Synergistic effect of sodium ions on the virulence of Beauveria bassiana against Solenopsis invicta

To address the limitations of Beauveria bassiana in the biological control of Solenopsis invicta—notably its slow insecticidal action and unstable efficacy—this study investigated the synergistic potential of metal ion salts in enhancing fungal virulence, with the goal of developing a green, low-cost control strategy.

Using a laboratory-based spray bioassay, we systematically evaluated the lethal effects of six metal ion salts—NaCl, KCl, CaCl₂, MgCl₂, (NH₄)₂SO₄, and FeCl₃—at concentrations of 100 and 150 mmol/L when applied in combination with Beauveria bassiana (1×10⁷ spores/mL) against Solenopsis invicta workers. Furthermore, the influence of pH (under acidic, neutral, and alkaline conditions) on the synergistic efficacy of the same metal ion salt was examined.

None of the metal ion salts exhibited significant insecticidal activity when applied alone. However, when combined with Beauveria bassiana, NaCl demonstrated pronounced synergism, achieving a 97% mortality rate at 150 mmol/L within 96 h. Through concentration gradient optimization, a 125 mmol/L NaCl formulation in combination with the fungal suspension resulted in 100% mortality, with a markedly accelerated kill rate—88.3% cumulative mortality observed within 48 h. Bioassays conducted under different pH conditions revealed that, at this optimal concentration, NaCl exerted the strongest synergistic effect under neutral conditions (pH ≈ 7), achieving 100% mortality within 96 h—significantly higher than under acidic or alkaline conditions.

Sodium chloride effectively enhances both the virulence and killing speed of Beauveria bassiana against Solenopsis invicta, with its synergistic efficacy being highly dependent on environmental pH and peaking under neutral conditions. This simple, readily available, and cost-effective combination presents a viable synergistic strategy for the green biological control of S. invicta and offers significant potential for laboratory screening, biopesticide formulation, and field application.