Efficacy, selectivity, and herbicide concentrations following a whole-lake 2,4-D application targeting Eurasian watermilfoil in two adjacent northern Wisconsin lakes

The herbicide 2,4-D (2,4-dichlorophenoxy acetic acid) has been used to control the nonnative aquatic plant Eurasian watermilfoil (Myriophyllum spicatum; EWM) since the 1950s. Although published research evaluates the herbicide's predicted and observed concentration and exposure times in both laboratory and field settings, few data are available evaluating selectivity and long-term efficacy as well as herbicide concentration behavior following large-scale, whole-lake applications. A controlled study was conducted on 2 adjacent oligo-mesotrophic northern Wisconsin lakes to determine the potential efficacy and selectivity of large-scale and low-dose 2,4-D applications. Initial 2,4-D concentrations in both treated lakes were approximately 100g/L higher than the nominal lakewide targets of 500 and 275g/L, respectively, and the herbicide dissipated and degraded more slowly than predicted. A lakewide regression model relating 2,4-D concentration at monitoring sites to days after treatment (DAT) found the mean half-life of 2,4-D to be 34-41 DAT, and the threshold for irrigation of plants not labeled for direct treatment with 2,4-D (<100g/L) was not met until 50-93 DAT. In the lake treated at the higher 2,4-D rate, EWM was not detected for 3 consecutive years posttreatment. Additionally, several native monocotyledon and dicotyledon species also showed sustained significant declines posttreatment. This study is the first to link field-collected 2,4-D concentration measurements to selectivity and long-term efficacy in EWM control following whole-lake management efforts. Although multiyear EWM control was achieved with these single low-dose applications, longer than expected herbicide persistence and impacts to native plants demonstrate the challenges facing aquatic plant managers and the need for additional field studies.