Effects of in-lake and shoreland variables on Eurasian watermilfoil (Myriophyllum spicatum L.) and milfoil weevil (Euhrychiopsis lecontei Dietz) abundance in Wisconsin lakes

Abstract

The invasive aquatic macrophyte Eurasian watermilfoil (Myriophyllum spicatum L.) (Haloragaceae) is a serious nuisance in water bodies throughout its non-native range. Studies seeking a feasible biological control agent for M. spicatum have focused on an aquatic weevil, Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae), which is native to much of the United States. The objectives of this study were to assess the density of E. lecontei in 14 Wisconsin lakes, assess the relationships between a wide variety of limnological and geographical variables and densities of E. lecontei or M. spicatum, and assess whether density of E. lecontei or abundance of the M. spicatum fungal pathogen Mycoleptodiscus terrestris (Gerd.) Ostazeski correlated with the M. spicatum density index in the study lakes. Lakewide E. lecontei average densities varied between zero and 1.37 per stem, and substantial variation within lakes was observed. Out of a total of 27 weevil surveys conducted on 14 lakes over two years, 17 surveys found live individuals of E. lecontei. Evidence of E. lecontei feeding activity was observed on the M. spicatum stems collected during all 27 surveys of the 14 study lakes, indicating a widespread distribution of E. lecontei throughout Wisconsin. The majority of limnological and geographical characteristics were not statistically correlated with E. lecontei density or the M. spicatum density index. E. lecontei was negatively correlated with coarseness of substrate texture. All other variables tested were insignificant. M. spicatum showed significant positive correlations with variables related to human disturbance, suggesting that preservation of natural habitat around lakes and minimization of disturbance could reduce the likelihood of nuisance M. spicatum populations. Neither abundance of M. terrestris nor density of E. lecontei correlated significantly with the M. spicatum density index. The host-specificity and broad distribution of E. lecontei in Wisconsin make this insect a strong candidate for biological control of M. spicatum. Further research into predation of E. lecontei is needed to better understand the ideal conditions in which to employ E. lecontei as a biological control tool.