The exotic Eurasian water-milfoil (EWM) is widespread in the US and Canada (49 states and 3 provinces) and has colonized hundreds of lakes in Wisconsin. Rapid spring growth allows this submersed plant to out-compete native plants and form dense surface mats that interfere with human activities. Nuisance levels of EWM are often treated with herbicides or mechanically-harvested, procedures that have both high costs and untoward side effects. Biological control is an enticing alternative treatment. The native milfoil weevil (Euhrychiopsis lecontei), which co-exists with native water-milfoils, has been shown by others to have promise as a biological control agent: the milfoil weevil prefers EWM over native plants and its stem-boring larvae cause EWM stems to collapse. In some lakes, weevil densities are high enough that EWM biomass is low. But the conditions that allow control are poorly understood. We tested the effectiveness of milfoil weevils for control of EWM under natural lake conditions in a 3-year field experiment. Four lakes were selected from nearby lakes having both abundant EWM and lake associations willing to forego other treatment methods during the course of our experiment. EWM beds were mapped and initial plant surveys conducted. In 2013 we stocked weevils in 2 randomly-chosen beds and left 2 additional beds as controls in each of the 4 lakes. During June and August in 2013-15, we collected data on plant diversity, EWM stem density, biomass of EWM and native plants, and weevil abundance by stage in each of the 16 study beds.
Results/Conclusions
Background weevil densities varied widely among the weevil beds (range 0-3.2 weevils/stem) and were often greater than the densities stocked. Thus the treatment imposed in our study system was weak relative to natural conditions. Not surprisingly, biomass of both native plants and EWM were independent of stocking treatment. Nevertheless, weevil damage to EWM was common and its extent was strongly correlated to weevil density. Although EWM biomass in June was negatively correlated to weevil density, subsequent growth of EWM each summer was independent of weevil density, suggesting that weevils had little influence on EWM growth in our study lakes. Significant lake*treatment interactions suggest that lake-specific factors influence the response to augmentation of milfoil weevils. This work highlights the importance of carefully considering lake conditions that may influence the efficacy of stocking for biological control.