Effect of some UV light absorbers on the photostabilization of azadirachtin, a neem-based biopesticide

Photostabilization of neem-based azadirachtin insecticide (AZ-A) applied onto glass surfaces was studied in the presence of three UV absorbers, 2,4-dihydroxybenzophenone (Uvinul M-41)0, UM), 4-aminobenzoic acid (PABA) and Fluorescent brightener-28 (FB-28), a stilbene disulfonic acid derivative. The UV absorber, UM, provided excellent protection, increasing the dissipation half-life (DT50) of pure AZ-A from 3.87 to 22.54 d. Photostabilization due to PABA was marginal. The UV absorber, FB-28 acted as an effective photosensitizer, reducing the DT50 of AZ-A from 3.87 to (1.31 d. Similar trends in DT50 were observed when the three UV absorbers were added to four formulations containing AZ-A. It was found that for effective photostabilization, both AZ-A and the UV absorber must have matching UV spectra with similar λmax. The mechanism of photostabilization was likely due to either energy transfer from AZ-A to the UV absorber and/or competitive absorption of UV photons by the absorber. Photoinstability of AZ-A in the presence of FB-28 was due to energy transfer from the activated FB-28 to AZ-A molecules. Based on the UV spectral data, UV protectants can be selected and matched to stabilize UV-labile pesticides.