PHOTOSTABILIZATION OF THE BOTANICAL INSECTICIDE AZADIRACHTIN IN THE PRESENCE OF LECITHIN AS UV PROTECTANT

The phytochemical insecticide, azadirachtin (AZ), undergoes UV-induced photodegradation. Using the isomer AZ-A as a standard, its photochemi cal stability was studied with and without adding lecithin surfactant as a UV protectant. Standard solutions of pure AZ-A and Margosan-O(R) were prepared in methanol-hexane with (AZ-A:lecithin, 1:2 by weight) a nd without lecithin, applied separately onto glass plates and maple (A cer L.) foliage and exposed to radiant energy under controlled conditi ons. Noticeable photostabilization of AZ-A was achieved in the samples containing lecithin compared to AZ-A samples without the lecithin add itive. First-order kinetic evaluation of the data showed that the DT,, (half-life) and C (rate constant) values for AZ-A with and without le cithin on glass plates were 5.68 d and 0.122, and 5.42 d and 0.128, re spectively. The corresponding values for the Margosan-O formulation we re 7.37 d and 0.094, and 6.24 d and 0.111. The DT50 and C values for t he pure AZ-A on maple foliage with and without lecithin were 8.77 d an d 0.079, and 6.54 d and 0.106, respectively. The corresponding values for the Margosan-O formulation on foliage were 8.35 d and 0.083, and 7 .45 d and 0.093. The kinetic data gave quantitative information regard ing the photostabilization of AZ-A in the presence of lecithin. Good U V protection can only be achieved if the additive has the matching lam bda(max) of AZ-A. The mechanism of photostabilization of AZ-A in the p resence of lecithin was due to either energy transfer from the excited AZ-A to lecithin and/or competitive absorption of UV photons by the l atter.