John G. Skogerboe

Small-Plot, Low-Dose Treatments of Triclopyr for Selective Control of Eurasian Watermilfoil

ABSTRACT Small-plot treatments of triclopyr were conducted on Lake Minnetonka and Lake Minnewashta, MN, during June 1998 to investigate the herbicides potential to selectively control Eurasian watermilfoil (Myriophyllum spicatum L.) at low doses. Applications were made on 1-ha plots with rates based on plot type: references (0 mg acid equivalent (ae)·L−1), protected plots (0.5 mg ae·L−1), semi-protected plots(1.0 mgae·L−1), and unprotected plots (1.5 mgae·L−1). Plot protection was a function of potential mixing in the water column. Herbicide residues were monitored to determine dissipation 1 through 72 h post treatment. Samples were analyzed with both a high performance liquid chromatography technique and an enzyme-linked immunosorbent assay method. Results from these two analytical techniques were compared, and found equivalent (R2 = 0.96). Triclopyr had a relatively short half-life for each treatment (3.5 hr at 0.5 mg ae·L−1, 2.9 hr at 1.0 mg ae·L−1, and 4.2 hr at 1.5 mg ae·L−1). At 8 weeks post treatment, there was a 30 to 45% reduction in Eurasian watermilfoil distribution. Greatest Eurasian watermilfoil control was achieved in plots using higher triclopyr rates. Frequency of native plants decreased by 24% in the untreated reference plot, 20% in the 0.5 mg ae·L−1 plot and 6% in the 1.0 and 1.5 mg ae·L−1 plots. Mean species per point, however, either increased or remained unchanged in seven of the nine treated plots. Decline of native plants may be partially attributed to the onset of fall senescence. Larger contiguous areas, higher triclopyr rates, and sequential applications may be required to enhance Eurasian watermilfoil control in small-plot, partial lake treatments.

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

Abstract 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 herbicides 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 100 μg/L higher than the nominal lakewide targets of 500 and 275 μg/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 (<100 μg/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.

Evaluation of Transparency and Light Attenuation by Aquashade

ABSTRACT Aquashade™, a colorant and dye registered by the US Environmental Protection Agency (USEPA) for use in aquatic environments to manage plant growth, does not reduce the visibility of water that otherwise would meet safe swimmming standards. This product was examined for its potential impact on underwater visibility in surface waters used for recreational swimming. Aquashade™ treatments of 0, 1, and 5 mg L−1 were evaluated in 3-m deep outdoor tanks containing reservoir water. Transparency and light attenuation were examined using a Secchi disk and an underwater photo synthetically active radiation (PAR) meter. In the first experiment, filtered lake water was used, while in the second, bentonite clay was added to provide a light transparency of approximately 1 m. In filtered lake water, Aquashade™ treatments (1 mg L−1 and 5 mg L−1) reduced light transparency by approximately 10% and 30%, respectively, but the transparency (272 and 194 cm, respectively) was well above typical international (100 cm) a...