Seasonal changes in the abundance of the herbivorous moth larvae Acentria ephemere/la on submersed macrophytes in Lake Constance were studied between 1998 and 2000. Acentria were found feeding on many pondweeds (Potamogeton lucens, P. pectinatus, P. perfoliatus), Ceratophyllum demersum and Myriophyllum spicatum, but not on Chara spp., Najas marina ssp. intermedia or Elodea nuttallii. During the vegetated periods of 1998 and 2000 the abundance of larvae on P. perfoliatus and M. spicatum increased exponentially from approx. 10 ind.lm 2 to a maximum of 10,000 ind.lm 2 or 20-165 ind.lg dm. The much lower abundance in 1999 may be a result of the massive flooding of the lake in spring. Our data suggest that at least part of the Acentria population in Lake Constance may have two generations per year. Larvae use predominantly the upper shoot sections during the active feeding period and move to the lower stern sections for winter diapause. Herbivory caused substantial damage especially to the apical meristems of P. perfoliatus and M. spicatum. This study indicates that aquatic shredder-herbivores may have astronger impact on submersed macrophytes than previously considered.

Abstract-Larvae of Acentria ephemerella live fully submerged, feeding on submersed aquatic angiosperms such as pondweeds (Potamogeton spp.) and Myriophyllum spicatum. Only the latter contains high concentrations of hydrolyzable tannins known to interfere with the growth of insect herbivores. We tested whether larvae grow faster on Potamogeton perfoliatus or M. spicatum and whether this is due to polyphenols in their food source. Larvae originating from the same egg clutch grew faster and larger on P. perfoliatus than on M. spicatum. The same growth response was observed with larvae that spent winter diapause on either P. perfoliatus or M. spicatum. These larvae were fed either with their host plant or the other macrophyte. No prior feeding effect was found, but growth of larvae reared on M. spicatum was less than when grown on P. perfoliatus. Larvae from another egg-clutch reared on M. spicatum, either from lake or cultivated in aquaria, exhibited reduced growth on the lake plants. P. perfoliatus contained less than 1% and M. spicatum (aquarium or field material) between 5 and 9% phenolic compounds. No differences in nitrogen content of leaves were found, but apical shoot sections of M. spicatum exhibited a significantly higher nitrogen content than P. perfoliatus. Our results indicate that hydrolyzable tannins are responsible for the reduced growth of Acentria when fed with M. spicatum.