Effects of hydrolysable tannins on a herbivorous insect: fate of individual tannins in insect digestive tract |
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Authors: | Juha-Pekka Salminen Kyösti Lempa |
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Institution: | (1) Laboratory of Physical Chemistry, Department of Chemistry, University of Turku, FIN-20014 Turku, Finland, FI;(2) Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland, FI |
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Abstract: | Summary. We investigated the effects of four chemically characterised galloylglucoses (GGs, a subgroup of hydrolysable tannins) and
their hydrolysis product, gallic acid (GA), on consumption and performance of larvae of the autumnal moth Epirrita autumnata. Larvae were fed with birch (Betula pubescens) leaves that had been painted individually with each of the compounds at two levels, 5 and 20 mg/g. In addition, we investigated
the fates of the leaf-painted GGs and GA in the E. autumnata digestive tract by comparing phenolics in leaves consumed and in faeces. In general, GGs reduced leaf consumption by E. autumnata during the second and fourth instars, although there was high compound- and instar-specific variation. However, GGs did not
affect the leaf consumption rates by the most voracious fifth instar larvae. This resulted in approximately the same loss
of total biomass by the experimental tree, regardless of the nature and level of GGs enriched to its foliage. The characteristic
fate of hydrolysable tannins, i.e. hydrolysis, was evidenced in the larval digestive tract for three of the four leaf-painted
GGs. In addition to hydrolysis, the almost total absence of GGs in larval faeces was presumably related to the oxidation of
GGs. The dose-dependent excretion percentage of ingested GA showed that it's faecal content should not be used, although it
commonly is, to calculate the level of GG hydrolysis. Moreover, by comparing the non-uniform appearance of faecal tetragalloylglucoses,
whether ingested as such or hydrolysed from pentagalloylglucose, we concluded that a major part of oxidation of GGs occurs
before their hydrolysis in the digestive tract of E. autumnata. Criticism against the common use of tannic acid, a heterogeneous mixture of GA and GGs, in ecological studies is presented.
Received 15 May 2002; accepted 16 July 2002 |
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Keywords: | , Hydrolysable tannins —, galloylglucoses —, tannic acid —, metabolism —, birch —, Betula pubescens —, Epirrita autumnata |
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