The water veneer Acentria ephemerella exhibits in its larval stages a distinct seasonal & spatial distribution on macrophyte patches, which are both food and habitat for Acentria, in Lake Constance, Germany. Acentria larvae cause massive defoliations of Potamogeton perfoliatus during the active feeding period from late May to September and begin then to hibernate in stems of aquatic macrophytes. Mass outbreaks during summer and certain life-history characteristics (e.g. female flightlessness, male-biased sex ratio) suggest that Acentria can be considered as an aquatic analogue to the Gypsy moth. In field samples a highly male-biased sex-ratio (75% and higher) is found. Mesocosm and aquarium experiments show that fish predation by sticklebacks (Gasterosteus aculeatus L.) strongly reduces population growth of Acentria. Furthermore, stickleback predation and/or density-dependant mortality in the larval stage result in a male-biased sex ratio. Hence, fish predation plays a major role in controlling mass outbreaks directly and indirectly by shifting the sex-ratio towards male dominance. Female larvae are possibly more active und may therefore suffer higher mortality than male individuals. We use sex chromatin staining to analyse the primary sex ratio of freshly-hatched larvae as well as the sex-ratios of different larval size classes to study the age / size dependency of sex specific mortality in the field.