Combining Metarhizium anisopliae with spinetoram synergistically suppresses Frankliniella occidentalis via reciprocal enhancement

The combination of insecticides and entomopathogenic fungi, valued for both high efficacy and environmental compatibility, has become an important strategy for sustainable pest management, and their efficient combinations and underlying mechanisms have emerged as key research topics. Metarhizium anisopliae CQMa421 combined with compatible spinetoram exerts synergistic insecticidal activity against Frankliniella occidentalis, enhancing pest susceptibility to spinetoram, suppressing second-generation nymph, pupa and adult populations, and reducing the overall pupation rate. Mechanistically, the combined treatment upregulated the resistance target nAChRα6 and disrupted the activity of resistance-associated detoxifying enzymes (GST, CarE, POD, and CYP450) compared with spinetoram, while downregulating immune genes (GNBP1/2, Spätzle1/2, MyD881, Defensin, and PPO) compared with CQMa421. Accordingly, the median times for spore germination (GT50) and appressorium formation (AT50) of CQMa421 on the thrips cuticle were significantly shortened. Field trials showed that the combination (1 × 107 conidia/mL CQMa421 + 10 mg/L spinetoram) provided superior rapid and persistent control (90.67% at 3 d; 86.33% at 14 d) compared with spinetoram at 20 mg/L (74.33% and 43.33%, respectively). Consistently, in the continuous control trial (six applications of the combined treatment vs nine of spinetoram), the combined treatment maintained significantly lower thrips population densities during peak infestation. This study shows that CQMa421 combined with spinetoram disrupts detoxification enzyme activity, enhances pesticide sensitivity in F. occidentalis, and facilitates early cuticular colonization by CQMa421, achieving synergistic control efficacy through reciprocal enhancement. This synergism was validated under field conditions. These findings provide a theoretical and practical basis for effective, safe field fungal-chemical control.