Predicting Eurasian watermilfoil's (Myriophylum spicatum L.) distribution and response to biological control in Fall River, California

Authors: Spencer, David; Carruthers, Raymond

Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2012
Publication Date: 12/1/2013
Citation: Spencer, D.F., Carruthers, R.I. 2013. Predicting Eurasian watermilfoil's (Myriophylum spicatum L.) distribution and response to biological control in Fall River, California. Journal of Aquatic Plant Management. 51:7-14.

Interpretive Summary: Eurasian watermilfoil (Myriophyllum spicatum L.), was first observed in Fall River, California in approximately 2001. Its presence has had impacts on this spring-fed river which runs through a valley where agriculture is an important activity. In order to assist in the development of a management plan for dealing with this invasive aquatic weed, we determined Eurasian watermilfoil abundance and distribution during 2009 and 2010. We also determined water temperature and the levels of an important plant nutrient (total phosphorus) throughout a 21 mile long portion of the river. Eurasian watermilfoil frequency increased in the river downstream from the Spring Creek Bridge. Based on total phosphorus concentrations and computer simulations using a published growth model, Eurasian watermilfoil should be able to grow upstream of the bridge as well. High water levels which prevent boats from passing under this bridge may limit the upstream spread of this aquatic weed. One management proposal is to introduce the Eurasian watermilfoil weevil as a biological control. Calculation of the number of degree-days available for watermilfoil weevil growth indicate that the weevil will only achieve 3 or more generations per growing season at points downstream of the confluence of the Tule River. Since this only represents about one-third of the portion of the river that Eurasian watermilfoil inhabits or may potentially inhabit, it is not likely that the milfoil weevil will have a significant impact on it.

Technical Abstract: Eurasian watermilfoil (Myriophyllum spicatum L.), was first observed in Fall River, California in approximately 2001. Its presence has had impacts on the river. During 2009 and 2010 we determined Eurasian watermilfoil abundance and distribution. We also determined water temperature and total P concentration throughout a 37 km portion of the river. Eurasian watermilfoil frequency increased in the river downstream from the Spring Creek Bridge. Based on total P concentrations and simulations from a published growth model Eurasian watermilfoil should be able to grow upstream of the bridge as well. High water levels which prevent boats from passing under this bridge may limit the upstream spread of this weedy aquatic plant. One management proposal is to introduce the Eurasian watermilfoil weevil (Euhryciopsis lecontei) as a biological control. Degree-day calculations indicate that the weevil will only achieve 3 or more generations per growing season at points downstream of the confluence of the Tule River. Since this only represents about one-third of the portion of the river that Eurasian watermilfoil inhabits or may potentially inhabit, it is not likely that the milfoil weevil will have a significant impact on it.