The Milfoil Weevil (Euhrychiopsis lecontei) as a Potential Biological Control for Eurasian Watermilfoil in Washington State

Eurasian watermilfoil (Myriophyllum spicatum) is an aquatic plant native to Europe, Asia and northern Africa. This plant was introduced into North America in the 1940's and has spread rapidly across the United States and Canada. In Washington State, Eurasian watermilfoil is present in over 100 lakes and rivers, particularly along the Columbia and Okanogan Rivers and the Interstate 5 corridor. Eurasian watermilfoil has become a nuisance in many lakes and rivers. The plant reproduces quickly and can spread easily into new locations through fragmentation. In addition, it can create dense mats that can interfere with water circulation, increase the rate of lake sedimentation, change water chemistry, displace native plants, and create aquatic habitats that may be unsuitable for fish and invertebrates. Eurasian watermilfoil can also hinder boating, swimming and fishing and is considered by many property owners as undesirable.

Several methods are being used in Washington State to control Eurasian watermilfoil. These include mechanical harvesting, chemical treatment and biological control (grass carp). Since 1996, we have been evaluating the milfoil weevil Euhrychiopsis lecontei as a potential biological control. This weevil has been associated with declines of Eurasian watermilfoil in the United States (e.g. Vermont, Minnesota, Wisconsin). Studies in Vermont found that both adults and larvae negatively impact Eurasian watermilfoil by reducing its biomass and causing the plant to lose buoyancy and collapse. Furthermore, research in Minnesota has shown that the weevil is a watermilfoil specialist. Surveys conducted in Washington in 1993 indicated that the weevil was present in the state. During 1996-1997, we conducted a comprehensive survey of 50 waterbodies in Washington to determine 1) the weevil's distribution and 2) environmental factors related to that distribution. Overall, including the 1993 results, weevils were found in 21 lakes and rivers; two of which were in western Washington (Lake Meridian and Sawyer Lake, King County) and the remainder in eastern Washington. Weevils were associated with both Eurasian and northern watermilfoil (M. sibiricum). The water quality data collected in 1996 and 1997, suggested that in Washington the weevil was primarily present in waterbodies with pH >= 8.2 and specific conductance >= 0.2 mS cm-1 (i.e., hard-water lakes and rivers). Furthermore, we found that weevil presence was correlated with waterbody location (eastern versus western Washington) and surface water temperature (Logistic regression: Chi-square=24.3, P<=0.001).

Currently our evaluation has focused on 1) estimating weevil abundance in 13 lakes, 2) characterizing the weevil's terrestrial and aquatic habitat, 3) comparing the weevil's developmental performance (egg to adult development) among watermilfoil plants (Eurasian and northern) from different lakes, and 4) developing criteria for selecting lakes that may be suitable for weevil treatments. To date, our abundance estimates range from undetectable levels to 0.6 weevils (eggs, larvae, and adults) per stem. In addition, we are currently processing and analyzing data collected in 1999 and 2000 in order to characterize the weevil's habitat, compare the weevil's developmental performance, and develop guidelines for selecting potential weevil treatment lakes.