Tropical Plant Extracts as Sources of Grain-Protectant Compounds Against Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae)

The toxicity of organic plant extracts to Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) was assessed for three tropical plant species: branches, leaves, and seeds of Annona montana (Annonaceae), branches of Aristolochia paulistana (Aristolochiaceae), and leaves and branches of Casearia sylvestris (Salicaceae). The screening assay resulted that the extracts of A. montana seeds obtained with hexane (LC50 = 534.75 mg kg−1 and LT50 = 6.10 days) and with dichloromethane (LC50 = 424.67 mg kg−1 and LT50 = 5.03 days) were the most promising treatments, followed by the extract prepared from A. montana leaves with hexane (LC50 = 837.70 mg kg−1 and LT50 = 4.90 days). Moreover, extracts (at 1,500 mg kg−1) prepared from C. sylvestris branches with dichloromethane and A. paulistana with hexane caused significant mortality (37% and 41.5%, respectively) beyond sublethal effects on S. zeamais. Therefore, based on the biological assays, extraction yield, and evaluation of the chromatographic profile of the crude extracts by TLC, the hexane extract of A. montana seeds was selected and fractioned using liquid-liquid partitioning. The hydroalcoholic fraction caused mortality of 55.5%, significantly superior to dichloromethane fraction, which caused 35.5% of mortality. Chemical analyses (1H NMR, HPLC, and TLC) were performed, and the results showed the presence of alkaloids and acetogenins in both active fractions, which have been associated with bioactivity. Therefore, extracts prepared from A. montana seeds (especially) is an interesting source of new compounds with promising grain-protectant properties.