Virulence of the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae to sweet potato weevil Cylas puncticollis and effects on fecundity and egg viability
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The virulence of eight isolates of Metarhizium anisopliae and four isolates of Beauveria bassiana (Ascomycota: Hypocreales) to adult Cylas puncticollis (Coleoptera: Curculionidae) was tested in the laboratory. Insects were sprayed with a standard concentration of 1.0 × 107 conidia mL−1 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% mortality for the 12 isolates varied between 9.7 and 18.5 days. Four isolates, M. anisopliae International Centre of Insect Physiology and Ecology (ICIPE) 18 and ICIPE 62 and B. bassiana ICIPE 275 and ICIPE 114 were selected for dose–response mortality [lethal concentration to 50% mortality (LC50)] bioassays. Five concentrations (1.0 × 106, 3.0 × 106, 1.0 × 107, 3.0 × 107 and 1.0 × 108 conidia mL−1) of both fungal species were used. B. bassiana ICIPE 275 was the most active isolate with LC50 value of 0.7 × 106 conidia mL−1. The effect of fungal infection on feeding, fecundity and egg viability of C. puncticollis adult females was also investigated under laboratory conditions. M. anisopliae isolate ICIPE 18 and B. bassiana isolate ICIPE 114 were tested for feeding experiment using six concentrations (0, 1.0 × 106, 3.0 × 106, 1.0 × 107, 3.0 × 107 and 1.0 × 108 conidia mL−1). For reproduction potential (fecundity and egg viability) bioassays, five concentrations (0, 1.0 × 106, 3.0 × 106, 1.0 × 107 and 3.0 × 107 conidia mL−1) of M. anisopliae isolate ICIPE 18 were used. Adult sweet potato weevils (SPWs) treated with M. anisopliae at the concentrations of 3.0 × 107 and 1.0 × 108 conidia mL−1 consumed significantly less food than weevils in the control and B. bassiana treatments at all the concentrations, except at the higher concentration of 1.0 × 108 conidia mL−1, 14 days post-treatment. Female weevils in the control treatments laid more eggs than fungus-treated females. Percentage egg viability differences between controls and fungus treatments were significant at all the concentrations tested, 10 days post-treatment. These results show that B. bassiana and M. anisopliae are pathogenic to SPWs and infection can reduce feeding, fecundity and egg viability.