural pest control involves changing or modifying cultivation practices which directly or indirectly reduce the pest population. Cultural practices, such as crop rotation, intercropping, mulching, sanitation, etc., were the earliest control measures advocated for reducing sweet potato weevil damage.
Rotations of crops, such as growing sweet potatoes in a field only once every 5 years (TAC, 1954), avoiding planting of sweet potatoes in the same area for two successive years (Ballou, 1915; Chittenden, 1919; Edwards, 1930; Holdaway, 1941) or planting rice between two sweet potato crops (Franssen, 1935) have long been suggested.
The usefulness of crop rotation with rice in controlling the weevil was investigated in two experiments, each lasting 17-18 months in Taiwan (Talekar, 1983). The results obtained were variable dependent on the proximity of the source of weevil infection. Sweet potato weevil control was acceptable in a field planted away from a weevil-infested field, whereas the tubers were heavily infested when the fields were adjacent to each other.
Little research information is available on this approach for the management of sweet potato weevil. In one experiment in Taiwan, sweet potato was planted between two rows of each of 68 crop species and weevil infestations of the roots were monitored. Intercropping with chickpea (Cicer arietinum), coriander (Coriandrum sativum), pumpkin (Cucurbita moschata), radish (Raphanus sativus), fennel (Foeniculum vulgare), blackgram (Vigna mungo) and yardlong bean (Vigna unguiculata ssp. sesquipedalis) reduced weevil infestations considerably. However, intercropping with blackgram, fennel, pumpkin, and yardlong bean also reduced sweet potato yields (AVRDC, 1988). Similarly, Singh et al. (1984) observed reduced weevil damage when sweet potato was intercropped with proso millet (Panicum miliaceum) and sesame (Sesamum indicum). It is uncertain if the reduced yield (smaller or fewer roots) contributed to the lower weevil infestations. More research on the effects of intercropping on weevil damage and root yield is needed.
Soil cracks are the major route of weevil access to roots. The enlargement of roots, especially in cultivars which set roots near the soil surface, and soil moisture stress can produce cracks and increase exposure of roots to the weevil. The absence of cracks denies the weevil access to the roots. For example, in Taiwan, less damage by C. formicarius occurs during the rainy season when soil cracks are minimal (AVRDC, unpublished data). Similarly, the African sweet potato weevil [C. puncticollis] which causes damage similar to that by C. formicarius in Nigeria is less damaging during the wet season than during the dry season (Hahn and Leuschner, 1982). This is presumed to be due to the absence of soil cracks due to adequate soil moisture in the wet season as opposed to the dry season. Others have reported similar findings (Leuschner, 1982; Rajamma, 1983; Sutherland, 1986b). Prevention of soil cracking by hilling the area around the plant or irrigating frequently, are also suggested as an important method of reducing weevil damage (Franssen, 1935; Holdaway, 1941; Sherman and Tamashiro, 1954). Two experiments were conducted in Taiwan to study the potential of mulch for reducing sweet potato weevil infestations. Mulching materials, plastic film or rice straw, were spread over the planted area located in the vicinity of a weevil source, shortly after planting. Plastic film and rice straw mulch reduced weevil infestations as compared with non-mulched plots (AVRDC, 1988). Mulches conserved soil moisture and minimized soil cracking. The physical cover made by mulching materials further reduced access of roots to the weevil even if the soil cracked.
Sanitation practices or clean cultivation, especially for the control of an insect that has limited flying activity, may help protect the crop from insect infestation. These practices played an important role in pest control until the introduction and widespread use of chemical insecticides. A variety of sanitation methods have been recommended for weevil control, and in some locations they are even legally enforced (Karr, 1984).
Destruction of crop residues
Destroying any crop residues left in the field after harvest is important because weevils survive in roots and stems and infest succeeding or neighbouring sweet potato plantings (Chittenden, 1919; Franssen, 1935; Eddy et al., 1943). Crop rotation, in most cases, serves this purpose. However, in areas where sweet potato is a staple food and is planted year-round, rotation is not always possible.
Flooding of infested fields was tested in Taiwan to induce rotting of the left-over plant materials and thereby reduce weevil densities from one planting to the next (Talekar, 1990). Two or more weeks of flooding considerably reduced the emergence of volunteer sweet potato plants. Few plants emerged from flooded fields and these plants harboured few weevils. Conversely, a large number of volunteer plants grew in the non-flooded control plots, all of which were infested with weevils. These data show that flooding of fields between two consecutive sweet potato crops may reduce the immediate source of weevils from the field. This approach is considered in areas where rotation is not possible.
C. formicarius lays eggs in the vines, especially older portions in the absence of storage roots or when the roots are inaccessible (AVRDC, unpublished data). Planting of infested vines may spread the weevil infestation. Therefore, the use of weevil-free sweet potato cuttings is often advised (Ballou, 1915; Franssen, 1935; Tucker, 1937; Holdaway, 1941). Weevil-free cuttings can be produced by dipping them in a suitable insecticide solution before planting.
Recent findings in Taiwan showed that the cuttings (25-30 cm long) taken from fresh terminal growth, even from an infested crop, were rarely infested with weevils, whereas older portions of the stem were. The probability of finding weevils inside the stems decreased in younger cuttings (AVRDC, 1990). This was further confirmed in a related study where 1 to 8 week-old weevil-free plants were exposed to the weevil in the field. The numbers of weevils in vines increased with increase in vine age (r = 0.92**) (AVRDC, 1990). These results indicate that carry-over of the weevil from an infested crop to the new planting can be reduced by carefully selecting fresh cuttings for planting a new crop.
Control of alternative hosts
Several species of Ipomoea in addition to sweet potato, and a few related convolvulaceous plants are also alternative hosts of C. formicarius. Sutherland (1986b) listed 30 such species and four additional ones were recently found to harbour the weevil in Taiwan (AVRDC, 1989). A more complete and correct list of host plants of C. formicarius was presented by Austin et al. (1991). Among the convolvulaceous hosts, the insect overwhelmingly prefers sweet potato (Cockerham, 1943). The presence of alternative hosts, most of which are perennial, is important in the infestation of sweet potato weevil. Removal of these hosts growing in the vicinity of sweet potato plantings is recommended as a control measure (Gonzales, 1925; Franssen, 1935; Cockerham, 1943; Subramanian, 1959; Ho, 1970; Jayaramaiah, 1975; Wood, 1976). Indiscriminate elimination of wild Ipomoea, in pursuit of removing weevil sources, however, may lead to undesirable ecological effects. Availability of sex pheromone will aid considerably in quickly attracting weevils out of 'weevil-positive' Ipomoea, and only these plants will need to be eliminated. Alternatively all Ipomoea can be eliminated for one cropping season and allowed to grow in the subsequent seasons, once the area is free of the weevil. In this manner it is possible to eradicate the weevil with concentrated efforts. It has been shown in Taiwan that the removal of alternative hosts and volunteer sweet potato plants reduced the level of weevil infestation (Talekar, 1983).
Other cultural practices which may help reduce weevil damage and which are often advocated are: planting cuttings deep in the soil (Holdaway, 1941), use of deep-rooted cultivars (Franssen, 1935), and harvesting the crop as soon as it has developed roots of acceptable size (Edwards, 1930; Holdaway, 1941; Sherman and Tamashiro, 1954; Sutherland, 1986a). Planting weevil-resistant sweet potato cultivars also represents a potential cultural control method, however, a cultivar with a reliable level of resistance to the weevil is not yet available (Talekar, 1987b).
During the past 50 years, numerous attempts have been made to find sources of resistance mainly to Cylas species and to incorporate the resistance in agronomic cultivars. This line of research has been followed mainly at USDA laboratories, and at the International Institute of Tropical Agriculture (IITA) in Nigeria and AVRDC in Taiwan since their establishment in the early 1970s. However, despite these efforts, not a single sweet potato cultivar has been bred using previously identified sources of resistance, which is grown in any appreciable area to control Cylas species. Moderate levels of resistance have only been obtained in localized growing areas. Efforts to find resistant cultivars have been thwarted by the differences in weevil infestation among trials, locations, seasons, and at times among replicates of a single accession in a trial, among plants in the same plot, and even among storage roots within one plant (Talekar, 1982, 1987a). Environment seems to play a very significant role in host plant-insect pest interaction between weevil and the sweet potato (Talekar, 1987b). Transgenic cultivars containing Bacillus thuringiensis (Bt) endotoxin genes may confer resistance to C. formicarius (Garcia et al., 2000).
Numerous chemical insecticides have been tested for the control of C. formicarius despite the hidden mode of the insect's life cycle, which may thwart efforts to control this weevil by conventional insecticides. Sutherland (1986a) listed 59 different insecticides, including botanicals of unknown chemical composition, that were tested against sweet potato weevil. These chemicals, most of which were applied as post-planting foliar sprays, resulted in varying levels of control.
Pre-plant insecticide applications have been used to exterminate weevils from the planting material (vine cuttings) before planting. Insecticides with adequate water solubility are presumably transported through the vine and kill the weevils in that plant part. This type of treatment is usually more economical than post-plant insecticide applications, and if combined with proper sanitation and other measures to prevent immigration of weevils from infested plants, may result in satisfactory control of the weevil (Sherman, 1951; Sherman and Mitchell, 1953; Sherman and Tamashiro, 1954; Wolcott and Perez, 1955; Talekar, 1983).
Control of the weevil is difficult with conventional spraying, dusting, fumigation or side-dressing of insecticide granules with presently available insecticides, once weevils are present within the crown or the tuberous root. Control achieved by post-plant applications appears to be due to mortality of weevil adults searching for feeding or oviposition sites. Movement of adult weevils may facilitate the contact between the toxicant and the insect, thereby resulting in insect mortality. Several researchers have obtained satisfactory control of the weevil by spraying vines or soil around stems (Waddill, 1982; CTCRI, 1982; 1985; Rajamma and Padmaja, 1983). This method of control, however, requires frequent applications in order to kill adults that might migrate from other areas. This view concurs with that of Sakae (1988) in Japan. Frequent spraying of insecticides, however, is not cost-effective due to the low market price for sweet potato in developing countries.
The existence of a female sex pheromone in sweet potato weevil was demonstrated (AVRDC, 1976; Coffelt et al., 1978; Russo, 1973) and Heath et al. (1986) isolated, identified and synthesized the chemical (Z)-3-dodecen-1-ol(E)-2-butenoate. This chemical has great potential for attracting male sweet potato weevils (Proshold et al., 1986; Jansson et al., 1992) and reducing the weevil populations in the field (Talekar and Lee, 1989; Braun and van de Fliert, 1999). Weevil density in fields was notably decreased in a mass trapping trial in Japan (Yasuda, 1995). Systematic trapping with pheromone resulted in a 8.5-10% decrease in sweet potato tuber damage in a Chinese study (Li, 1998). Because of its potency and relatively long persistence, this chemical may be used in various ways to combat sweet potato weevil in an integrated programme (Talekar, 1990).
There are several reports of predators and parasites attacking sweet potato weevil (Waterhouse and Norris, 1987). Jansson (1991) gave an up-to-date list of predators, parasites, pathogenic fungi, bacteria and nematodes that attack Cylas species. Hardly any efforts have been made to introduce these natural enemies to combat sweet potato weevils.
The fungus Beauveria bassiana is produced in large quantities and used intensively for the control of the sweet potato weevil in Cuba, where sprays of the fungus have largely replaced the use of insecticides (Castellon et al., 1992). Synthetic sex pheromone can be used to attract male C. formicarius to traps containing B. bassiana, in which the weevils become auto-infected and subsequently spread infection to both sexes in field populations (Yasuda, 1999).
Among the three predators, only Pheidole megacephala is reported to be an effective biological control agent of C. formicarius in Cuba (Castiñeiras et al., 1982). This predator was more effective than chemical insecticides at controlling sweet potato weevil. Root yields in plots where P. megacephala was released to control weevils were 21.5 t/ha compared with only 7.8 t/ha in plots that relied solely on chemical insecticides (Morales, 1988). More recently, Castellón (1990) reported that the predatory ant, Tetramorium guineense, is as effective as P. megacephala in Cuba.
Integrated Pest Management
No specific control measures, used singly, can provide adequate control of C. formicarius where sweet potato is grown throughout the year and the weevil is endemic. However, a combination of tactics can give satisfactory control of the pest. With the exception of biological control and applications of insecticides, all other control measures are fully compatible. Biological control agents are quickly eliminated by chemical insecticides.
Two principal sources of weevil that play an important role in the infestation of new sweet potato planting are; (1) carry-over of the insect in cuttings taken from old infested fields and (2) immigration of the weevil from alternative hosts or weevil-infested crops to the new planting. For the successful control of the weevil, these two weevil sources must be attended to. Integration of several control components has potential in reducing and possibly preventing the crop being infested by the weevil from these two sources. Practicality of such IPM was successfully demonstrated on farmers' fields in Taiwan (Talekar et al., 1989).
The International Potato Center has tested in Cuba, a strategy for implementing the integrated management of the sweet potato weevil on a large scale in close collaboration with the Instituto de Investigaçion de Viandes Tropicales ( INIVIT), its Cuban counterpart. In a 3-year period, the programme was implemented on more than 30,000 ha. Damage was reduced from 40-50% to 4-8% and the number of insecticide sprays was reduced from 10-12 per season to none in 1996 (except for localized applications around pheromone traps) (Alcázar et al., 1997).
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