Assessment of the pathogenicity of oil-based formulation of Cordyceps fumosorosea combined with thiamethoxam against Diaphorina citri

Mycopesticides are crucial for integrated management of the Asian citrus psyllid (ACP, Diaphorina Citri), yet highly virulent formulations remain inadequately studied. This study aimed to optimize an oil-based formul ation of Cordyceps fumosorosea strain SCAU-CFDC01—selected for its high pathogenicity against ACP—and evaluate its synergistic potential with thiamethoxam. Key optimized parameters included corn germ oil, Tween 80 (12%), carboxymethylcellulose sodium (6%), organoclay (2%), and UV-531 (4%). Compared to unformulated conidia, the oil-based formulation of C. fumosorosea significantly increased virulence against both ACP fifth-instar nymphs and 6-d-old adult females, as indicated by increased mortality rates and decreased LT50 values. Furthermore, sublethal concentrations (LC25) of thiamethoxam synergistically enhanced mortality induced by the oil-based formulation in both life stages. Combined treatment of thiamethoxam LC25 with the oil-based formulation (1 × 106 conidia ml−1) also significantly reduced female longevity and fecundity. Lastly, molecular analysis revealed downregualtion of key genes associated with juvenile hormone signaling (Met, Kr-h1) which is a crucial reproductive regulator, and other 3 reproductive related genes (DcVg1-like, DcVgA1-like, and DcVgR) after treatments with thiamethoxam LC25 and oil-based formulation (1 × 106 conidia ml−1). These results demonstrate the synergistic potential of combining C. fumosorosea oil-based formulation with thiamethoxam for effective ACP control, offering a promising integrated pest management strategy.