Preparation of a long‐lasting tablet of spinosad microspheres and its residual insecticidal efficacy against the yellow fever mosquito, Aedes aegypti (Diptera: Culicidae) larvae

Abstract BACKGROUNDS In order to provide a long‐lasting formulation for spinosad (SP) targeting larval stages of Aedes aegypti (Linnaeus) and others alike, a SP tablet was developed based on microspheres, using polylactic acid as inside coating material. The microspheres were encapsulated using polyethylene glycol and 1‐hexadecanol to form a sustained‐release SP tablet. Micromorphology, active ingredient loading, structure identification, photolysis resistance and biological activity were evaluated in this report. RESULTS (i) The SP microspheres had an average particle size of 6.16 ± 2.28 μm, low adhesion and good dispersion as evaluated by scanning electron microscopy and morphology. (ii) The average active ingredient loading and encapsulation of SP microspheres were 32.80 ± 0.74% and 78.41 ± 2.22%, respectively. (iii) The chemical structure of encapsulated SP was confirmed by Fourier transform infrared and 1 H‐nuclear magnetic resonance. (iv) The photostability of the microspheres and the tablets were evaluated. The results showed that DT 50 (time required to dissipate 50% of the mass originally present) of SP was 0.95 days in microspheres and 6.94 days in tablets. (v) The long‐term insecticidal activity of SP tablets was investigated, and the tablet had a long‐lasting activity against the mosquito larvae, showing 100% larval mortality for 63 days. CONCLUSIONS The study provided a new long‐lasting formulation of SP, which displayed good efficacy in the control of Ae. aegypti larvae. © 2024 Society of Chemical Industry.