Major Pests and Diseases of the sweet potato (Ipomoea batatas)
IN Guyana Sweet Potato is the second most widely grown root tuber after yams and cassava. The crop is largely produced under traditional, low input technology and consequently, often suffers poor quality yields. Much of this is due to inadequacies in crop management practices.
Economic damage by diseases, pests and weeds of sweet potato is relatively moderate, although sweet potato weevil can be a menace in some regions, if the problem is not identified early, and remedial action not implemented in a timely manner.
INSECT PESTS OF SWEET POTATO (IPOMOEA BATATAS) IN GUYANA
Sweet Potato Weevil (Cylas formicarius)
Among the 300 insect and mite species that feed on sweet potato in the tropics and subtropics, only sweet potato weevil (Cylas formicarius) is the most destructive insect pest. No resistant source is available.
Cultural Control: Integrated pest management for this insect is recommended, consisting of the following measures: • Crop rotation; • Eradication of Ipomoea weeds; • Use of clean planting material; • Deep planting; • Regular hilling to fill soil cracks around plants; and • Use of sex pheromone which is effective to trap male weevils; Chemical Control: • Pre-plant slip treatment using Triazophos 40%EC; and • Soil treatment using Basudin 60 per cent EC once monthly.
Vine Borer Symptoms (Omphisa anastomosalis)
O. anastomosalis adults are white with a brownish-yellow pattern on the wing. They lay slightly domed, greenish eggs with a flat base, on the upper and lower surfaces of the leaf and on petioles. The eggs are usually laid singly and egg incubation lasts for about one week. Soon after hatching, the larvae bore into the stems and gradually eat their way down the vines. Full-grown larvae are 30 mm long and light purple, although they may also be yellowish-white. The head capsule is brown, the ventral surface and legs are white, and the back and lateral sides have yellowish-brown grooves.
The larval period usually lasts 30-35 days but may vary between 21 and 92 days depending upon temperature. Pupation usually takes place in the vine but larvae may also bore into storage roots and pupate when roots are close to the soil surface. The adults are active at night. Both males and females mate at one to six days old.
Cultural Control: • Integrated Pest Management; and • Crop rotation Chemical Control: • The use of Sevin and/or Decis gives effective control of O. anastomosalis, increasing the yield of treated plants. • Systemic insecticides such as Carbofuran are applied to the soil in the vicinity of the main stem. The chemical is translocated, killing the insect larvae boring in the stem, but does not come into contact with natural enemies. However, treatment with Carbofuran is barely economical because the price of sweet potato roots in local markets is low
Cricket Gryllotalpa spp. (Orthoptera: Gryllotalpidae) Acheta spp.
(Orthoptera: Gryllidae) Mole crickets, which have heavily sclerotised front legs that are adapted for digging, are usually common in sandy soils. All crickets are nocturnal, feeding at night and secluded by day, under the soil. They feed at or slightly below the soil surface and can cause considerable damage before being discovered. Crickets spend their entire life cycle below the soil, which may go through a period of approximately 28 -35 days. They are termed soil insects.
Adult Cricket Sympton
Crickets eat tubers of sweet potatoes. Fully grown crickets are brown in colour and are about 2.5 – 3.5 cm long. The various species of these insects usually live in the soil, bushes and under decaying crop residues and vegetation.
Cultural control: • The areas where sweet potatoes are grown should receive full sunlight, kept clean of weeds and all crop residues should be removed and burnt. • Proper land preparation serves to control weeds, diseases, and soil insects, and also helps in the destruction of large soil clods, which act as hiding places for cricket. • Good field sanitation- rid the field of weeds and plants residues from previous crops. Chemical control: • Any approved soil insecticide at the recommended rate may be applied, such as Basudin 60% E.C (Diazinon) or Vydate L 40 percent E.C at the rate of 10 mls to 4500 mls water to cultivated areas.
White Flies Bemisia tabaci (Homoptera: Aleyrodidae)
These insects are in fact bugs. The adults are white moth-like insects (Figure 4) that fly upwards from the plant when disturbed. They are about 2 mm in length and their wings are covered with a white waxy powder. The pinhead-size nymphs are oval and flattened and are attached to the leaf surface until maturity. All stages of this pest can be found on the underside of leaves. Nymphs and adults feed by sucking plant sap, resulting in leaves becoming mottled, yellow and brown before dying. Feeding whiteflies excrete honeydew on the leaf surface which encourages the growth of sooty mould, thus hampering photosynthesis. Ants are also attracted to the honeydew. This pest is also a vector of viral diseases. The life cycle may be completed in about 28-35 days.
Cultural Control: • Do not plant a new crop next to one which is mature: The common practice of having mature crops adjacent to newly planted ones makes management of the pest very difficult since the cycle of the pest is never broken. • An integrated control strategy is necessary for the effective management of this pest. • Good farm sanitation, including removal of weeds around the cultivation, is necessary since weeds serve as hosts for the pests. Chemical Control: • Several new generation insecticides are now available for the effective control of white flies. Targeting both nymphs and adults with soap-based products should be applied very early in the morning or late in the evening. Other chemicals which may be used include Admire, Pegasus and/ or Basudin and Vydate L at 10 mls to 4500 mls water.