Selection of Beauveria bassiana strains against the two spotted spider mite, Tetranychus urticae Koch in laboratory and greenhouse trials

Abstract

South African strains (62) of Beauveria bassiana (Balsamo) Vuillemin were screened for pathogenicity against two spotted spider mite, Tetranychus urticae Koch, in laboratory bioassays on bean leaves, Phaseolus vulgaris L., under greenhouse conditions. In the first bioassay, strains of B. bassiana were applied at a single concentration of 107 conidia ml-1. A mortality mite percentage between 4-92.5% was observed, with 40% of strains causing mortality levels higher than 50%. The median lethal times (LT50) ranged between 5.5 - 8.6 days. The six most virulent strains were compared in a second screening, together with the commercial strain PPRI 5339. Five concentrations (2 × 104 to 2 × 108conidia ml-1) on female mites and three concentrations (2 × 106; 2 × 107 and 2 × 108conidia ml-1) on eggs were used. Mortality of mites assessed indicated that Strain PPRI 7315 and Strain PPRI 7861 performed similarly and were the most efficient, causing mite mortality of >80%, 9 days after inoculation, at the highest concentration, with a median lethal concentration (LC50) of 1.13 × 106 and 1.22 × 106 conidia ml-1, respectively. Both strains performed better than the commercial strain (PPRI 5339) in vitro and showed good control of T. urticae during greenhouse trials. Key words: Beauveria bassiana, biological control, control efficacy, strain selection,Tetranychus urticae.