The development of azadirachtin as a soil-applied, granular insecticide
The aim of this project was to develop azadirachtin as an insecticide that is applied to the soil, using a granular formulation, for root uptake and subsequent systemic plant protection. A method was developed whereby azadirachtin could be rapidly isolated to approximately 95% purity using flash chromatography. This material was used in all subsequent chemical and biochemical studies. To increase the speed of crude extract analysis, a colorimetric technique was assessed to rapidly quantify azadirachtin. However, this method was generally unsuitable for the requirements of this project because it was non-specific and not stable. Granular formulations based on sodium alginate, starch-kaolin and poly(e-caprolactone), and containing different neem seed extracts were successfully prepared. These granules exhibited differnces in the rate of azadirachtin release into water. Additives such as kaolin clay and rapeseed oil could be used to modify the speed of release. Following application to soil, the position of granules did not affect release rates. However, granule application method was shown to affect the rate at which the limonoid was accumulated within the nasturtium plants. Azadirachtin was shown to be moderately water-soluble (1.29 g/l). During mixing studies between distilled water and n-octanol, the limonoid partitioned more favourably into the non-aqueous phase at a ratio of 7:1. Based on calculated Koc values (<40), azadirachtin was classified as very highly soil mobile. Adsorption occurred principally to the organic matter of soils. Clay minerals were comparably non-sorbent. Desorption from both of these sites occurred readily. Azadirachtin was not persistent within soil where the limonoid’s DT50 was as short as 1.06 days. Initial breakdown resulted in the acetyl moiety being cleaved from the molecule. In addition, azadirachtin was shown to exhibit a pH sensitive hydrolytic degradation. The limonoid’s half-life in solution ranged from 57 days at pH 5 to 7.15 hours at pH 9. In conclusion a suitable granule for a controlled-release of azadirachtin was developed.