METABOLISM OF FLAVONOIDS BY LEPIDOPTERAN PESTS OF SOYBEAN Spodoptera frugiperda AND Anticarsia gemmatalis

Abstract

The soybean plant (Glycine max L. Merryl) produces a range of flavonoids known to have harmful effects towards insect herbivores. However, Spodoptera frugiperda and Anticarsia gemmatalis (Lepidoptera: Noctuidae) are important insect pests known to cause damage in the soybean culture in Brazil. Surprisingly little is known about how these insects cope with plant secondary metabolites, and how their metabolism explains their host-plant choices and their status as agricultural pests. The present work aims to investigate the main metabolic pathways used by these insects to minimize the harmful effects of quercetin and rutin, used here as model compounds representing flavonoid aglycones and glycosides, respectively. S. frugiperda and A. gemmatalis caterpillars fed on artificial diets containing quercetin and rutin, and feces extracts were analyzed by UPLC-QToF. Interpretation of chromatograms and MS spectra suggested that both species metabolize quercetin by glycosylation, methylation and sulfonation, with S. frugiperda also performing phosphorylation. Some of these reactions take place sequentially, generating products such as methylglucosides, sulfoglucosides, phoshpoglucosides, methyl sulfoglucosides, methyl sulfo-derivatives, diglucosides, and triglycosides. In total, 23 quercetin metabolites were detected for S. frugiperda, and 18 for A. gemmatalis. For experiments with rutin, 7 metabolites were detected in feces of S. frugiperda, and 4 in A. gemmatalis. These include methyl and glucosyl derivatives of rutin, and metabolites from quercetin, possibly arising from rutin hydrolysis in vivo and its subsequent metabolism as quercetin. These results show that the metabolism of flavonoids in these insects constitutes a network of complementary conjugation reactions, arguably resulting in metabolites that are less toxic and/or easier to excrete. Future experiments are going to aim the elucidation of the structures of insect metabolites and the identity of the enzymes and genes implicated in such metabolic pathways.