A three-year evaluation of triclopyr for selective whole-bay management of Eurasian watermilfoil on Lake Minnetonka, Minnesota

Abstract

Impact of whole-bay, low-dose triclopyr applications for selective control of Eurasian watermilfoil (Myriophyllum spicatum: EWM) was evaluated on Lake Minnetonka, Minnesota, from 2011 to 2013. To assess invasive and native plant frequency and abundance over multiple seasons following management, we collected plant frequency, herbicide concentration, biomass, and hydroacoustic data. Two enclosed bays, St. Albans (64 ha) and Grays (64 ha), were treated at 300 g/L. St. Albans was treated in late May 2011 and 2013 and Grays was treated in late May 2012. Triclopyr half-lives ranged from 8.6 to 12.1 days. A larger, more open bay, Gideons (133 ha), was treated by targeting 50 ha of EWM beds at 1500 g/L in early June 2011. Triclopyr half-lives in Gideons treatment blocks averaged 3.7 days with a bay-wide half-life of 9.4 days. Near complete loss of EWM in the 3 bays was observed the year of treatment. Increased EWM frequency was observed the following June and August; however, EWM remained a minor component of bay-wide biomass (<2%). Number of points with native plants, mean native species per point, and native species richness in the bays were not reduced following treatment. Native species decreasing in frequency included Myriophyllum sibiricum, Zosterella dubia, Elodea canadensis, and Potamogeton zosteriformis. Most native plants showed no significant posttreatment change in frequency. Hydroacoustic data did not indicate bay-wide decreases in percent coverage or biovolume. Treatments provided up to 2 seasons of EWM control without reducing the overall distribution and abundance of native plants.