Field studies on flooding and survival of overwintering large heath butterfly Coenonympha tullia larvae on Fenn's and Whixall Mosses in Shropshire and Wrexham, U.K

1. Peatland rehabilitation projects may involve large increases in water levels if Sphagnum-based vegetation is to be restored over areas where vegetation has largely been destroyed by commercial peat-cutting processes. While this can result in the creation of cotton-sedge tussocks Eriophorum vaginatum on which large heath butterfly Coenonympha tullia larvae live, the water level changes can also lead to these tussocks being subjected to long periods of winter flooding. 2. Field trials were carried out over two winters (1996-97 and 1997-98) (1) to test laboratory predictions of the survival of C. tullia larvae under wet and dry conditions. (2) to record the behavioural responses of larvae to flooding, and (3) to determine whether vertebrate predation affected larval survival significantly. 3. Field trials confirmed the results from earlier laboratory work in that they showed that the submergence of overwintering C. tullia larvae has a marked impact on their survival. During the 1996-97 winter, 35% of the larvae placed in wet areas survived compared with 63% placed in dry areas. During the 1997-98 winter, none of the C. tullia larvae placed in wet areas survived compared with 54% of the larvae placed in dry areas. Because the depth of flooding in the wet areas was much greater in the 1997-98 winter than in the 1996-97 winter, the results from this study may reflect the survival rates of wild C. tullia larvae under moderate (1996-97 data) or severe (1997-98 data) levels of flooding. 4. The behavioural data from the field also support earlier laboratory findings and suggest that C. tullia can move in order to stay above the water surface. No evidence was found that vertebrate predation was a significant factor affecting larval survival. 5. These results suggest that raised water levels threaten C. tullia populations, but that the maintenance of suitable E. vaginatum tussocks in drier areas may ensure that a proportion of the C. tullia population survives even the wettest years.