The relationship between water quality, watermilfoil frequency, and weevil distribution in the State of Washington

During the summer of 1997, we surveyed 50 waterbodies in Washington State to determine the distribution of the aquatic weevil Euhrychiopsis lecontei Dietz. We collected data on water quality and the frequency of occurrence of watermilfoil species within selected watermilfoil beds to compare the waterbodies and determine if they were related to the distribution of E. lecontei. We found E. lecontei in 14 waterbodies, most of which were in eastern Washington. Only one lake with weevils was located in western Washington. Weevils were associated with both Eurasian (Myriophyllum spicatum L.) and northern watermilfoil (M. sibiricum K.). Waterbodies with E. lecontei had significantly higher (P < 0.05) pH (8.7 ± 0.2) (mean ± 2SE), specific conductance (0.3 ± 0.08 mS cm^-1) and total alkalinity (132.4 ± 30.8 mg CaCO₃ L^-1). We also found that weevil presence was related to surface water temperature and waterbody location (X²₂ = 24.3, P ≤ 0.001) and of all the models tested, this model provided the best fit (Hosmer-Lemeshow goodness-of-fit X²₈ = 4.0, P= 0.9). Our results suggest that in Washington State E. lecontei occurs primarily in eastern Washington in waterbodies with pH ≥8.2 and specific conductance ≥0.2 mS cm^-1. Furthermore, weevil distribution appears to be correlated with waterbody location (eastern versus western Washington) and surface water temperature.