Chitosan-based nano-delivery systems exhibit potential toxic effects on the larvae of Harmonia axyridis through multiple exposure pathways.

BACKGROUND Despite advancements in nanopesticide technology addressing low pesticide utilization, environmental risks on predatory insects remain understudied. Natural enemies face dual exposure through direct contact with pesticides or indirectly via residues present in their prey. RESULTS This study developed microscale (MPs, D50 = 2.93 μm) and nanoscale delivery system (NPs, D50 = 219.41 nm) for delivery of lambda-cyhalothrin (λ-Cy). The system was fabricated by coordinating chitosan with manganese (Mn2+) ions and cross-linking this with glutaraldehyde. Contact toxicity assay on the 2nd-instar larvae of Harmonia axyridis Pallas (Coleoptera: Coccinellidae) revealed MPs exhibited higher median lethal concentration (LC50) (9.55 mg L-1) when compared to NPs (3.67 mg L-1) and emulsifiable concentrate (EC, 2.34 mg L-1) of λ-Cy. Moreover, abdominal LC50 values were two to three orders of magnitude lower than pronotum exposure. Under secondary exposure, MPs reduced Acyrthosiphon pisum Harris (Hemiptera: Aphididae) predation by H. axyridis larvae to 19.33% at 12 h (60% lower than control), whereas NPs and EC suppressed predation by >90%. CONCLUSION Although MPs might contribute to enhancing biological safety to a certain extent, both micro- and nanoscale systems posed potential exposure-related toxicological risks to predatory natural enemies. Therefore, more systematic and comprehensive risk assessments are required to ensure their ecological safety. © 2025 Society of Chemical Industry.