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Plantwise Technical Factsheet

sweet potato weevil (Cylas formicarius)

Host plants / species affected
Calystegia sepium (great bindweed)
Colocasia esculenta (taro)
Cuscuta (dodder)
Ipomoea (morning glory)
Ipomoea aquatica (swamp morning-glory)
Ipomoea batatas (sweet potato)
Ipomoea cairica (five-fingered morningglory)
Ipomoea pes-caprae (beach morning glory)
Ipomoea purpurea (tall morning glory)
Ipomoea quamoclit (Cupid's-flower)
Jacquemontia tamnifolia (Smallflower morningglory)
Pharbitis nil (Japanese morning glory)
List of symptoms/signs
Leaves  -  external feeding
Roots  -  external feeding
Roots  -  internal feeding
Stems  -  distortion
Stems  -  external feeding
Stems  -  internal feeding
Symptoms
C. formicarius adults feed on the epidermis of vines, scraping oval patches off young vines and petioles. Adults also feed on external surfaces of storage roots resulting in round feeding punctures. These punctures are deeper than oviposition punctures. The developing larvae tunnel the vines and tuberous roots resulting in significant damage. Frass is deposited in tunnels. In response to the damage, tuberous roots produce terpene-like chemicals which render the damaged root inedible, even at low concentration and low levels of insect damage (Sato et al., 1982). Feeding inside the vines causes malformation, thickening and cracking of the vine (Sherman and Tamashiro, 1954). Leaves may become pale green, and growth and overall vigour of the plant is adversely affected (Trehan and Bagal, 1958). Occasionally adult weevils feed on leaves chewing away portions of leaf lamina or scraping small patches of major veins and petioles.
Prevention and control

Introduction

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 Control

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.

Crop rotation

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.

Intercropping

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.

Mulching

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

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.

Clean cuttings
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).

Host-Plant Resistance

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. Nonetheless, 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. 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).

Chemical Control

Always use pesticides in a lawful manner which is consistent with the product’s label. Failure to follow the label may result in crop injury, poor control or residue problems. Consult the list of registered pesticides for your country to determine which products are legally allowed for use. Certain pesticides are acknowledged to present particularly high levels of hazards to human health and/or the environment according to internationally accepted classification systems (see the Plantwise pesticide red list), and as a consequence their use is not advised.
For further information, we recommend you visit the following resources:


Impact
Introduction

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 and several African countries. It has recently been inadvertently introduced in two South American countries, Venezuela and Guyana. Few areas in the above regions where sweet potato is grown are free from its destruction. The crop losses from weevil damage range from 5 to 80%, with weevil 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.

Asia

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.

Africa

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.

Related treatment support
Plantwise Factsheets for Farmers
Moreno, O. L.; CABI, 2012, English language
Moreno, O. L.; CABI, 2012, Spanish language
Mtui, H. D.; Ndomba, R.; CABI, 2013, English language
Mtui, H. D.; Ndomba, R.; CABI, 2013, Swahili language
Mpimpa, S.; CABI, 2013, English language
 
Pest Management Decision Guides
Mwangi, J.; Khalid, I.; CABI, 2014, English language
Spencer, J. D.; Johnson, R. A. B.; Swaray, J. M.; Kpana, F.; CABI, 2012, English language
CABI; CABI, 2017, Portuguese language
Macharia, E.; Ndungu'u, B. N.; Otipa, M.; CABI, 2014, English language
Nuñez, C.; Fortin, C.; Castillo, A.; Castillo, E.; CABI, 2013, Spanish language
 
External factsheets
Biovision Factsheets, Biovision Foundation, 2012, English language
TNAU Agritech Portal Crop Protection Factsheets, Tamil Nadu Agricultural University, English language
TNAU Agritech Portal Crop Protection Factsheets, Tamil Nadu Agricultural University, Tamil language
CARDI Factsheets, CARDI, English language
Clemson Cooperative Extension Factsheets, Clemson University Cooperative Extension, 2009, English language
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