RESTORING NATIVE VEGETATION IN A EURASIAN WATER MILFOIL DOMINATED PLANT COMMUNITY USING THE HERBICIDE TRICLOPYR

In an effort to evaluate the selective control of the exotic weed Eura sian water milfoil (Myriophyllum spicatum I,.) and to assess the recov ery and restoration of the native submersed plant community, a 6-ha ri ver and 4-ha cove plot were treated with the herbicide triclopyr at ap plication rates of 2.5 and 1.75 mg/l, respectively, in the Pend Oreill e River, WA, in August 1991. Water exchange half-lives within the plot s were measured using rhodamine WT dye (river, t(1/2) = 20 h; cove, t( 1/2) = 52 h), and triclopyr dissipation rates were also calculated (ri ver, t(1/2) = 19 h; cove, t(1/2) = 53 h). Triclopyr concentrations wer e below the proposed potable water tolerance level (0.5 mg/l) within t he river treatment plot by 3 days after treatment (< 0.01 to 0.41 mg/l ), and 675 m downstream of that plot by 1 day after treatment (< 0.01 to 0.47 mg/l). Following the cove treatment, triclopyr residues ranged from 0.12 to 0.29 mg/l by 7 days after treatment, and from < 0.01 to 0.06 mg/l as close as 150 m downstream from the plot. Eurasian water m ilfoil biomass was reduced by 99% in the treated plots at 4 weeks post -treatment, remained low one year later (river treatment, 28% of pretr eat levels; cove treatment 1% of pre-treat levels) and was still at ac ceptable levels of control at two years post-treatment (river treatmen t, 47% of pre-treat levels; cove treatment, 24% of pre-treat levels). The four-week post-treatment efficacy results verified triclopyr conce ntration/exposure time relationships for controlling Eurasian water mi lfoil developed under laboratory conditions. Non-target native plant b iomass increased 500-1000% by one year posttreatment, and remained sig nificantly higher in the cove plot at two years after treatment. Nativ e species diversity doubled following herbicide treatment, and the res toration of this robust community delayed the re-establishment and dom inance of Eurasian water milfoil for three growing seasons. (C) 1997 b y John Wiley & Sons, Ltd.