Physicochemical comparison of chitin characteristics in three major stored-product beetle pests: Implications for biofumigant toxicity.

The present study investigates the chitin properties of stored-product insect pests and their association with the
fumigant toxicity of garlic essential oil. Chitin isolates of Callosobruchus maculatus, Sitophilus oryzae, and Tribolium
castaneum adults were characterized using FT-IR, XRD, EA, SEM-EDS, and NMR techniques. Fumigant
toxicity assay was performed under airtight condition in glass vial. The S. oryzae contains highest chitin content
(19 %), followed by T. castaneum (10 %) and C. maculatus (8 %). The degree of crystallinity was lower in
C. maculatus (67.13 %) than in S. oryzae (77.05 %) and T. castaneum (76.56 %). Morphologically, C. maculatus
chitin displayed a flat lamellar surface with pores, while S. oryzae and T. castaneum exhibited densely arranged
microfibrils based surfaces. Fumigant toxicity assays revealed varied susceptibility levels, C. maculatus exhibited
higher susceptibility (0.27 μL/L air of LC50) compared to S. oryzae and T. castaneum (14.35 and 3.74 μL/L air of
LC50, respectively) to garlic essential oil. The higher chitin content, greater crystallinity, and densely arranged
structures in S. oryzae might contribute to its tolerance towards fumigant. Additionally, physico-chemical
properties and penetration potentiality of the bioactive constituents might be linked to the toxicity in insects.
Understanding these relations can enrich knowledge of chitin's role in fumigant toxicity mechanism.