A field test on the effectiveness of milfoil weevil for controlling Eurasian watermilfoil in Wisconsin lakes

We tested the effectiveness of milfoil weevils ( Euhrychiopsis lecontei ) for reducing biomass of Eurasian watermilfoil ( Myriophyllum spicatum , EWM) under natural lake conditions in a 3-year field experiment. In each of four lakes, we randomly chose two EWM beds for stocking and two beds as controls. A total of ca. 40,000 weevils were added to the eight stocked beds. During June and August 2013–2015, we measured plant diversity, biomass of EWM and native plants, and weevil abundance in the 16 study beds. Background weevil densities varied widely among beds and were often greater than the densities stocked. Weevil stocking had no significant effect on EWM or native plant biomass. Nevertheless, weevil damage to EWM was common and its extent appeared strongly related to observed densities of the weevil. ANCOVA results indicated that weevil density was a significant predictor of EWM biomass in both June and August, but not on growth during summer. Overall, our study found that weevil density is an important factor for predicting EWM biomass, while weevil density is likely affected by a large number of environmental factors. This work highlights the importance of carefully considering lake conditions that may influence the efficacy of stocking for biological control.