Biological control of Eurasian watermilfoil by Euhrychiopsis lecontei: assessing efficacy and timing of sampling

The milfoil weevil, Euhrychiapsis lecontei Dietz, is a biological control agent for Eurasian watermilfoil (EWM; Myriophyllum spicatum L.), a nuisance aquatic macrophyte. EnviroScience, Inc. (Stow, OH) rears and stocks E. lecontei for management of EWM infestations. Here, we analyze data collected by EnviroScience, Inc. from treatment (weevil-stocked) and control (unstocked) EWM beds in 30 Michigan and Wisconsin lakes over six years. Initial and final EWM and weevil densities were compared with lake-specific average and maximum lake depths, and lake surface area. The analyses showed substantive variability of weevil efficacy to control EWM. No significant associations were seen between average or maximum lake depth or lake surface area on final weevil densities or plant density changes. Only the number of days between initial and final surveys and timing of final data collection proved significant in determining final EWM densities. As more time passed between surveys, final EWM densities significantly decreased at treatment sites, possibly due to weevils, with a much smaller decrease at control sites. Also, EWM densities declined at both treatment and control sites when final survey data were collected after the start of September, a phenomenon not seen when final data were collected earlier in the year. These declines at control sites were likely due to plant senescence. Lake managers utilizing E. lecontei should consider the length of time between EWM surveys, as well as the timing of data collection, to avoid the confounding effect of plant senescence on data interpretation.