Comprehensive Dissipation of Azadirachtin in Grapes and Tomatoes: The Effect of Bacillus thuringiensis and Tentative Identification of Unknown Metabolites.

Abstract

Neem oil is a biopesticide normally applied together with Bacillus thuringiensis (Bt). However, neither its dissipation nor the influence of Bt has been previously evaluated. In this study, dissipation of neem oil was investigated when it was applied alone or together with Bt at 3 and 22 °C. A methodology involving solid-liquid extraction and liquid chromatography-high-resolution mass spectrometry was developed for that purpose. The method was validated obtaining recoveries from 87 to 103%, with relative standard deviations lower than 19% and limits of quantification from 5 to 10 μg/kg. Azadirachtin A (AzA) dissipation was fit to a single first order, being faster when neem oil was applied together with Bt and at 22 °C (RL50 = 12-21 days) than alone and at 3 °C (RL50 = 14-25 days). Eight related compounds were found in real samples with similar dissipation curves compared to AzA, and five unknown metabolites were identified in degraded samples, with increasing concentrations during parent compound degradation.