Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Cylas formicarius
(sweet potato weevil)

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Datasheet

Cylas formicarius (sweet potato weevil)

Summary

  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Host Animal
  • Preferred Scientific Name
  • Cylas formicarius
  • Preferred Common Name
  • sweet potato weevil
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Arthropoda
  •       Subphylum: Uniramia
  •         Class: Insecta

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Pictures

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PictureTitleCaptionCopyright
Sweet potato weevil. Adult length 4.80-6.75 mm.
TitleAdult
CaptionSweet potato weevil. Adult length 4.80-6.75 mm.
CopyrightPeter A.C. Ooi/CABI BioScience
Sweet potato weevil. Adult length 4.80-6.75 mm.
AdultSweet potato weevil. Adult length 4.80-6.75 mm.Peter A.C. Ooi/CABI BioScience
Cylas formicarius (sweet potato weevil); adult, dorsal view.
TitleAdult
CaptionCylas formicarius (sweet potato weevil); adult, dorsal view.
Copyright©Georg Goergen/IITA Insect Museum, Cotonou, Benin
Cylas formicarius (sweet potato weevil); adult, dorsal view.
AdultCylas formicarius (sweet potato weevil); adult, dorsal view.©Georg Goergen/IITA Insect Museum, Cotonou, Benin
Sweet potato weevil larva. The full-grown larva is 7.5-8.0 mm long.
TitleLarva
CaptionSweet potato weevil larva. The full-grown larva is 7.5-8.0 mm long.
CopyrightPeter A.C. Ooi/CABI BioScience
Sweet potato weevil larva. The full-grown larva is 7.5-8.0 mm long.
LarvaSweet potato weevil larva. The full-grown larva is 7.5-8.0 mm long.Peter A.C. Ooi/CABI BioScience
Root damage by larvae of the sweet potato weevil.
TitleDamage symptoms
CaptionRoot damage by larvae of the sweet potato weevil.
CopyrightPeter A.C. Ooi/CABI BioScience
Root damage by larvae of the sweet potato weevil.
Damage symptomsRoot damage by larvae of the sweet potato weevil.Peter A.C. Ooi/CABI BioScience

Identity

Top 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

EPPO code

  • CYLAFO (Cylas formicarius)

Taxonomic Tree

Top 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 Nomenclature

Top of page The subspecies Cylas formicarius elegantulus was created for the New World weevil to separate it from the Cylas formicarius formicarius of the Old World. But numerous sex pheromone studies indicate that these two groups are the same species and not two subspecies (NS Talekar, AVRDC, Taiwan, personal communication, 1996). Wolfe confirmed this with taxonomic studies (Wolfe, 1991).

Description

Top of page Adults

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.

Larvae

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.

Pupae

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.

See Kemner (1924) for detailed morphological descriptions

Distribution

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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 Table

Top 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.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

BangladeshPresentAhmad and Hossain, 1979; CABI/EPPO, 2004; EPPO, 2014
British Indian Ocean TerritoryPresentCABI/EPPO, 2004; EPPO, 2014
Brunei DarussalamWidespreadWaterhouse, 1993; CABI/EPPO, 2004; EPPO, 2014
CambodiaWidespreadCABI/EPPO, 2004; EPPO, 2014
ChinaWidespreadCABI/EPPO, 2004; EPPO, 2014
-FujianPresentCABI/EPPO, 2004; EPPO, 2014
-GuangdongPresentCABI/EPPO, 2004; EPPO, 2014
-GuangxiPresentCABI/EPPO, 2004; EPPO, 2014
-GuizhouPresentCABI/EPPO, 2004; EPPO, 2014
-HainanPresentCABI/EPPO, 2004; EPPO, 2014
-Hong KongPresentCABI/EPPO, 2004; EPPO, 2014
-HunanPresentCABI/EPPO, 2004; EPPO, 2014
-JiangsuPresentCABI/EPPO, 2004; EPPO, 2014
-JiangxiPresentCABI/EPPO, 2004; EPPO, 2014
-ShandongPresentCABI/EPPO, 2004; EPPO, 2014
-SichuanPresentCABI/EPPO, 2004; EPPO, 2014
-YunnanPresentCABI/EPPO, 2004; EPPO, 2014
-ZhejiangPresentCABI/EPPO, 2004; EPPO, 2014
Christmas Island (Indian Ocean)PresentCABI/EPPO, 2004; EPPO, 2014
Cocos IslandsPresentCABI/EPPO, 2004; EPPO, 2014
IndiaWidespreadCABI/EPPO, 2004; EPPO, 2014
-Andaman and Nicobar IslandsPresentCABI/EPPO, 2004; EPPO, 2014
-Andhra PradeshPresentCABI/EPPO, 2004; EPPO, 2014
-Arunachal PradeshPresentCABI/EPPO, 2004; EPPO, 2014
-AssamPresentCABI/EPPO, 2004; EPPO, 2014
-BiharPresentCABI/EPPO, 2004; EPPO, 2014
-ChhattisgarhPresentNetam et al., 2008
-GoaPresentCABI/EPPO, 2004; EPPO, 2014
-GujaratPresentCABI/EPPO, 2004; EPPO, 2014
-KarnatakaPresentCABI/EPPO, 2004; EPPO, 2014
-KeralaPresentCABI/EPPO, 2004; EPPO, 2014
-Madhya PradeshPresentCABI/EPPO, 2004; EPPO, 2014
-MaharashtraPresentCABI/EPPO, 2004; EPPO, 2014
-ManipurPresentCABI/EPPO, 2004; EPPO, 2014
-MeghalayaPresentCABI/EPPO, 2004; EPPO, 2014
-NagalandPresentCABI/EPPO, 2004; EPPO, 2014
-OdishaPresentCABI/EPPO, 2004; EPPO, 2014
-RajasthanPresentCABI/EPPO, 2004; EPPO, 2014
-Tamil NaduPresentCABI/EPPO, 2004; EPPO, 2014
-Uttar PradeshPresentCABI/EPPO, 2004; EPPO, 2014
-West BengalPresentSoumik et al., 2006
IndonesiaWidespreadCABI/EPPO, 2004; EPPO, 2014
-Irian JayaPresentCABI/EPPO, 2004; EPPO, 2014
-JavaPresentCABI/EPPO, 2004; EPPO, 2014
-MoluccasPresentCABI/EPPO, 2004; EPPO, 2014
-Nusa TenggaraPresentEPPO, 2014
-SumatraPresentCABI/EPPO, 2004; EPPO, 2014
JapanRestricted distributionCABI/EPPO, 2004; EPPO, 2014
-HonshuPresentCABI/EPPO, 2004; EPPO, 2014
-KyushuPresentYasuda, 1989; CABI/EPPO, 2004; EPPO, 2014
-Ryukyu ArchipelagoPresentCABI/EPPO, 2004; EPPO, 2014
-ShikokuPresentCABI/EPPO, 2004; EPPO, 2014
Korea, Republic ofAbsent, intercepted onlyCABI/EPPO, 2004; EPPO, 2014
LaosRestricted distributionCIE, 1993; Waterhouse, 1993; CABI/EPPO, 2004; EPPO, 2014
MalaysiaWidespreadCABI/EPPO, 2004; EPPO, 2014
-Peninsular MalaysiaPresentHo, 1970; CABI/EPPO, 2004; EPPO, 2014
-SabahPresentCABI/EPPO, 2004; EPPO, 2014
-SarawakPresentCABI/EPPO, 2004; EPPO, 2014
MaldivesPresentCABI/EPPO, 2004; EPPO, 2014
MyanmarPresentGhosh, 1940; Waterhouse, 1993; CABI/EPPO, 2004; EPPO, 2014
PakistanPresentCABI/EPPO, 2004; EPPO, 2014
PhilippinesPresentBernardo, 1986; Waterhouse, 1993; CABI/EPPO, 2004; EPPO, 2014
SingaporePresentWaterhouse, 1993; CABI/EPPO, 2004; EPPO, 2014
Sri LankaPresentCABI/EPPO, 2004; EPPO, 2014
TaiwanPresentTalekar and Lee, 1989; CABI/EPPO, 2004; EPPO, 2014
ThailandWidespreadWaterhouse, 1993; CABI/EPPO, 2004; EPPO, 2014
VietnamPresentHoang et al., 1989; Waterhouse, 1993; CABI/EPPO, 2004; EPPO, 2014

Africa

CameroonPresentCABI/EPPO, 2004; EPPO, 2014
ChadPresentCABI/EPPO, 2004; EPPO, 2014
Congo Democratic RepublicPresentCABI/EPPO, 2004; EPPO, 2014
EthiopiaPresentCABI/EPPO, 2004
GhanaPresentCABI/EPPO, 2004; EPPO, 2014
KenyaPresentNderitu, 1990; CABI/EPPO, 2004; EPPO, 2014
LiberiaPresentCABI/EPPO, 2004; EPPO, 2014
LibyaPresentCIE, 1993; CABI/EPPO, 2004
MadagascarPresentCABI/EPPO, 2004; EPPO, 2014
MauritiusPresentCABI/EPPO, 2004; EPPO, 2014
MozambiquePresentCABI/EPPO, 2004; EPPO, 2014
RéunionPresentCABI/EPPO, 2004; EPPO, 2014
SenegalPresentCABI/EPPO, 2004; EPPO, 2014
SeychellesPresentCABI/EPPO, 2004; EPPO, 2014
SomaliaPresentCABI/EPPO, 2004; EPPO, 2014
South AfricaRestricted distributionCABI/EPPO, 2004; EPPO, 2014
SudanPresentCABI/EPPO, 2004; EPPO, 2014
SwazilandPresentCABI/EPPO, 2004; EPPO, 2014
TanzaniaPresentCABI/EPPO, 2004; EPPO, 2014
UgandaPresentIngram, 1967; CABI/EPPO, 2004; EPPO, 2014
ZimbabwePresentCABI/EPPO, 2004; EPPO, 2014

North America

MexicoPresentCABI/EPPO, 2004; EPPO, 2014
USARestricted distributionCABI/EPPO, 2004; EPPO, 2014
-AlabamaPresentKarr, 1984; CABI/EPPO, 2004; EPPO, 2014
-ArkansasPresentASPB, 1920; CABI/EPPO, 2004; EPPO, 2014
-FloridaWidespreadJansson et al., 1991; CABI/EPPO, 2004; EPPO, 2014
-GeorgiaPresentMullen, 1984; CABI/EPPO, 2004; EPPO, 2014
-HawaiiPresentSherman and Tamashiro, 1954; CABI/EPPO, 2004; EPPO, 2014
-LouisianaPresentRolston, 1984; CABI/EPPO, 2004; EPPO, 2014
-MississippiPresentCABI/EPPO, 2004; EPPO, 2014
-New MexicoPresentCABI/EPPO, 2004; EPPO, 2014
-North CarolinaPresentSorensen, 1987; CABI/EPPO, 2004; EPPO, 2014
-South CarolinaPresentJones et al., 1980; CABI/EPPO, 2004; EPPO, 2014
-TexasPresentJansson et al., 1992; CABI/EPPO, 2004; EPPO, 2014

Central America and Caribbean

AnguillaPresentCABI/EPPO, 2004; EPPO, 2014
Antigua and BarbudaPresentCABI/EPPO, 2004; EPPO, 2014
BahamasPresentCABI/EPPO, 2004; EPPO, 2014
BarbadosAbsent, invalid recordEPPO, 2014
BelizePresentCABI/EPPO, 2004; EPPO, 2014
Cayman IslandsPresentCABI/EPPO, 2004; EPPO, 2014
CubaPresentCABI/EPPO, 2004; EPPO, 2014
Dominican RepublicPresentCABI/EPPO, 2004; EPPO, 2014
GuadeloupePresentDenon et al., 2009; EPPO, 2014
GuatemalaPresentCABI/EPPO, 2004; EPPO, 2014
HaitiPresentCABI/EPPO, 2004; EPPO, 2014
JamaicaPresentCABI/EPPO, 2004; EPPO, 2014
Netherlands AntillesPresentCABI/EPPO, 2004; EPPO, 2014
Puerto RicoPresentCABI/EPPO, 2004; EPPO, 2014
Saint Kitts and NevisRestricted distributionCABI/EPPO, 2004; EPPO, 2014
Saint LuciaPresentIntroduced Invasive CABI/EPPO, 2004; EPPO, 2014
Trinidad and TobagoPresentCABI/EPPO, 2004; EPPO, 2014
United States Virgin IslandsPresentCABI/EPPO, 2004; EPPO, 2014

South America

GuyanaPresentCleare, 1927; CABI/EPPO, 2004; EPPO, 2014
VenezuelaPresentGuagliumi, 1965; CABI/EPPO, 2004; EPPO, 2014

Europe

NetherlandsAbsent, intercepted onlyEPPO, 2014

Oceania

American SamoaPresentCABI/EPPO, 2004; EPPO, 2014
AustraliaRestricted distributionCABI/EPPO, 2004; EPPO, 2014
-Australian Northern TerritoryPresentCABI/EPPO, 2004; EPPO, 2014
-New South WalesPresentHamilton and Toffolon, 1986; CABI/EPPO, 2004; EPPO, 2014
-QueenslandPresentTryon, 1900; CABI/EPPO, 2004; EPPO, 2014
Cook IslandsPresentCABI/EPPO, 2004; EPPO, 2014
FijiPresentCABI/EPPO, 2004; EPPO, 2014
French PolynesiaPresentCABI/EPPO, 2004; EPPO, 2014
GuamPresentFullaway, 1912; CABI/EPPO, 2004; EPPO, 2014
KiribatiPresentCABI/EPPO, 2004; EPPO, 2014
Marshall IslandsPresentCABI/EPPO, 2004; EPPO, 2014
Micronesia, Federated states ofPresentCABI/EPPO, 2004; EPPO, 2014
New CaledoniaPresentCABI/EPPO, 2004; EPPO, 2014
New ZealandAbsent, intercepted onlyCABI/EPPO, 2004; EPPO, 2014
NiuePresentCABI/EPPO, 2004; EPPO, 2014
Northern Mariana IslandsPresentCABI/EPPO, 2004; EPPO, 2014
PalauPresentCABI/EPPO, 2004; EPPO, 2014
Papua New GuineaPresentSutherland, 1985; CABI/EPPO, 2004; EPPO, 2014
SamoaPresentCABI/EPPO, 2004; EPPO, 2014
Solomon IslandsPresentCABI/EPPO, 2004; EPPO, 2014
TongaPresentCABI/EPPO, 2004; EPPO, 2014
TuvaluPresentCABI/EPPO, 2004; EPPO, 2014
VanuatuPresentCABI/EPPO, 2004; EPPO, 2014
Wallis and Futuna IslandsPresentCABI/EPPO, 2004; EPPO, 2014

Hosts/Species Affected

Top of page 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.

Host Plants and Other Plants Affected

Top of page

Growth Stages

Top of page Flowering stage, Fruiting stage, Post-harvest, Vegetative growing stage

Symptoms

Top of page 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.

List of Symptoms/Signs

Top of page
SignLife StagesType
Leaves / external feeding
Roots / external feeding
Roots / internal feeding
Stems / distortion
Stems / external feeding
Stems / internal feeding

Biology and Ecology

Top 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.

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).

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.

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Beauveria Pathogen Adults
Beauveria bassiana Pathogen Adults
Bracon Parasite Larvae
Bracon cylasovorus Parasite Larvae
Bracon mellitor Parasite Larvae
Bracon punctatus Parasite Larvae
Drapetis exilis Predator Larvae
Euderus purpureas Parasite Larvae
Fusarium Pathogen Adults
Heterorhabditis Parasite Larvae
Heterorhabditis bacteriophora Parasite
Heterorhabditis heliothidis Parasite
Metapelma spectabile Parasite Larvae
Metarhizium anisopliae Pathogen Adults
Pheidole megacephala Predator
Rhaconotus Parasite Larvae
Rhaconotus menippus Parasite Larvae

Notes on Natural Enemies

Top of page The natural enemies of C. formicarius are poorly known so that it is not possible to evaluate them properly. Among the three predators only Pheidole megacephala is reported to be an effective natural 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). A similar role in Cuba is played by the predatory ant Tetra morium guineense (Castellón, 1990). The two predatory ants, P. megacephala and T. guineense, cannot co-exist in the same sweet potato field. In Florida, where P. megacephala is found, this predator is not an important natural enemy of C. formicarius (Jansson, 1991).

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.

Plant Trade

Top of page
Plant parts not known to carry the pest in trade/transport
Bulbs/Tubers/Corms/Rhizomes

Impact

Top of page 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.

Detection and Inspection

Top of page Colour: Bluish-black abdomen with brick reddish orange legs, antennae and thorax
Size: 4.8-6.75 mm long
Shape: Ant-like
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/Conditions

Top of page C. formicarius is very similar to the African sweet potato weevil, Cylas puncticollis. The latter is darker and blue-black in colour. It is also slightly larger, about 6-8 mm long.

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 Control

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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

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 application
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).

Post-plant application
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.

Sex pheromone
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). 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).

Biological Control

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. Sprays of the fungus have largely replaced the use of insecticides (Castellón et al., 1992).

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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