The Geographic Distribution of the Aquatic Milfoil Weevil (Euhrychiopsis lecontei) and Factors Influencing its Density in Wisconsin Lakes

Abstract

The exotic aquatic macrophyte, Eurasian watermilfoil (Myriophyllum spicatum L.) has invaded waterbodies throughout the United States and Canada and become a nuisance plant. Euhrychiopsis lecontei (Dietz), a native aquatic weevil has been associated with natural declines of M spicatum and has the potential to be used as a biological control agent. The objectives of this study were I) to determine the geographic distribution of E. lecontei in Wisconsin lakes with M spicatum, and 2) to assess geographical and limnological characteristics associated with E. lecontei presence or absence and abundance. M spicatum beds in 46 lakes were surveyed to determine the presence or absence of E. lecontei. E. lecontei distribution was found to be widespread across the state with 45 new records identified in lakes with M spicatum. Relations between lake and macrophyte characteristics and E. lecontei abundance were assessed in 31 lakes. E. lecontei densities were determined by collecting a total of 120 milfoil stems per lake and counting E. lecontei on each stem. E. lecontei densities varied from 0.0 to 2.5 weevils per stem across the lakes and varied among individual M spicatum beds within the lakes. Most geographical and limnological variables were collected at the lake level while other habitat variables were collected within the M spicatum beds (i.e., at the bed level). Only some bed level variables correlated with E. lecontei density including the distance from shore and depth of the milfoil beds, the number of apical tips per milfoil plant, the percentage of broken apical tips per milfoil stem and the percentage of natural shoreline adjacent to the milfoil bed. The widespread distribution of E. lecontei and its herbivory on M spicatum make it an excellent organism to evaluate as a biological control agent. Factors influencing its density and predator-prey dynamics could provide insight into their potential as biological control agents for M spicatum.