How to cope with a superior enemy? Plant defence strategies in response to annual herbivore outbreaks

1. The perfoliate pondweed Potamogeton perfoliatus L. constitutes large monospecific macrophyte patches in many Central European lakes. Correlative evidence from the field suggests that P. perfoliatus is under an increasing grazing pressure during its short vegetation period fromMay to September due to seasonal outbreaks of the aquatic moth Acentria ephemerella Denis & Schiffermüller. We used a mesocosm experiment to determine the influence of A. ephemerella herbivory onP. perfoliatus shoot development and resting bud production and to study the defence strategies of this macrophyte. 2. Herbivory resulted in a reduction of the P. perfoliatus vegetation period by more than 2 months thereby reducing the average resting bud size and the overall resting bud biomass sevenfold. This suggests that besides its severe immediate effects, herbivory affects P. perfoliatus growth and dynamics also during the subsequent season. 3. As a response to herbivory P. perfoliatus translocated nutrients (phosphorus (P) and nitrogen (N)) from leaves towards buds.Acentria ephemerella larvae had a high P content, implying P limitation of larval growth especially within the herbivory treatment. This suggests that at least the P translocation from leaves towards resting buds may be viewed as an anti-predator strategy rather than as a nutrient conservation strategy. 4. Acentria ephemerella herbivory changed the allocation strategy of P. perfoliatus in the size versus number of resting buds: only the number, but not the size of resting buds was reduced under shoot grazing by Acentria ephemerella. This change in the number versus size trade-off might allow the plant to produce aminimum resting bud size necessary for successful sprouting in the next spring. 5. Synthesis. Overall, our results suggest an escape syndrome (after Agrawal & Fishbein, Ecology, 87 (2006) S132) as a defence strategy against herbivory for P. perfoliatus, consisting of a shortening of the growth period, a translocation of nutrients and a change in allocation strategy. The increased plant senescence that was accompanied by the shortening of the growth period has further implications for the usage of macrophyte patches as a habitat for invertebrates and fishes and for the structure of littoral food webs.