April 2008
GeoResources Institute Report 5028

"Assessment of Eurasian watermilfoil (Myriophyllum spicatum L.) Populations in Lake
Pend Oreille, Idaho for 2007"

John D. Madsen and Ryan M. Wersal
GeoResources Institute, Box 9652, Mississippi State, MS 39762-9652

ABSTRACT

Lake Pend Oreille is a freshwater lake located in northern Idaho and is the largest in the
state encompassing approximately 91,000 acres (40,500 hectares), of which 25,300 acres are
considered to be littoral zone habitat. In recent years the non-native aquatic plant Eurasian
watermilfoil (Myriophyllum spicatum) has invaded and established populations in several areas
of the lake. 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. One component of this program is to determine the extent of Eurasian watermilfoil
infestations by conducting comprehensive surveys of susceptible or suspected water-bodies.
Surveys of the littoral zone of Lake Pend Oreille were conducted in June and August of 2007 to
determine the distribution of Eurasian watermilfoil and native plant species in both the river and
lake. In addition to the littoral surveys, assessment surveys were conducted pre and post
deployment of control techniques for Eurasian watermilfoil to quantitatively compare the
efficacy of triclopyr, fluridone, and diver-operated suction dredging.
Lake Pend Oreille has a diverse aquatic plant community with approximately 51 species
identified during the littoral surveys. Plant species richness increased from the June survey to
the August survey. Native plant species inhabited water depths between 5 and 12 feet although
some species were found to depths of 30 feet. Eurasian watermilfoil typically inhabited water
depths of 10 to 20 feet. Lake-wide, the presence of Eurasian watermilfoil significantly increased
from the time of the pre-treatment survey to the post treatment survey when all control
techniques were analyzed. The increase in Eurasian watermilfoil is attributed to poor efficacy of
the fluridone and diver dredge treatments, and expansion from untreated areas. The presence of
Eurasian watermilfoil during the pre and post treatment surveys of fluridone treated areas was 45
and 40%, respectively; which was not a statistically-significant change. Eurasian watermilfoil
presence in diver-operated suction dredging areas was 36% and 46% respectively for the pre and
post treatment surveys. Triclopyr however, was effective at reducing the presence of Eurasian
watermilfoil in treated areas by 70%. Eurasian watermilfoil presence during the pre-treatment
survey of triclopyr-treated areas was 61% (598 acres) and was reduced to 18% (180 acres)
during the post treatment survey.
Fluridone treatments did not appear effective at the time of evaluation. Fluridone
typically requires 60 days of exposure for maximum effectiveness; however, Eurasian
watermilfoil plants typically show symptoms and bleaching when exposed to lethal
concentrations of fluridone, and these symptoms were rarely observed. Diver dredging was not
effective in reducing the presence of Eurasian watermilfoil. If left untreated, the current 990
acres of Eurasian watermilfoil could expand in one season to encompass 1,200 to 1,500 acres;
the use of herbicides is prohibiting further spread of this species. The techniques deployed in
2007 averaged $797 treated acre-1 which prohibited the treatment of more infestations in the
lake. Additional herbicides, combinations of herbicides, application times, and other
management strategies should be evaluated in the future. Monitoring and assessment is
necessary and should continue to ensure a successful Eurasian watermilfoil control program.