Estimating Lake-wide Watermilfoil Weevil (Euhrychiopsis lecontei) Density: The Roles of Quadrat Size, Sample Size, and Effort

摘要

Eurasian watermilfoil (Myriophyllum spicatum L.), a nonin-digenous macrophyte in North America, can reach nuisance levels in freshwater systems. The watermilfoil weevil (Euhry-chiopsis lecontei) has been employed as a biological control agent for Eurasian watermilfoil. If lake-wide watermilfoil suppression is desired, then E. lecontei research needs to be focused at this spatial scale. However, previous studies and current management methods estimate E. lecontei density with limited sampling, and lake-wide studies are rare. In a single watermilfoil-infested lake previously stocked with E. lecontei, we (1) determined which of four quadrat sizes was most appropriate for sampling E. lecontei, (2) calculated the optimum number of samples required to estimate E. lecontei abundance at a lake-wide scale with varying precision, and (3) estimated the cost associated with each required sample size. Analysis of variance showed that differences between quadrat sizes were not significant (p > 0.9775), likely due to E. lecontei's highly variable distribution. Next, we collected lake-wide samples with a 0.1 m2 quadrat to estimate E. lecontei density. Power analysis concluded that highly precise estimation (±20% of the true mean) of lake-wide E. lecontei abundance required a large sample size (>300) and substantial effort (261 h), and that reduced precision (±50% of the true mean) required 49 samples and 41.8 h. The patchy distribution of E. lecontei makes highly precise lake-wide density estimation difficult,implying that researchers and managers will need to either accept lower precision associated with their sampling or largely increase sample size and effort when estimating lake-wide E. lecontei density.