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

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.