Abundance of milfoil weevil in Wisconsin lakes: Potential effects from herbicide control of Eurasian watermilfoil

摘要

Although extensive research has revealed the milfoil weevil (Euhrychiopsis lecontei to have potential for biological control of Eurasian watermilfoil (EWM, Myriophyllum spicaturn), herbicides are widely used for controlling this invasive aquatic plant. We hypothesized that density, of the milfoil weevil would be lower in lakes with a history of treating EWM with herbicides than in untreated lakes. We surveyed 3 groups of lakes in the Northern Highland Lake District of Wisconsin to see if densities of the milfoil weevil and extent of weevil damage to milfoil depended on history of herbicide control in the lakes or on species of milfoil (EWM or northern watermilfoil, NWM, Myriophyllum sibiricum). We detected milfoil weevils in 28 of 36 lakes surveyed. Despite healthy EWM beds, mean weevil density in lakes that had herbicide treatment of EWM within the past 10 yr was only one-fifth of that in comparable lakes without a recent history of herbicide control. Densities were also significantly higher in untreated EWM lakes than in lakes with only NWM and no EWM. The frequency of milfoil showing evidence of weevil damage depended strongly on the density of the milfoil weevil. The pattern of low weevil density in treated lakes may be explained either by slow recolonization of weevils after destruction of host EWM or by historically low densities of weevils releasing EWM to grow in abundance, triggering aggressive treatment with herbicides.