Bioactivity and physical investigation of a novel fluid bed-dried Bacillus thuringiensis kurstaki formulation

The intensive damage caused by citrus pests such as Phyllocnistis citrella and Prays citri requires the use of efficient, safe, and eco-friendly control measures. This study reports the development, characterization, and evaluation of a novel fluid bed-dried formulation based on Bacillus thuringiensis kurstaki (Btk) strains, namely Lip and Blb1, produced via submerged fermentation on wheat bran. The concentrated pellets using Lip achieved mean values of 9.93 Log₁₀ CFU/g dry matter (gdm) spores and 117 mg/gdm proteins, with post-formulation of 9.51 Log₁₀ CFU/gdm and 15 mg/gdm, respectively. Number- and volume-based size distribution functions were analysed for dry formulations (Btk Lip, Btk Blb1, and Delfin®) and solubilized suspensions, demonstrating critical safety and application criteria associated with "fine" and "coarse" populations. For the Delfin® suspension, remaining "coarse" particles (>100 µm: 21.7 %) were specifically reported. Laboratory bioassays against P. citrella, confirmed no significant differences across all formulations, whereas for P. citri, and Spodoptera frugiperda the lethal concentration of 50 % (LC₅₀) of Delfin® was significantly lower than Lip- and Blb1 formulations (p < 0.001). In a field trial against P. citrella, Lip and Blb1 induced significantly faster mortality (LT₅₀ = 1.67 d and 1.61 d, respectively) than Delfin® (LT₅₀ = 2.89 d, p < 0.001). This study addresses whether fluid bed-dried formulations of locally isolated Btk strains produced on wheat bran-based media can combine physicochemical robustness, effective dispersion, and high insecticidal efficacy against citrus pests, offering a sustainable alternative to commercial Btk products.