Photo 1. Adult West Indian sweetpotato weevil, Euscepes
postfasciatus, showing distinctive white patches on the wing
West Indian sweetpotato weevil
Euscepes batatae; previously Euscepes postfasciatus
Widespread. Asia (Japan), North America (California, Hawaii), Caribbean, South America, Europe (Portugal), Oceania. It is recorded from Cook Islands, Federated States of Micronesia, Fiji, French Polynesia, Guam, Kiribati, New Caledonia, New Zealand, Northern Mariana Islands, Palau, Papua New Guinea, Samoa, Tonga, Vanuatu, and Wallis & Futuna.
Sweetpotato, and wild relatives in the genus Ipomoea.
The larvae or grubs do the most damage, and this is similar to that caused by the sweetpotato weevil, Cylas formicarius. The larvae tunnel through the base of the stem (crown) and through the storage roots. In the storage roots, tunnelling produces chemicals called terpenes, which give the flesh an unpleasant taste. The adults also feed on the shoots and young stems, but the damage is small compared to that on storage roots and crown.
The eggs are round, yellowish, laid singly in pits in the stems (always at the nodes, the junction of stems and petioles) or in storage roots. After laying the eggs the pits are sealed with a faecal plug. Storage roots are preferred to stems. The eggs hatch in about 10 days, and the legless grubs moult four times over the next 20-30 days, reaching a length of about 8 mm. The pupae are white, about 5 mm long. After another 10 days the adults emerge. They leave the storage roots and stems by chewing exist holes. Adults are 3.5-4 mm long and 1.5 mm wide, reddish brown to greyish black, covered with stiff hairs, with two white patches on the wing cases (Photos 1&2); they are thought to live for about 6 months.
There is no evidence that the weevils spread by flying; however, this needs to be checked. It is more likely that they crawl short distances from weedy borders, and are spread longer distances in cuttings and in storage roots used for planting. Hot and dry weather favours weevil development. At 27-30°C, the life cycle takes about 30 days.
There is little information on the damage caused by this weevil, compared to that done by Cylas formicarius (see Fact Sheet no. 29). However, in Papua New Guinea is appears to be more common in storage roots than Cylas, in both highland and lowlands areas. As Euscepes is common in other Pacific island countries, the damage done by this weevil needs to be verified to find out which is the more important.
Look at the base of the vine (called the crown) for small holes and damage to the stem. Break the stem to look for tunnels, rots and larvae. Dig up a storage root and look for damage to the skin, and feeding tunnels of the larvae inside. Detection of early infestation is not easy as adults are most active at night, and they pretend to be dead when disturbed.
Use the guide prepared by the Australian Museum to distinguish between the larvae and pupae of Cylas and Euscepes1.
The unrestricted movement of plant propagating material (cuttings, shoots and storage roots) has the potential of further spreading the weevil, and should be done with caution. The FAO/IBPGR Technical Guidelines for the Safe Movement of Sweet Potato Germplasm (http://www.bioversityinternational.org/e-library/publications/detail/sweet-potato/) should be followed. Note, the weevil has not been recorded from Solomon Islands.
Little is known about natural enemies. Infections from the fungus Beauveria sp. are reported from Peru and Japan. However, the weevil causes greatest damage during dry weather, so strains of Beauveria are needed that persist and are pathogenic during such times. In Cuba, nests of ants (e.g., Pheidole, the big-headed ant) are put in gardens in rolls of banana leaves to control weevils.
Cultural control methods offer the best chance of reducing populations of the weevil to acceptable levels. Hygiene measures and the use of cuttings free from adults and eggs are two of the most important cultural control strategies:
None have been reported, although germplasm collections, such as that at the International Potato Center (CIP), Peru, have been screened for resistance.
AUTHOR Grahame Jackson
Information from CABI (2014) Euscepes postfasciatus (West Indian sweet potato weevil) Crop Protection Compendium. (www.cabi.org/cpc); and from 1Reid C Identification of sweetpotato weevil species in the west Pacific. Australian Museum. Sydney. Photos 1&2 Gerald McCormack, Cook Islands Biodiversity & Natural Heritage. (http://cookislands.bishopmuseum.org/).
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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