Combinedeffectsof<italic>Serratiamarcescens</italic>S-JS1withfiveinsecticidesagainst<italic>Laodelphaxstriatellus</italic>andtheeffectofS-JS1onactivitiesofrelativeenzymesin<italic>L.striatellus</italic>

The objective of this study is to evaluate the efficacy of combinations of Serratia marcescens isolate S-JS1 and insecticides against Laodelphax striatellus, and to investigate the effect of S. marcescens on the detoxifying and protective enzymes of L. striatellus. Third-instar nymphs of L. striatellus were used in all experiments. The efficacy of 5 insecticides (spirotetramat, thiamethoxam, imidacloprid, acetamiprid, and chlorpyrifos) alone and mixed with S. marcescens against L. striatellus were compared by spraying method. After L. striatellus feeding on S. marcescens-treated rice seedling, the activity changes of carboxylesterase (CarE), glutathione-S-transferases (GSTs), peroxidase (POD), and superoxide dismutase (SOD) in it were determined. The results showed that combinations of S. marcescens with insecticide of any concentration resulted in higher L. striatellus mortality compared to the effect achieved by insecticide alone. Compared to the mortality effect against L. striatellus using 1.25 mg/L thiamethoxam (44.24%) or 25 mg/L spirotetramat (49.22%), applications of 10⁹ cfu/mL S. marcescens + 1.25 mg/L thiamethoxam for 3 days (65.58%) or 10⁹ cfu/mL S. marcescens + 25 mg/L spirotetramat for 5 days (76.27%) show significantly higher activities. And interactions resulted in a synergistic responses (χ² < 3.84, observed mortality – expected mortality > 0). Other combination treatments of S-JS1 with insecticide of any concentrations all exhibited additive interactions. 12 and 24 h after L. striatellus feeding on S. marcescens-treated rice seedling, there was no significant difference in CarE activity between the treatment group and the control group. The activity of GSTs increased initially, then decreased. The activities of POD and SOD were always significantly lower than those in the control. Therefore, S. marcescens S-JS1 may weaken the defense activity of L. striatellus against insecticides. This study provides reference for exploring the combined applications of entomopathogenic bacteria and insecticides, and may provide a novel clue for the control of L. striatellus.