Lethal and sublethal toxicities of Annona sylvatica (Magnoliales: Annonaceae) extracts to Zabrotes subfasciatus (Coleoptera: Chrysomelidae: Bruchinae)
Keywords:
Mexican bean weevil, bioactivity, allelochemicals, acetogenins, alkaloids, lignansAbstract
Plant secondary metabolites comprise a diverse range of compounds (allelochemicals) that affect insect–plant interactions; many function in plant defense against herbivory. Thus, allelochemicals constitute an important source of insecticidal molecules that potentially can be used in different forms in integrated pest management programs. The objective of this study was to evaluate the bioactivity of ethanolic extracts and partially purified fractions of these extracts obtained from the leaves, branches, and seeds of Annona sylvatica A. St.-Hil. (Magnoliales: Annonaceae), a native Brazilian species, against Zabrotes subfasciatus (Boheman) (Coleoptera: Chrysomelidae: Bruchinae). In the screening assay, the ethanolic extract of A. sylvatica seeds was the most promising treatment, causing lethal (LC50 = 753.47 and 701.06 mg kg−1 for males and females, respectively) and sublethal effects, mainly oviposition deterrence (EC50 = 438.70 mg kg-−1). On the other hand, ethanolic extracts prepared from branches and leaves caused only sublethal effects including mainly oviposition deterrence (EC50 = 1,168.90 and 1,010.70 mg kg−1, respectively) and reduction in number of offspring. Based on these results, the extracts were submitted to liquid–liquid partitioning, and their fractions were tested against Z. subfasciatus to verify their bioactivity. Overall, the results of the fraction bioassays showed evidence of synergistic interactions among compounds of different chemical classes and polarities. Chemical analyses of active fractions revealed the presence of triglycerides, alkaloids, and acetogenins in the seed fractions; alkaloids, lignans, and long-chain fatty acid ethyl esters in the branch fractions; and glycosides, flavonoids, terpenoids, and long-chain fatty acid ethyl esters in the leaf fractions. Thus, A. sylvatica is an interesting and potentially important source of structurally diverse grain-protective compounds.
Resumo
Metabólitos secundários de plantas compreendem uma gama diversificada de compostos (aleloquímicos) que, entre outras funções, atuam na regulação das interações inseto-planta e, muitos deles, atuam na defesa da planya contra a herbivoria. Assim, aleloquímicos constituem uma importante fonte de moléculas inseticidas que podem ser utilizadas em programas de manejo integrado de pragas (MIP) em diferentes formas. Portanto, o objetivo deste estudo foi avaliar a bioatividade de extratos etanólicos e frações parcialmente purificadas desses extratos obtidos de folhas, de ramos e de sementes de Annona sylvatica A.St.-Hil. (Magnoliales: Annonaceae), uma espécie nativa do Brasil, sobre Zabrotes subfasciatus (Boheman) (Coleoptera: Chrysomelidae: Bruchinae). Na triagem inicial, o extrato etanólico de sementes foi o tratamento mais promissor, causando efeitos letal (CL50 = 753,47 e 701,06 mg kg−1, respectivamente, para machos e fêmeas) e subletais, especialmente deterrência de oviposição (EC50 = 438,70 mg kg−1). Por outro lado, os extratos etanólicos preparados a partir de ramos e de folhas causaram apenas efeitos subletais, principalmente deterrência de oviposição (EC50 = 1168,90 e 1010,70 mg kg−1, respectivamente) e efeito na progênie. Com base nesses resultados, os extratos foram submetidos à partição líquido-líquido e as frações obtidas testadas sobre Z. subfasciatus para verificar sua bioatividade. De um modo geral, os resultados dos bioensaios com as frações mostraram evidências de sinergismo entre compostos de diferentes classes químicas e polaridades. As análises químicas mostraram a presença de triglicerídeos, alcaloides e acetogeninas nas frações ativas do extrato de sementes; alcaloides, lignanas e ésteres etílicos de ácidos graxos de cadeia longa nas frações ativas do extrato de ramos; e glicosídeos, flavonoides, terpenoides e ésteres etílicos de ácidos graxos de cadeia longa nas frações ativas do extrato de folhas. Assim, A. sylvatica é uma fonte interessante de compostos estruturalmente diversificados com propriedades protetoras de grãos.
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