Athogenicity of entomopathogenic fungi beauveria bassiana (balsamo) and metarhizium anisopliae (metschnikoff) and potential of cultural and chemical control measures to cylas puncticollis (boheman) on sweetpotato tubers
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Sweetpotato (Ipomoea batatasy is one of the most important staple foods in Kenya, covering around 75,000 ha each year. The potential to increase its production is limited by damage from sweetpotato weevils, Cylas puncticollis and C. brunneus, which are present in all the sweetpotato-growing regions in Kenya, causing up to 90010 damage in the absence of adequate control measures. The management components of these insects include cultural control, host plant resistance, biological control, sex pheromones and chemical control. This study has focused on evaluating the efficacy of entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae under laboratory conditions and the potential of cultural and chemical control measures under field conditions in order to develop an integrated sweetpotato weevil management strategy. In the laboratory, four isolates of B. bassiana and eight isolates ofManisopliae were tested to determine their pathogenicity to the adult C. puncticollis. Insects were sprayed with a standard concentration of 1.0 x 107 conidia mrl using Burgerjon's spray tower. All the isolates tested were pathogenic to C. puncticollis. Mortality varied between 77.5 and 84.2% with isolates of B. bassiana and between 62.5 and 89.2% with isolates of M anisopliae, 26 days post-treatment. The lethal time to 50 percent mortality (LT5o)for the twelve isolates varied between 9.7 and 18.9 days. Four isolates, Manisopliae ICIPE 18 and ICIPE 62 and B. bassiana ICIPE 275 and ICIPE 114 were selected for dose-response mortality (LC50)bioassays because of their ability to grow fast on the insect cadaver. Five concentrations (1.0 x 106,3.0 x 106, 1.0 x 107,3.0 X 107 and 1.0 x 108 conidia ml") of both fungal species were used. Beauveria bassiana ICIPE 275 was the most active isolate with LC50value of 2.2 x 106 conidia mrl. The effect of fungal infection on feeding, fecundity and egg hatchability of C. puncticollis adults was also investigated under laboratory conditions. Metarhizium anisopliae isolate ICIPE 18 and B. bassiana isolate ICIPE 114 were tested for feeding experiment using six concentrations (0, 1.0 x 106, 3.0 x 106 , 1.0 x 107 , 3.0 x 107 and 1.0 x 108 conidia mrl. For fecundity and egg hatchability bioassays, five concentrations (0, 1.0 x 106 , 3.0 x 106 , 1.0 x 107 and 3.0 x 107 conidia ml") of M. anisopliae isolate ICIPE 18 were used. Feeding by the adult sweetpotato weevil generally decreased following fungal infection and M anisopliae significantly reduced the fecundity rate in the C. puncticollis adult female. Egg hatchability was dose-dependent with the highest concentration ofJ.O x 107 conidia ml-1producing the lowest hatchability. Field studies revealed that, C. puncticollis was associated with 8 wild host plants besides its primary suitable host, the sweetpotato (J. batatasi. The crop was also found to be associated with 18 insect species. Further studies indicated that combination of cultural practices and pesticide application are effective in suppressing weevil population resulting to less damage and significant yield insome sweetpotato cultivars. Results obtained from laboratory experiment suggest that M anisopliae and B. bassiana are potential biocontrol agents for the management of C. puncticollis on sweetpotato while field experiment suggests that C. puncticollis is vulnerable to suppression by cultural practices and conventional insecticide application. In general, laboratory and field results reveal that more than one application method is vital in managing sweetpotato weevil thus; integrated pest management strategy remains the potential option in controlling the weevil.