Entomopathogenic Nematodes as a Tool for the Management of Sweetpotato Weevil Cylas Puncticollis Boheman in Kenya

Abstract

Entomopathogenic nematodes Heterorhabditis indica, Steinernema karii and Steinernema yirgalemense from Kenya were tested against the sweet potato weevil Cylas puncticollis Boheman in laboratory bioassays and in the field for three consecutive seasons. The trials were carried out at the Kenya Agricultural Research Institute, Kabete. Sweet potato varieties KSP20, K004 (Kakamega 4) and (K10) Kemb 10 were also evaluated for susceptibility to the weevil in the field for one season and Kemb 10, the most susceptible variety tested in subsequent seasons. The assessments were carried out during growth, at harvest and one month after harvest. The conventional pesticide bifenthrin, the entomopathogenic nematode Steinernema carpocapsae ex UK strain and control were used for comparative purposes. Mean percent weevil larval mortality was 76, 71 and 48 for H. indica, S. yirgalemense and S. karii respectively and adult mortality was 12.5% for H. indica, S. yirgalemense and 25% for S. karii. Kemb 10 was the most susceptible sweet potato variety, K004 the most tolerant and KSP20 moderately tolerant. Nematodes reduced weevil infestations only slightly if initial infestations were over 80% and the weather conditions dry. The nematodes however reduced the rate of weevil development in tubers in the store by 75%. Nematodes were as effective as bifenthrin when weevil infestations were moderate and the weather cool and moist. S. carpocapsae the ex-UK strain was the most effective on the weevil overall but H. indica was the most effective indigenous nematode. H. indica when used with appropriate insecticides in a sequential manner was an effective strategy for sweet potato weevil management.