Efficacy of Two Serotypes of Bacillus thuringiensis on the African Bollworm, Helicoverpa armigera Hiibner (Lepidoptera: Noctuidae), on Snap Bean (Phaseolus vulgaris L.) in the Central Rift Valley of Ethiopia

Abstract

African bollworm (ABW), Helicoverpa armigera Hiibner, is an important insect pest inflicting damage on snap bean (Phaseolus vulgaris ^ p r o d u c e d as vegetable in the Central Rift Valley (CRV) o f Ethiopia. The efficacy o f two serotypes o f Bacillus
thuringiensis (Bt) namely kurstaki (Delfin*) and aizawai (P ro b it'j, were evaluated against African bollworm at Ethio-Vegfru farm, Koka, in the Central Rift Valley. The experiment was laid out in a factorial randomized complete block design with three
replications. The two serotypes o f Bt were applied at three rates (0.25, 0.5 and 075 kg ha'1) and at two different application frequencies (4 and 7 day intervals) from flower initiation until harvest. Treatments were applied five and three times for the 4 and 7
days application intervals, respectively. Deltamcthrin (Decis*’ 25 EC) applied at 0.3 L ha'1 weekly and untreated control were included as checks. Data on number o f ABW larvae, pod damage both by number and weight and marketable yield were recorded.
Damage level was rcduccd with increased rate and frequency o f application. The high rate of Dclfin applied both at 4 and 7 days interval and the high rate o f Probit applied at 4 days interval significantly reduced pod damage compared with the untreated control.
Delfin at the high rate performed significantly better than Probit and its low rate treatments in reducing infestation level and minimizing yield loss. In conclusion, application o f high rate o f Delfin weekly from flower initiation up to harvest for three
times was the most effective in reducing the pod damage and associated yield loss, and can be used in place o f pyrethroid insecticides such as deltamethrin that are currently in use in the central rift valley.