Phenolic Compounds and Their Fates In Tropical Lepidopteran Larvae: Modifications In Alkaline Conditions

Abstract

Lepidopteran larvae encounter a variety of phenolic compounds while consuming their host plants. Some phenolics may oxidize under alkaline conditions prevailing in the larval guts, and the oxidation products may cause oxidative stress to the larvae. In this study, we aimed to find new ways to predict how phenolic compounds may be modified in the guts of herbivorous larvae. To do so, we studied the ease of oxidation of phenolic compounds from 12 tropical tree species. The leaf extracts were incubated in vitro in alkaline conditions, and the loss of total phenolics during incubation was used to estimate the oxidizability of extracts. The phenolic profiles of the leaf extracts before and after incubation were compared, revealing that some phenolic compounds were depleted during incubation. The leaves of the 12 tree species were each fed to 12 species of lepidopteran larvae that naturally feed on these trees. The phenolic profiles of larval frass were compared to those of in vitro incubated leaf extracts. These comparisons showed that the phenolic profiles of alkali-treated samples and frass samples were similar in many cases. This suggested that certain phenolics, such as ellagitannins, proanthocyanidins, and galloylquinic acid derivatives were modified by the alkaline pH of the larval gut. In other cases, the chromatographic profiles of frass and in vitro incubated leaf extracts were not similar, and new modifications of phenolics were detected in the frass. We conclude that the actual fates of phenolics in vivo are often more complicated than can be predicted by a simple in vitro method.

New approaches to tannin analysis of leaves can be used to explain in vitro biological activities associated with herbivore defence

“…Some of the eucalypt samples in our study possessed very high oxidative activity (up to 94 mg g −1 DM gallic acid equivalents). This is at the higher end of values that have been reported in other plant species (Vihakas et al ., , ). In theory, the oxidative capacity of polyphenols could affect plant resistance to insect herbivory (Appel, ), but this has yet to be demonstrated conclusively.…”

“…Instead, the alkaline conditions may promote the oxidation of tannins and other phenolic compounds, leading to the formation of oxygen radicals and, consequently, cell damage (Appel, 1993). For example, the larvae of some tropical lepidopteran species were found to contain the oxidation products of polyphenols in their frass (Vihakas et al, 2015). Likewise, Lymantria dispar, Orgyia Leucostigma and Malacosoma distria caterpillars that fed on maple leaves with high concentrations of ellagitannins had high levels of semiquinone radicals in their midguts together with increased protein carbonyl contents that suggested increased oxidation of the proteins in the gut (Barbehenn et al, 2005(Barbehenn et al, , 2013.…”

Ellagitannin Digestion in Moth Larvae and a New Dimeric Ellagitannin from the Leaves of Platycarya strobilacea

“…Production of 3 suggested the oxidative degradation of HHDP groups. It is known that the pH of midgut fluids of insect caterpillars is alkaline [ 18 , 19 , 27 , 28 ], and our previous study demonstrated that dehydrohexahydroxydiphenoyl (DHHDP) esters, the oxidized form of HHDP esters, decompose and generate 3 under weakly alkaline conditions [ 19 , 21 ].…”

“…In addition, production of brevifolin carboxylic acid ( 3 ) and its decarboxylation product brevifolin ( 4 ) suggest that a part of the HHDP esters is oxidatively degraded in the larval gut, since the production mechanism of 3 from HHDP esters include the oxidative process [ 21 ]. Previously, Vihakas et al showed that phenolic profiles of the frass of lepidopteran larvae fed on leaves containing ETs were similar to those of the alkali-treated extracts of the leaves, and they suggested that ETs were modified by the alkaline pH of the larval gut [ 19 ]. Because tannins are susceptible to autoxidation under alkaline conditions, 3 in the insect tea may by generated by autoxidation.…”

“…Recent studies have confirmed the same metabolism of ETs in human intestinal microflora [ 13 , 14 ], and this has had a considerable impact on biological studies of ETs. From the viewpoint of chemical ecology, degradation of ETs by insects are also important; however, details of the degradation products are not sufficiently clarified [ 18 , 19 ]. In this study, we investigated insect tea produced in Southwest China that is made from the frass of moth larvae.…”