The aquatic moth Acentria ephemerella as a key species in submerged aquatic vegetation : direct and trait-mediated interactions with predators and food plants

The perfoliate pondweed Potamogeton perfoliatus constitutes large mono-specific macrophyte patches in many Central European lakes. Correlative evidence from the field suggests that Potamogeton is under an increasing grazing pressure during its short vegetation period from May to September due to seasonal outbreaks of the aquatic moth Acentria ephemerella. We used a mesocosm experiment to determine the influence of Acentria herbivory on Potamogeton shoot development and resting bud production and to study the defence strategies of this macrophyte. Herbivory resulted in a reduction of the Potamogeton vegetation period by more than two months, thereby reducing the average resting bud size and the overall resting bud biomass seven-fold. This suggests that, besides its severe immediate effects, herbivory will affect Potamogeton growth and dynamics also during the subsequent season. As a response to herbivory Potamogeton retranslocated nutrients (phosphorus (P) and nitrogen (N)) from leaves towards resting buds and changed its allocation strategy in the size versus number of resting buds. This change in the number versus size trade-off might allow the plant to produce a minimum resting bud size necessary for successful sprouting in the next spring. Acentria larvae had a high P content implying P limitation of larval growth especially within the herbivory