Verh. Internat. Verein. Limnol. 28 1–5 Stuttgart, July 2002 Investigation on competitors and predators of herbivorous aquatic Lepidoptera (Acentria ephemerella) on submersed macrophytes in a large prealpine lake E. M. Gross and R. Kornijów Introduction Methods 2 Submersed macrophytes offer a large colonization Lake Constance is the second largest (571 km ) pre- area for many macroinvertebrates. Numerous studies alpine lake in Europe. The lake has three major investigated the interaction between plant species, basins, the Überlinger See, the Upper Lake and the growth and leaf form and the abundance and species Lower Lake. The Lower Lake (71 km2) is shallow composition of invertebrates (e.g. SOSZKA 1975, (max. depth 46 m, 28% littoral area defined as area KORNIJÓW & KAIRESALO 1992). Grazers and above 10 m water depth). This part supports luxuri- detrivores assist the plants by removing epiphyton ous submersed macrophyte growth. and allowing more light to reach the photosynthetic In order to determine the seasonal and spatial tissue. Only recently has the paradigm that aquatic dynamics of Acentria ephemerella, macrophyte stands angiosperms are not subjected to a high degree of at different sites in the Überlinger See and Lower herbivory been challenged (LODGE 1991, NEWMAN Lake Constance were sampled regularly during the 1991, KORNIJÓW 1996, LODGE et al. 1998). A vegetation period from 1998 to 2000 (GROSS et al. in detailed study of the literature reveals that aquatic prep.). The samples for analyses of the animal guild insects were frequently observed feeding on sub- structure were collected during August–September mersed macrophytes. For example, Acentria ephemer- 2000 from Potamogeton perfoliatus, P. pectinatus and ella, a true shredder–herbivore, was found associated Myriophyllum spicatum at the western tip of the with pondweeds and other macrophytes in several island, from two different locations. They were situ- studies (BERG 1942, MÜLLER-LIEBENAU 1956, SOSZKA ated about 300 m from each other, at a depth of 1975). Acentria seems to prefer large lakes to small between 2 and 2.5 m, depending on the water gauge ponds as habitat (JOHNSON et al. 1998). Data on its level of the lake. abundance in different lakes range from a few dozen In addition to the 0.1-m2 samples taken for the to many hundred species per square meter (SOSZKA evaluation of Acentria abundance, one or two shoots 1975, HEDAL & SCHMIDT 1992, BÄNZIGER 2000, of each macrophyte were carefully selected at the JOHNSON et al. 2001, GROSS et al. in prep.). sediment interface and pushed gently in an upward In Lake Constance, Acentria is the major inverte- moving sweep net (mesh width 240 µm) to avoid brate herbivore and regularly develops exponentially loss of highly mobile macroinvertebrates. Three rep- increasing densities during the vegetation period licate samples from each macrophyte were collected (GROSS et al. in prep.). However, information is still at each sampling location . lacking on the relationship between the abundance All samples were taken snorkelling. Samples were of this herbivore and the resulting damage to differ- concentrated in mesh beakers and transferred with ent macrophyte species (JOHNSON et al. 2001, GROSS water into 1-L sealable (Ziploc ™) bags. After sort- et al. 2001). Very little is also known on the interac- ing, the macroinvertebrates were preserved in 70% tions with other invertebrates, especially competitors ethanol. The absolute number of Dreissena polymor- and predators. Therefore, the aim of this study was pha is probably underestimated because not all speci- to learn the guild structure of the epiphytic macroin- mens were preserved. The dry weight of the macro- vertebrate communities inhabiting various elodeids phyte shoots was determined after drying at 100 °C in Lake Constance with analyses of potential com- for 48 h. The density of macroinvertebrates was cal- petitors and predators of Acentria. culated per 100 g of plant dry weight. 0368-0770/02/0028-0001 $ 1.25 ©2002 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 2 Verh. Internat. Verein. Limnol. 28 (2002) Results ironomus albipennis (81%, 24% and 71% by numbers, respectively). The second most Density of Acentria numerous group was the herbivores, repre- In 1998, Acentria developed densities, in dense sented by only two species, among which Acen- stands of both Potamogeton perfoliatus and tria ephemerella predominated (98%, 100% Myriophyllum spicatum, of 10–8000 ind./m2 and 87%) over Macroplea appendiculata (Chry- lake area. That year exhibited extremely low somelidae). Predators occurred in relatively low summer water levels allowing lush macrophyte numbers and only on P. perfoliatus and M. spi- growth. In late spring and early summer of catum. 1999, Lake Constance exhibited a very high water level (2 m higher than normal for this Discussion time of the year) due to massive precipitation in Acentria ephemerella is an important inverte- the northern catchment area. This severely brate herbivore in Lower Lake Constance, caus- interfered with normal submersed macrophyte ing extensive damage to aquatic vegetation. growth, which usually starts in May. Conse- Since its abundance increases almost exponen- quently, maximum densities of Acentria found tially during the vegetation period, other mem- 2 in 1999 reached only about 100 ind./m from bers of the macroinvertebrate community are August until the end of October. During 2000, potential competitors or predators of this spe- water levels were close to the long-term annual cies. In previous studies, the only predators mean, and pondweeds developed normally. found associated with Acentria were planaria Only M. spicatum failed to build up the dense (Dugesia sp.) and dytiscid larvae (BUCKINGHAM stands observed in the previous years and & ROSS 1981). In studies of lakes in the US, reached only 150–175 g DW per m2 in Septem- Acentria was frequently associated with the her- ber 2000 compared with ca. 350 g DW per m2 bivorous cucurlionid Euhrychiopsis lecontei (26 in 1998. Acentria density dropped from 2420 out of 34 lakes shared both herbivores ind./100 g DW at the end of September 1998 (JOHNSON et al. 1998). to 243 ind./100 g DW at the same time in Many species classified in this study as omni- 2000. The mean summer percentage of Acen- vores consume plant tissue, also, although in tria in the total density of epiphytic fauna on small quantities. They can interact with Acen- Potamogeton perfoliatus, P. pectinatus and Myrio- tria in various ways. For example, the predomi- phyllum spicatum in 2000 differed considerably nating photophilous larvae Endochironomus and amounted to 11%, 31% and 1.3%, respec- albipennis occur mainly in the upper stem sec- tively. tions of submersed macrophytes (ZHGARIEVA 1982), the same habitat used by Acentria Guild structure of the animal epiphytic communi- (GROSS et al. 2001). They overwinter in the ties bottom sediments but live on various macro- The analysis of the plant-associated macrofauna phytes during the vegetation period, with maxi- occurring on the vegetation during late summer mum abundance during midsummer 2000 exhibited a diverse community, composed (KORNIJÓW 1992). The larvae may benefit from of 27 taxa, of which 25 were found on Potamo- herbivory by Acentria due to damage of the geton perfoliatus, 6 on P. pectinatus and 12 on leaves causing leaching of cell sap and the pro- Myriophyllum spicatum. The total mean faunal duction of faecal pellets still rich in nutrients, densities amounted to 2819, 397 and 2137 which can lead to the subsequent higher pro- ind./100 g DW, respectively. duction of bacterial and algal epiphytes. On the The communities can be pooled into three other hand, Acentria benefits from epiphyte groups: vascular plant feeders (two species), grazers since they remove dense layers of algae, omnivores (18 taxa) and predators (seven taxa). especially filamentous species, which can The most abundant were omnivores (Table 1) severely interfere with the feeding on macro- with predominating chironomid larvae Endoch- phyte tissue. E. M. Gross & R. Kornijów, The effects of A. ephemerella on submersed macrophytes 3 Table 1. List of invertebrates (per 100 g DW) associated with the herbivorous Acentria ephemerella (Lepi- doptera: Pyralidae) on Potamogeton perfoliatus, P. pectinatus and Myriophyllum spicatum in late summer in Lower Lake Constance. *Indicates potential predators. Taxa P. perfoliatus P. pectinatus M. spicatum Hirudinea Erpobdella sp.* 17 Glossiphonia heteroclita* 1 Helobdella stagnalis* 17 Oligochaeta Stylaria lacustris 921 Naididae n. det 84 71 Gastropoda Bithynia tentaculata 106 Gyraulus albus 3 Planorbis planorbis Bivalvia Dreissena polymorpha 140 95 306 Sphaeridae 2 Crustacea Gammarus sp.* 10 Hydracarina* 63 Ephemeroptera Caenis sp. 7 Zygoptera Trichoptera Athripsodes atterimus l. 53 44 37 Athripsodes atterimus p. 57 9 332 Trichoptera non. det. 6 Lepidoptera Acentria ephemerella l. 283 123 12 Acentria ephemerella p. 34 16 Chrysomelidae Macroplea appendiculata 511 Chironomidae Ablabesmyia sp.* 7 Corynoneura lobata 5 Cricotopus sp. (gr. silvestris) 25 7 Endochironomus albipennis 1801 43 1163 Limnochironomus sp. 39 11 14 Orthocladiinae non. det. 5 Paratanytarsus sp. (gr. lauterborni) 671 Parachironomus varus 10 42 Psectrocladius gr. sordidellus 4 Chironomidae pupa 185 35 Fauna total per 100 g DW 2819 397 2137 4 Verh. Internat. Verein. Limnol. 28 (2002) Colonization of macrophytes by Dreissena Science Foundation (DFG) to EMG (SFB 454, TP should not interfere