Operational control of Eurasian watermilfoil and impacts to the native submersed aquatic macrophyte community in Lake Pend Oreille, Idaho

Authors: Madsen, John; WERSAL, RYAN - Lonza Corporation; WOOLF, THOMAS - Idaho State Department Of Agriculture

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2015
Publication Date: 6/1/2015
Citation: Madsen, J.D., Wersal, R.M., Woolf, T.E. 2015. Operational control of Eurasian watermilfoil and impacts to the native submersed aquatic macrophyte community in Lake Pend Oreille, Idaho. Invasive Plant Science and Management. 8:219-232.

Interpretive Summary: Lake Pend Oreille is a large fluctuating reservoir that has a species-rich aquatic plant community that shifts from early to late season species. The management of Eurasian watermilfoil has promoted growth of native plants that may limit future re-infestation. Eurasian watermilfoil was reduced by 63% at sites treated before September 2008. Results suggest that fluridone applications should be limited to areas of low water exchange to enhance efficacy. The combination of 2,4-D and endothall applied in areas of higher rates of water exchange resulted in significant reductions in of Eurasian watermilfoil, though the addition of a contact herbicide may have resulted in selectivity temporary loss of some native species. The use of triclopyr also resulted in a reduction of Eurasian watermilfoil frequency in treatment plots, with less impact to native plants. The combination of triclopyr and endothall initially looks useful in areas of high water exchange, though additional studies are needed. Based on our analyses the presence of Eurasian watermilfoil was more detrimental to the native plant community than the herbicide treatments. Assessments should consider not only the time of treatment, but the response time of the target plant to each individual herbicide applied. The full plant response may not be seen until the following season.

Technical Abstract: Lake Pend Oreille is the largest (36,000 ha or 91,000 acres) freshwater lake in Idaho. Approximately 27% or 10,000 ha of the lake is littoral zone habitat supporting aquatic plant growth. Eurasian watermilfoil (Myriophyllum spicatum L.) has invaded large areas of this littoral zone habitat, with early estimates suggesting approximately 2,000 ha by the mid 2000’s. Idaho State Department of Agriculture developed a state-wide eradication program in response to the threats posed by Eurasian watermilfoil, which attempts to quantify Eurasian watermilfoil infestations and impacts on the native plant community. Littoral zone point intercept surveys were conducted in 2007 and 2008 to monitor the trends in aquatic plant community structure and assess management strategies against Eurasian watermilfoil. Lake Pend Oreille has a species-rich aquatic plant community of over 50 species. Lake-wide, the presence of Eurasian watermilfoil significantly decreased from 2007 (12.5%) to 2008 (7.9%). The native plant community has remained stable from 2007 to 2008 despite lakewide management activities. In managed areas, the frequency of Eurasian watermilfoil during the 2008 assessment was 23.6% following herbicide applications. This represents a 63% reduction in Eurasian watermilfoil presence from the 2007 (64.5%) survey. When 2,4-D was combined with endothall the presence of Eurasian watermilfoil declined from 63% (2007) to 36.5% in 2008. Eurasian watermilfoil treated with triclopyr also declined significantly, 64% to 18.2%. When all treatment methods were pooled and compared to areas that were not treated the presence of Eurasian watermilfoil was significantly greater (52.5%) in untreated areas as opposed to treated areas (23%). The removal of Eurasian watermilfoil resulted in an increase in native species in most areas. Currently, there is as little as 500 acres of Eurasian watermilfoil remaining which represents an overall reduction of 90% in approximately 7 years of management.