Lake Pend Oreille is the largest (36,000 ha or 91,000 ac) freshwater lake in Idaho. Approximately 27% or 10,000 ha of the lake is littoral zone habitat supporting aquatic macrophyte growth. Eurasian watermilfoil has invaded large areas of this littoral zone habitat, with early estimates suggesting approximately 2,000 ha by the mid 2000s. 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 its effects on the native plant community. Littoral zone point intercept surveys were conducted in 2007 and 2008 to monitor the trends in aquatic macrophyte community structure and assess management strategies against Eurasian watermilfoil. Lake Pend Oreille has a species-rich aquatic macrophyte community of more than 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 lake-wide management activities. In managed areas, the frequency of Eurasian watermilfoil during the 2008 assessment was 23.6% after 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 with 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 200 ha of Eurasian watermilfoil remaining, which represents an overall reduction of 90% in approximately 7 yr of management.

Nomenclature: Fluridone; triclopyr; 2,4-D; endothall; Eurasian watermilfoil, Myriophyllum spicatum L. MYSP2.

Management Implications: Lake Pend Oreille is a large fluctuating reservoir with a species-rich aquatic macrophyte community that shifts from early- to late-season species. The management of Eurasian watermilfoil has promoted growth of native plants. 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 water exchange rates resulted in significant reductions of Eurasian watermilfoil, although the addition of a contact herbicide may have resulted in a temporary loss of some native species. Triclopyr use also resulted in a reduction of Eurasian watermilfoil frequency in treatment plots, with less effect on native plants. The combination of triclopyr and endothall in areas of high water exchange was effective in a preliminary assessment, although additional studies are needed. On the basis of our analyses, the presence of Eurasian watermilfoil was more detrimental to the native plant community than the herbicide treatments.