Sweetpotato weevil, Cylas formicarius formicarius F is the most destructive pest of sweetpotato, Ipomoea batatas in tropical and subtropical regions. The larvae and adults feed on sweetpotato roots and crowns. For the past 12 years we have been screening sweetpotato germplasm for resistance to this pest in order to breed an agronomic cultivar with weevil resistance. Resistance screening is conducted at AVRDC and Penghu island in autumn and summer, respectively. Germplasm is planted between heavily infested source rows maintained in a weevil nursery. Roots are harvested, sliced into thin pieces, and the number of insects (larvae + pupae + adults) per unit weight, as well as the weight of damaged and healthy root slices, are recorded. The resistance rating is based mainly on the number of insects found in the root. Considerable differences in resistance rating are observed from location to location and season to season. Even at one location, tremendous differences are observed in weevil infestation of a particular entry among the replicates. Although over 1000 sweetpotato accessions have been screened, we have not found any that is consistently less damaged over several seasons at either location. Our recent screening of a population of I. trifida × I. batatas crosses show certain hybrids with high yield and low weevil infestation. These hybrids need to be evaluated further to confirm their resistance.

In a series of field experiments conducted since 1977, various cultural pest management techniques have been tested to reduce damage by the sweetpotato weevil, Cylas formicarius formicarius F. Planting rice after harvest of the weevil-infested crop does not reduce weevil damage in the ensuing sweetpotato crop if the source of weevil is present in the vicinity. Rice rotation initiated away from the weevil source, coupled with removal of the common wild-host Ipomoea sp., can reduce weevil infestation and maximize the effectiveness of carbofuran transplant dip treatment. In order to reduce the weevil source from the weevil-infested sweetpotato of the previous season, a 2-week flooding of the field soon after harvest practically eliminated the number of volunteer sweetpotato plants and the weevil. Covering the planted area with rice straw or plastic mulch reduced weevil infestation.