Evaluation of large-scale low-concentration 2,4-D treatments for Eurasian and hybrid watermilfoil control across multiple Wisconsin lakes

Nault ME, Barton M, Hauxwell J, Heath E, Hoyman T, Mikulyuk A, Netherland MD, Provost S, Skogerboe J, Van Egeren S. 2017. Evaluation of large-scale low-concentration 2,4-D treatments for Eurasian and hybrid watermilfoil control across multiple Wisconsin lakes. Lake Reserv Manage. 34:115129.Herbicides have been utilized for decades for nonnative milfoil control; however, limited literature is available examining large-scale herbicide applications, especially for commonly used herbicides such as 2,4-D (2,4-dichlorophenoxy acetic acid). Twenty-three lakes were studied pretreatment and posttreatment to monitor large-scale and low-concentration (lakewide rate: 73500 /L) 2,4-D dissipation and degradation patterns, and determine the efficacy and selectivity of these treatments for Eurasian watermilfoil (Myriophyllum spicatum; EWM) and hybrid watermilfoil (Myriophyllum spicatum x M. sibiricum; HWM) control. Measured mean surface concentrations averaged throughout the initial 2 weeks after treatment ranged from 119 to 544 /L. In addition, the threshold for irrigation of plants which are not labeled for direct treatment with 2,4-D (< 100 /L by 21 d after treatment) was exceeded in 18 of the 28 treatments. Calculated 2,4-D half-lives ranged from 4 to 76 d, and herbicide degradation was generally observed to be slower in oligotrophic seepage lakes. Year of treatment reductions in milfoil frequency ranged from 4 to 100%, with sustained multi-year control observed in some lakes. While good year of treatment control was achieved in all lakes with pure EWM populations, significantly reduced control was observed in the majority of lakes with HWM populations. Several native monocotyledon and dicotyledon species also showed significant declines posttreatment, with variation in recovery observed over time. Although target species control was achieved with some of these treatments, variation in herbicide persistence, reduced control in many HWM populations, and nontarget impacts to certain native plants demonstrate the need for additional research and field studies.