Delayed Response of Common Reed Phragmites australis to Herbivory as a Cause of Cyclic Fluctuations in the Density of the Moth Archanara geminipuncta

The moth Archanara geminipuncta (Haworth) is an important herbivore of common reed. The moth has one generation per year. The stem-boring larvae may kill up to 80% of the apical parts of reed shoots. In an experimental reed field in the Netherlands a fluctuation was found in the infestation of the reed during which the survival of the larvae became zero every three years. The aim of this study was to search for the factors responsible for this fluctuation. For survival the larvae of Archanara need reed shoots with a diameter of more than 5 mm. Reed is a rhizomatous plant and the reed shoots have an annual growth habit. The reaction of the plant to damage in a certain year is the formation of thinner shoots in the next year. It appeared that apart from the important influence of competition between the larvae at high densities, the delayed response of the plant to damage was very significant in causing the periodic (local) extinction. The possible adaptive nature of the variation in shoot diameter is discussed.