An Introduction to Eurasian Watermilfoil

Updated: Oct 8, 2019

Since its first documented occurrence in U.S. waterways in 1942, Eurasian watermilfoil (Myriophyllum spicatum) has proven to be devastating. It is now considered to be among the most threatening of all aquatic invasive species, and for good reason. Aside from being a major nuisance to boaters and anglers, it can significantly reduce the efficiency of drainage and irrigation canals. Because it is such a conspicuous bane to agricultural and recreational land users, public support for Eurasian watermilfoil eradication is often very strong [6].

eurasian watermilfoil

In Colorado, both the Department of Agriculture and the Division of Wildlife direct aquatic nuisance species management. Eurasian watermilfoil (hereafter referred to as EWM) is on the Colorado Division of Wildlife’s Aquatic Nuisance Species Watch List. EWM is designated as a List B species on the Colorado Department of Agriculture’s Noxious Weed List (USDA Code: MYSP2). Its List B designation signifies a potential need for control measures as advised under the state noxious weed management plan [9,17].

The Boulder Valley Conservation District has begun to work with local ditch companies to prevent new infestations [10]. The St. Vrain and Left Hand Water Conservancy District has proposed the use of screens to resist the spread of invasive species, including EWM, into St. Vrain State Park reservoirs [11]. EWM contamination containment programs and watercraft inspection have been implemented at a few state parks [13]. Boulder County wildlife restoration volunteer groups are now including EWM eradication efforts in their programs [19]. Yet, even with well-coordinated management strategies and heightened public awareness, this destructive weed is poised to spread into new areas.

A clear and growing threat

EWM is dispersed primarily through fragmentation. It can become established in waters ranging from <one foot to >20 feet depth. Though it causes the greatest disturbance at 6-15 feet depth, it can blanket shore bottoms at 3-20 feet depth [2]. Once introduced into a new environment, a small colony can competitively displace all other submerged vegetation within two to three years [15]. Even a moderate infestation can dramatically alter the physical, chemical and thermal conditions of riparian habitats, often to the detriment of native flora and fauna [1,2,7]. 

Many aquatic macrophyte species are known to secrete allelopathic substances into the surrounding waters. However, those of EWM (primarily its hydrolyzable polyphenols) are especially potent. Highly algicidal, these compounds strongly inhibit the growth of naturally occurring phytoplankton [21]. This results in an overall reduction of zooplankton, which ultimately impacts native fisheries.