Overwinter Habitat and the Relationship of Overwinter to In-lake Densities of the Milfoil Weevil, Euhrychiopsis lecontei, a Eurasian Watermilfoil Biological Control Agent

The native weevil Euhrychiopsis lecontei has been associated with declines of Eurasian watermilfoil (Myriophyllum spicatum). The weevil spends all summer on submersed plants, producing 3 to 6 generations. In September to November adult weevils move to shore where they overwinter in leaf litter at drier sites near the shoreline. Mean November shoreline densities from 1992-1998 at Lake Auburn (mean = 43 N/ m²) and Smith's Bay of Lake Minnetonka, (mean = 125 N/ m²) have ranged from zero to over 200 N/m². Overwinter mortality is not severe (survival was typically >60%). Adults collected from terrestrial habitat have developed flight muscles and limited flight has been observed in the spring but submersed adults in summer do not have developed flight muscles. Adults return to the water in spring and females begin to develop and lay eggs after the water temperature reaches 10-15C. Spring (May-June) and Fall (September) in-lake densities in these two lakes have ranged from zero to 40 N/m² and Lake Auburn typically had higher in-lake weevil densities (mean of 15 N/m² compared to 4 N/m² at Smith's Bay). There was no relationship between in-lake and shoreline densities at Lake Auburn, but Smith's Bay spring in-lake densities were correlated with spring shoreline densities. In-lake densities were not correlated between the two lakes but shoreline densities were correlated over time, suggesting that regional climatic factors may influence shoreline densities. Weevils disappeared from Lake Auburn in-lake samples in July 1998; no weevils were found there in shoreline or in-lake samples in 1999. In-lake factors such as fish predation may be more limiting than overwinter conditions.