The effects of flooding on the survival and behaviour of overwintering large heath butterfly Coenonympha tullia larvae

The rehabilitation of raised mires commonly requires water levels to be raised in order to reinstate Sphagnum-based raised mire vegetation over peatland areas which have previously been extensively worked by commercial peat cutters. Coenonympha tullia larvae living on cottonsedge tussocks Eriophorum vaginatum in the base of peat cuttings will be inundated by the rising water levels under such management regimes. The submergence of overwintering C. tullia larvae was found to have a marked impact on their long term survival. The numbers of larvae that survived to resume feeding the following spring was lower for all submergence times (3 to 108 days) compared with controls; no larvae survived a submergence period of 108 days. Total submergence had two effects. First, it caused an immediate increase in mortality after 7 days (with larval death occurring under water) and second, a longer term increase in mortality over the next few months. Behavioural tests suggested that C. tullia larvae usually respond to the raising of water levels by trying to climb out of the water. Results suggest that, on sites where the conservation of C. tullia is important, water levels should not be raised so high as to totally submerge E. vaginatum foodplants containing C. tullia larvae until E. vaginatum has spread sufficiently on the higher peat surface to provide a replacement habitat.