Effect of Emulsion Size and Shelf Life of Azadirachtin A on the Bioefficacy of Neem (Azadirachta indica A. Juss) Emulsifiable Concentrates

In a study of 33 recipes of neem oil based emulsifiable concentrates, the specific surface area of the emulsions and cream plus oil layer separation in emulsions at 24 h revealed a correlation of −0.6874 between them and correlations of −0.8940 and 0.6972, respectively, with bioefficacy (LC₅₀) against the 3-day-old second-instar larvae of the Bihar hairy caterpillar, Spilosoma obliqua Walker. Nearly 96−99% of azadirachtin A in emulsifiable concentrates (aza-A content = 617.93−1149.65 ppm) degraded during the heat stability test at 54 ± 1 °C for 14 days with half-lives ranging between 1.84 and 4.53 days. The LC₅₀ values against S. obliqua were, however, statistically at par in both the pre- and the post-heat-treated samples, suggesting a similar effect of azadirachtin A and its degradation products on the bioactivity. The half-life of azadirachtin A could be enhanced by storing the concentrates at lower temperatures. A low pH of the formulation solvent did not check the degradation of azadirachtin A, as reported with aqueous solutions in the literature.

Keywords: Azadirachta indica A. Juss; emulsifiable concentrate; azadirachtin A; particle size; shelf life; bioefficacy