IPM For Sweet Potato Weevil

The sweetpotato weevil, Cylas formicarius, is the most serious pest of sweet potato throughout the tropics and subtropics. While the adult weevil does minimal damage while feeding on the tuber surface or foliage, the larvae feed inside the roots and stems, often destroying entire tubers. Even slightly damaged roots are usually unsuitable for human consumption due to an off-flavor that develops as a result of the partial damage.

Damage occurs both in storage and in the field; therefore for successful control of the insect is needed in the field. Based on extensive research at the experiment station and on farmers fields, researchers at the Asian Vegetable Research and Development Center (AVRDC) have developed an effective and inexpensive control method.

Female weevils look for exposed roots to lay their eggs. The roots are usually found through cracks in overly dry soil. The female makes a small hole through the skin of the tuber and lays her eggs. The larvae that develop feed inside the root. Some eggs are also laid in the stem, but the resulting larvae feeding inside the stem only rarely reduce the yield.

Sweet potato weevils live and feed on all the Ipomea species, including the various morning glory varieties that are often found as weeds in the fields. Weevil infestations, therefore, usually come from infected planting material, planting in already infected fields, infected fields within 200 meters or infected morning glory plants found in the field borders.

Protection from the weevil is accomplished by minimizing the introduction of the weevil from these sources. First, the fields growing sweet potatos should be rotated whenever possible. If the fields cannot be rotated, they should be cleaned of small roots and stems that may be infected with the larvae. In those fields with adequate access to water, flooding the field for up to a week will cause the leftover debris to rot and kill the existing larvae. Avoid planting sweet potato next to other fields that have infestations or where control measures are not being undertaken. In smallholder areas, this may necessitate cooperation in the implementation of control measures that will benefit the whole community.

Throughout the year, keep other Ipomea species such as morning glory away from the vicinity of the fields. When the entire area is free of weevils, these flowers can be allowed to return. Use weevil free cuttings to plant new fields. This is extremely important. If cuttings cannot be obtained from fields with no weevil infestations, then cuttings should probably be treated by dipping in a suitable insecticide solution. Thoroughly soaking the infected cuttings in dilute (.01-.05% active ingredient) will kill any existing larvae or eggs in the cuttings. This treatment will not protect the plants against future infestations. Extreme caution and protective gloves and clothing should be used in handling the solution and the treated cuttings both during and after the dipping.

Post planting controls should include keeping the roots covered at all times. During drought or tuber enlargement extra care should be taken to ensure that cracks in the soil around the main stem are filled. One additional tool may soon become more available in the fight against the sweet potato weevil. Recently scientists have discovered a sex pheremone that attracts male weevils. By adequately deploying this chemical in traps around a sweet potato field, farmers should be able to prevent weevils from breeding. Small amounts of the chemical and plans for traps are available from AVRDC for field scientists and extension workers.

By following the above steps farmers should be able to dramatically reduce if not eradicate the sweet potato weevil in their communities. Denying the adult weevil access to Ipomea plants upon which to breed is the key to the success in this battle.

How to control sweetpotato weevil: a practical IPM approach. AVRDC 88-292. 1988.

For further info:

Dr. N.S. Talekar,AVRDC
PO Box 42 Shanua, Tainan
74199 Taiwan