Size-tunable carboxymethyl chitosan enabled nanoplatform of spinosad: Efficient pest control, enhanced plant growth, and reduced residues in soil.

Nanopesticide delivery systems represent a promising strategy to enhance pesticide utilization efficiency. Employing bioactive natural polymers as carriers enables the development of multifunctional nanoplatforms for sustainable agriculture. Herein, a series of size-tunable nanosuspensions of spinosad was fabricated using carboxymethyl chitosan as the carrier. Leaf-dipping bioassay revealed its size-dependent toxicity to Plutella xylostella, with a critical efficacy threshold at ~300 nm. The nanosuspension with a minimum particle size of 188 nm achieved an encapsulation efficiency of 94.1% and a loading content of 10.7%, demonstrating enhanced UV-shielding capacity and increased foliar retention. Complementary instrumental (FTIR, TGA, and DSC) analyses and molecular dynamics simulations identified physical adsorption and weak hydrogen bonding as the primary insecticide-carrier interactions. In vitro release tests displayed pH/temperature-responsive profiles, with kinetics well fitted to the Ritger-Peppas model (Fickian diffusion) or first-order model. Pot experiments demonstrated a 48 h LC₅₀ of 41.10 mg/L for a 21-day efficacy, significantly lower than that (135.27 mg/L) of commercial spinosad suspension concentrate (CSP). Compared with CSP, foliar spraying of the nanosuspension significantly promoted Chinese cabbage growth, with SPAD and crown width increased by 42.1% and 40.7% after 21 days, respectively. The half-life (5.3 d) of the nanosuspension in soil was approximately half that (9.9 d) of CSP. This work demonstrates the potential of natural polysaccharides as versatile carriers toward enhanced agrochemical delivery and sustainable pest management.