Potential of Selected Isolates of Entomopathogenic Fungi in Causing Lethal Infection against Various Developmental Stages of the Cocoa Pod Borer, Conopomorphacramerella (Snellen) (Lepidoptera: Gracill

The pathogenicity of three species comprising 15 isolates of entomopathogenic fungi was evaluated in the laboratory against the eggs, last instar larvae (prepupae), pupae and adults of the cocoa pod borer (CPB), Conopomorpha cramerella (Snellen). The fungal isolates used were from Metarhizium anisopliae [Ma(SBN), Ma(Chi), Ma(SIB), Ma(SB8), Mman, MaPs, Ma1, Ma2, MMaj, MaGm], Beauveria bassiana [BBgc, Bb1, BbPs, BbPc] and Iseria fumosoroseus (Pf). Effective isolates were also tested for their field efficacy against the adults. Larvae (prepupa) inoculated with 0.5 mL conidial suspension of 1x106 conidia mL-1 in 0.05% aqueous Tween 80 began to manifest external disease symptoms three days after inoculation (DAI). All the isolates could cause infection with cumulative larval mortalities ranging from 55-100% at a concentration of 1x106 conidia mL-1. Five superior isolates with the mean percentage mortalities in excess of 95% were BBgc, Ma(SBN), Ma(Chi), Ma(SB8) and Ma(SIB) with 100, 100, 97.8, 97.5 and 95.0% mortality, respectively. Adults inoculated with 0.5 mL of these five superior isolates at concentrations ranging from 1x104 to 1x108 conidia mL-1 began to show disease symptoms at 3 DAI. Six days after fungal application BBgc was the most efficacious recording 100% mortality followed by Ma(SBN) with 97.14%, Ma(SIB) with 94.29%, Ma(Chi) with 91.43% and Ma(SB8) with 82.86%. Eggs inoculated with 0.5 mL of 1x106 conidia mL-1 of the preceding two most effective isolates, BBgc and Ma(SBN), in 0.05% aqueous Tween 80 did not prevent hatching and subsequent boring of the pods, hatchability being 100%. Pupae inoculated with 0.5 mL conidial suspension of 1x107 conidia mL-1 using these isolates revealed that none could cause more than 50% infection at 7 DAI; however, isolate BBgc gave significantly higher mean percentage mortality of 43.12% compared to 26.3% for isolate Ma(SBN). Tests against the adults showed infected moths became moribund within 3-4 days and fully infected by the fifth day of treatment (DAT). Only isolate BbGc recorded complete mortality followed by Ma(SBN) with 97.14%, Ma(SIB) with 94.29%, Ma(Chi) with 91.43% and Ma(SB8) with 82.86% at 6 DAT. A field spray consisting of a suspension of 10 mL of 1x108 conidia mL-1 of isolates BbGc, Ma(SBN) and their 50:50 combination in 0.05% aqueous Tween 80 applied against caged adults significantly gave a complete kill compared to the control by the 6 DAT, and their efficacy was equal to that of the insecticide Decis®.