In western Europe the Marsh Warbler breeds mainly in rank vegetation on damp or seasonally flooded soils, and is particularly attracted to tall herbaceous vegetation such as nettles, meadowsweet, willowherbs and to young osiers and other low woody plants.
The nocturnal larvae are polyphagous, feeding on the leaves of a variety of woody and herbaceous plants, mainly Sorbus, Quercus robur, Salix, Corylus avellana, Stellaria, Filipendula ulmaria, Alnus glutinosa, Rubus and Populus.
The larvae feed on a wide range of plants, including Betula species (Betula verrucosa, Betula pubescens, Betula nana), Alnus incana, Salix caprea, Salix pentandra, Salix aurita, Salix cinerea, Salix phylicifolia, Populus tremula, Philadelphus coronarius, Rubus idaeus, Filipendula ulmaria, Sorbus aucuparia, Vaccinium uliginosum and Ledum palustre.