Limnological Institute, University of Konstanz, D-78457 Konstanz, Germany.
Plant-herbivore interactions in freshwater systems have only recently received attention. We are investigating herbivory of the submerged living caterpillar Acentria ephemerella in Lake Constance, Germany. A. ephemerella is a polyphagous species which was found mainly on Potamogeton pectinatus, P. perfoliatus and Myriophyllum spicatum. The abundances ranged from about 80 to 8000 specimens/m2 in 1998, depending on season. The abundance as well as the damage in the field was highest on P. perfoliatus, where almost all leaves exhibited herbivore damage. In laboratory choice tests, Potamogeton-species were preferred over M. spicatum. In no-choice tests the growth rate was significantly reduced with M. spicatum. This may be due to high amounts of polyphenolics in M. spicatum reaching up to 10% (dw). Such high amounts of polyphenolics are known to deter feeding of herbivores in terrestrial systems. Since A. ephemerella can fully develop on M. spicatum, there must be physiological adaptations to polyphenolics from this macrophyte. The alkaline gut of the caterpillar should prevent tannin-protein interactions but promotes on the other hand autoxidation, leading to reactive oxygene species. Measurements with microelectrodes revealed anoxic conditions in the gut of A. ephemerella, which should prevent the formation of these products. The role of tannin-tolerant microorganisms isolated from the gut of A. ephemerella is presently investigated.