Improving the efficiency of Beauveria bassiana applications for sustainable management of Plutella xylostella (Lepidoptera: Plutellidae) in West Africa

The effectiveness of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin isolates Bb11, Bb115, Bb116 and Bb362 against the diamond back moth (DBM), Plutella xylostella (L.) population from Cotonou, Benin, was investigated in the laboratory and on station. In the laboratory, six concentrations, 10(4), 10(5), 10(6), 10(7), 10(8) and 10(9) conidia/ml of each isolate were used to assess B. bassiana virulence. Third instar DBM larvae were inoculated with 2 mu l of each isolate's concentrations by topical application method. Control larvae were inoculated with sterilized Tween 80 solution (0.05%). Compared with other isolates, Bb11 was more virulent against DBM with LD50 value estimated to 4.96 x 10(6) conidia/ml and the highest rates of cadaver sporulation. The effectiveness of Bb11 against DBM on cabbage was subsequently assessed on station and on farm in Benin and Togo, at a low doses of 53 g/ha once or twice a week and a high dose of 315 g/ha weekly. Compared with unsprayed control and deltamethrin treatments, the dose of 53 g/ha of Bb11 applied twice a week at intervals of 4 days was able to reduce on farm the density of DBM by 83% and 93%, respectively. Also, the marketable cabbage yield obtained on farm with this dose was significantly higher compared with the other treatments, with a 199% and 452% increase in Danyi and Cotonou over the unsprayed control, respectively. These results confirm the better performance of Bb11 over other isolates, and also indicate that the newly investigated application dosage and frequency of Bb11 could be an efficient management option to control DBM in Benin and Togo.