| dc.description.abstract | The cowpea bruchid, Callosobruchus maculatus is a major pest of stored cowpea (Vigna unguiculata) in Kenya that has been reported to cause damage reaching 100% of stored cowpea grain. The cowpea bruchid is also known to decrease germination potential and reduce the commercial, nutritional and aesthetic value of the grain due to physical contamination by insects, eggs and excrement. There are several control methods for the cowpea bruchid including the use of the safe entomopathogenic fungi, Beauveria bassiana. Experiments were conducted at the KEPHIS laboratory to evaluate the morphological, growth characteristics of nine B. bassiana isolates. Evaluation of the spore concentration, viability, relative hyphal growth, rate of sporulation, and rate of conidial growth as well as virulence to the cowpea bruchid, C. maculatus was conducted for each isolate. The white to cream white colonies were characterized to be those of B. bassiana with conidia appearing round with single or branched conidiophores. The highest spore count and viability was recorded for isolate J59 and J57, respectively. The highest rate of sporulation was recorded for isolate BVT® while isolate BBC® recorded the highest rate of conidial growth. There were positive and negative correlation between the tested growth characteristic variables. Mortality of the cowpea bruchids was evaluated through mortality by immersion, effective dose, optimal dose and farmer storage simulation tests. Mortality rates increased by increasing concentration of B. bassiana conidia. The isolates J35, BBC®, J29, J39 and J59 recorded higher percentage mortality of C. maculatus at 4.86 × 107 cfu/ml concentration while J36, BVT® , J57, RI performed best at 4.86 × 106 cfu/ml concentration. The half and double rates recorded highest and lowest mortality of C. maculatus respectively for effective dose test. Isolate
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BVT® and J59 recorded the highest grain damage followed by control in assay one and two, respectively, while isolate J29 recorded the lowest grain damage in both assays when the isolates were used in suspension form. Isolate J57 recorded the highest grain damage followed by control in both assays, while isolate J59 recorded the lowest grain damage in both assays when the isolates were used in powder form. There were positive and negative correlations recorded between the growth characteristics and mortality of the cowpea bruchids. These findings provide preliminary pathogenicity status of B. bassiana isolates against C. maculatus in cowpea grain during storage and may in future be upscaled to field trials in view of commercial product development. | en_US |
