Preliminary studies on the use of Beauveria bassiana to control Sitophilus zeamais (Coleoptera: Curculionidae) in the laboratory

Abstract

The virulence of ten isolates of Beauvena bassiana (Balsamo) Vuillemin to the maize weevil, Sitophilus zeamais Motsch., was tested in the laboratory. All isolates tested were capable of infecting S. zeamais but their virulence, determined by adult mortalities and median lethal time, varied. A total of five (I89-481, I90-520, I89-477, I90-533, and I94-907) highly virulent, three (I92-736, I93-906 and I92-761A) intermediate and two (I93-868 and I93-870) weakly virulent isolates were identified for S. zeamais. The growth and sporulation rates and persistency of water formulated conidia of the most virulent isolates were also studied. I90-520 was selected for further work and dose-mortality relationships and the infectivity of dry conidia against Sitophilus zeamais were determined. The lowest dose (10⁴ conidia ml⁻¹) caused about 88% mortality within eight days. To assess the infectivity of a dry formulation of the isolate, conidial doses of 0, 0.1, 0.5 and 1 g of B. bassiana per 25 g of popping maize seeds were compared with pirimiphos-methyl at 10 ppm. This demonstrated the potential of dry conidia of the isolate to infect and cause mycosis to S. zeamais. Adult mortality of S. zeamais and damage to maize seeds treated with different levels of dry conidia were not statistically significantly different from pirimiphos-methyl-treated maize after 14 days of storage period, but the chemical pesticide killed all target insects within 24 h, whereas mortality began at day 3 for the best B. bassiana treatment.