Abstract

The virulence of aerial conidia of the entomopathogenic fungi Beauveria bassiana (Balsamo-Crivelli) Vuillemin (Hypocreales: Cordycipitaceae) has been documented on a very broad range of arthropods; however, the activity of blastospores against the two-spotted spider mite has been seldomly reported. Moreover, studies investigating the activity of blastospores under greenhouse conditions were not found in the literature. In this study, we tested the efficacy of blastospores and aerial conidia of two B. bassiana strains, against all life stages of Tetranychus urticae Koch (Acari: Tetranychidae). The concentration of 109 blastospores/ml of the Lebanese strain was the most efficient in laboratory experiments, and the recorded mortalities were 52, 67.9 and 95.3% in treated eggs, motile juveniles and adults, respectively. In greenhouse experiments, higher T. urticae mortalities were recorded among strawberry plants (Fragaria x ananassa) sprayed with blastospores of B. bassiana. Furthermore, the latter plants also recorded a higher yield (454.92 ± 5 g/plant) compared to infested control plants (144.42 ± 5 g/plant) during the growing season. In addition, this study also investigated for the first time the resistance development of T. urticae to different types of infective propagules of B. bassiana. Under selective pressure for 40 generations, the laboratory yielded aerial conidia (AC-TSM) and blastospores (BS-TSM) resistant strains of the mite recorded 28.8- and 8.4- fold change in LC50 values respectively when compared to the field strain; however, this resistance ratio was minimal when compared to those of chemical acaricides.