Investigation of the combined use of entomopathogenic microorganisms in the biological control of Western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae)

In our study, the combined use of entomopathogenic microorganisms in the control of Frankliniella occidentalis, a pest of significant importance to both the world and our country, was investigated. As a result of bioassays performed on larvae at a concentration of 108 cfu/ml, the isolates with the highest virulence were determined to be Serratia marcescens Se9 with a 54% mortality rate and Bacillus safensis Cq1 with a 51% mortality rate. For the adult stage, the isolates with the highest virulence were found to be S. marcescens Se9 (69.64%), B. safensis Cq1 (62.5%), and B. thuringiensis Sn10 (54%).For fungal isolates, the virulence of Metarhizium flavoviride As-18, Lecanicillium muscarium Pa3, and Beauveria bassiana Hp-4 on larvae was determined to be 92.1%, 85.1%, and 86.1%, respectively. Similar to larvae, the isolates with the highest virulence on adults were identified as M. flavoviride As18 (74.5%), B. bassiana Hp4 (70.5%), B. bassiana Pa4 (70%), and L. muscarium Pa3 (69.2%).The concentration trials of the bacteria revealed that B. safensis Cq1 isolate had the lowest LC50 value on larvae, while the LC50 value of S. marcescens Se9 isolate was not significantly different (p<0.05) from that of B. safensis Cq1. Due to its lowest LC50 value on adults, S. marcescens Se9 was selected as the primary bacterium for synergistic effect trials. For fungi, M. flavoviride As-18 isolate, which had the lowest LC50 value on both larvae and adults, was chosen as the primary fungus for synergistic effect trials.Nine different combinations of Se9 and As-18 isolates were prepared for synergistic effect trials, and efficacy trials were conducted on the pest's larvae. In the combination of LC25 values of Se9 and As-18 isolates (combination 7), an expected mortality rate of 43.32% was observed to be 47.63%. The cotoxicity factor for the combination was determined to be 9.94, indicating an additive effect. Similarly, additive effects were observed in combinations 8, 10, 11, and 12.Antagonistic effects were observed in combinations where S. marcescens Se9 isolate was included at LC25×100 concentration. The only synergistic effect in the combination trials on larvae was obtained with combination 9 [Se9 (LC25) + As-18 (LC25×100)]. In this combination, the expected mortality rate was 69.96%, while the observed mortality rate was 95.79%. The cotoxicity factor for the combination was calculated as 36.92, indicating a strong synergistic effect.