Distribution and abundance of the milfoil weevil, Euhrychiopsis lecontei, in Lake Minnetonka and relation to milfoil harvesting

The milfoil weevil, Euhrychiopsis lecontei (Dietz), native to North America, is a potential biological control agent for Eurasian watermilfoil (Myriophyllum spicatum L.). The milfoil weevil has been shown to control the plant when sufficient densities are attained and maintained throughout the summer. Mechanical harvesting of Eurasian watermilfoil has been shown to locally reduce the density of weevils in harvested plots, but the effects of broad-scale harvesting have not been evaluated. We determined weevil densities in unharvested plots in early-, midand late-summer in nine bays of Lake Minnetonka that had a range of harvest levels. Weevil densities were significantly negatively related to proportion of the bay harvested (r ² = 0.55. p = 0.02). No relationship between shoreline habitat and weevil density was found. Our results suggest that large-scale harvesting is detrimental to weevil populations and that harvesting and chemical control should be limited if the aim is to promote biological control.