Two South African Beauveria bassiana (Balsamo) Vuillemin (Hypocreales: Cordycipitaceae) strains, PPRI 7598 (non-commercial) and PPRI 7861 (commercial), were compared for virulence against the Russian wheat aphid (RWA), Diuraphis noxia (Kurdjumov). Pre-selection (21 isolates) of the most virulent strain was based on >95 % mortality of the greater wax moth, Galleria mellonella L. (Lepidoptera: Pyralidae) larvae and >90 % conidial germination. In comparison with the chemical standard, Aphox® WG (pirimicarb), which induced 100 % aphid mortality within 24 h, both B. bassiana strains were confirmed pathogenic to D. noxia over seven days post-inoculation. The B. bassiana strain PPRI 7598 caused a significantly higher cumulative mortality of 55.64 ± 2.32 % at 1 × 108 conidia/ml than the 43.1 ± 2.89 % recorded with PPRI 7861. Additionally, a significantly higher level of overt mycosis was recorded with PPRI 7598 (61.63 ± 1.66 %) in comparison with PPRI 7861 (52.06 ± 2.81 %), both at higher concentrations. The interaction between time (days) and treatment was significant, with an increase in aphid mortality over time. The results suggest that the two tested strains were significantly more pathogenic to aphids at the higher concentration (1 × 108 conidia/ml). In addition, these findings also direct our future research focus on exploration of PPRI 7598 against other cereal aphids, including oat aphid, Rhopalosiphum padi, and as endophyte for induced systemic resistance to insects and diseases.
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Vol. 28 • No. 2