Geosystems Research Institute Report 5032
"Aquatic Plant Community and Eurasian
watermilfoil (Myriophyllum spicatum
L.) Management Assessment in Lake Pend
Oreille, Idaho for 2008"
A Report to the Idaho State Department of Agriculture
John D. Madsen and Ryan M. Wersal
EXECUTIVE SUMMARY
Lake Pend Oreille is the largest (91,000 acres) freshwater lake in located Idaho. Approximately 27% or 25,000 acres of the lake is considered littoral zone habitat and can support the growth of aquatic plants. Eurasian watermilfoil (Myriophyllum spicatum L.) has invaded large areas of this littoral zone habitat and established growing populations. The economic and ecological threats posed by Eurasian watermilfoil have prompted the development of a state-wide eradication program led by the Idaho State Department of Agriculture. An important component of any program is to monitor and quantify the extent of Eurasian watermilfoil infestations as well as the native plant community. Pursuant to this monitoring objective a littoral zone survey was conducted in July of 2007 and 2008 on Lake Pend Oreille to monitor the trends in the aquatic plant community and to assess the efficacy of control techniques on Eurasian watermilfoil. In 2008, the herbicides triclopyr, 2,4-D, endothall, and combinations of endothall with either triclopyr or 2,4-D were evaluated.
Similar to 2007, the 2008 littoral survey found a diverse aquatic plant community. Differences between the 2007 and 2008 surveys were primarily the inclusion of an early season survey in 2007. This early survey resulted in the finding of several early season mud flat annual plant species that are not present later in the season. Lake-wide, the presence of Eurasian watermilfoil significantly decreased from 2007 (12.5%) to 2008 (7.9%). Overall, the native plant community has remained relatively stable from 2007 to 2008 as indicated by the year to year pairwise comparison of species and no significant changes in native species richness. There are several native species that increased in occurrence from 2007 to 2008, most notably northern watermilfoil (Myriophyllum sibiricum Komorov). Eurasian watermilfoil had a frequency of occurrence of 64.5% lake-wide in 2007 following the assessment survey. The frequency of occurrence was significantly reduced to 23.6% following treatments made in 2008 prior to September. The areas treated with 2,4-D were largely unprotected and subject to high flow and therefore the limited sites evaluated did not result in a significant reduction in Eurasian watermilfoil. 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%. There were few sites treated with triclopyr in combination with endothall; however the treatments made with this combination resulted in a significant decrease in the presence of Eurasian watermilfoil. When all treatment methods were pooled and compared to areas that were not treated until after September (an untreated reference for purposes of analysis), the presence of Eurasian watermilfoil was significantly greater (52.5%) in untreated areas as opposed to treated areas (23%). According to the 2008 assessment survey all treatments evaluated were successful in reducing the presence of Eurasian watermilfoil with the exception of 2,4-D; again however, we believe this is due to the fact that the majority of 2,4-D was applied after the assessment survey and analyses were conducted on plots in the river that were likely subjected to increased flow.
