Lower abundance of flood water mosquito larvae in managed wet meadows in the lower Dalälven floodplains, Sweden

To investigate the effect of vegetation management on mosquito abundance on the floodplains of the River Dalalven, central Sweden, we studied abundance of floodwater mosquito larvae (mainly Aedes sticticus) using a paired design, comparing flooded wet meadows that were mowed or grazed by cattle to those that were unmanaged. Two floods occurred during the study year (2012), the first in conjunction with the spring flood in May and the second after heavy rainfall in July. We used the standard mosquito dipper to estimate mosquito larval abundance along transects from the shore towards permanent water on each meadow. The number of mosquito larvae was on average lower in each managed wet meadow compared to the corresponding unmanaged meadow, both in May (71 % reduction, SD = 28 %) and in July (35 % reduction, SD = 79 %). However, there was substantial variation in the difference between managed and unmanaged meadows among pairs. We measured water depth, temperature, estimated proportion of open water surface, soil nutrient levels and micro-topology. Some of the variation in larval abundance could be explained by differences in local conditions. Mosquito larval abundance increased with nutrient levels and occurrence of tussocks, and decreased with water depth and percentage open water surface, especially among unmanaged wetlands. This study suggests that mowing or grazing may reduce the abundance of floodwater mosquito larvae in flooded wet meadows. In addition, vegetation management seem to have the greatest effect on mosquito larval abundance in nutrient-rich wet meadows where tussocks are abundant.