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Eurasian Watermilfoil 6 EURASIAN WATERMILFOIL R. L. Johnson1 and B. Blossey2 1 Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, USA 2 Department of Natural Resources, Cornell University, Ithaca, New York, USA PEST STATUS OF WEED Eurasian watermilfoil, Myriophyllum spicatum L., (Fig. 1) is a submersed aquatic plant that has become a major aquatic nuisance throughout much of North America. Plants are rooted at the lake bottom and grow rapidly creating dense canopies (Aiken et al., 1979). Eurasian watermilfoil is able to form dense beds (Fig. 2) with stem densities exceeding 300/m2 in shallow water (Aiken et al., 1979). Plants typically grow in water depths of 1 to 4 m, but have been found growing in water as deep as 10 m (Aiken et al., 1979). Conventional control efforts have been unsuccessful in providing more than short-term relief. Herbicide applications may suppress regrowth from as little as six weeks or up to one year (Aiken et al., 1979), but have considerable non-target effects (e.g., fish kills, increased algal growth, water supply contamination, Figure 1. Eurasian watermilfoil plant. Cornell University Research Ponds, Ithaca, New native macrophyte die-off). Mechanical harvesters, York, July 1997. (Photo courtesy of Robert L. rototillers, cultivators, barriers, dredges and other Johnson, Department of Ecology and physical control techniques have resulted in short Evolutionary Biology, Cornell University, term, often localized reductions of M. spicatum popu- Ithaca, New York) lations, but these methods are disruptive, costly, and labor intensive. For continued effect, they need to be maintained long term (Boylen et al., 1996). Nature of Damage Economic damage. High densities of Eurasian watermilfoil negatively affect wildlife and fish popu- lations and make recreational use difficult or impos- sible. Direct financial damages to recreation (boat- ing, swimming, fishing) have not been assessed. Eur- asian watermilfoil continues to be the most impor- tant waterweed in the continental United States with millions of dollars spent nationwide for control ef- Fig. 2. Dense Myriophyllum spicatum canopy at forts (U.S. Congress, Office of Technology Assess- lake surface. Cayuga Lake, Ithaca, New ment, 1993). In New York state alone, annual costs York, August 1989. (Photo courtesy of are estimated at $500,000. Robert L. Johnson) 79 Biological Control of Invasive Plants in the Eastern United States Ecological damage. With the expansion of Eur- asian watermilfoil, species diversity and abundance BACKGROUND INFORMATION of native macrophytes declines (Smith and Barko, ON PEST PLANT 1990; Madsen et al., 1991). Although in small tank Taxonomy experiments the native northern watermilfoil (Myriophyllum sibiricum Kom.) appears competi- Eurasian watermilfoil belongs to the watermilfoil tively superior, in the field, however, M. spicatum has family, Haloragaceae, which has two genera in the replaced M. sibiricum over much of the temperate eastern United States, Myriophyllum L. (10 species, range of this species in North America (Valley and the watermilfoils) and Proserpinaca L. (two species, Newman, 1998). Suppression of native macrophytes the mermaid-weeds) (Gleason and Cronquist, 1991). is enhanced through the formation of a Eurasian All species are hydrophytes with many finely divided watermilfoil canopy at the water surface, reducing leaves. All Haloragaceae species are herbs submersed light penetration. This canopy often forms early in in quiet waters or rooted on muddy shores. The simi- the season before native macrophytes reach their larity of the species has led to much confusion about maximum growth potential. Eurasian watermilfoil species identity, and most species in the family can- beds contain significantly fewer macroinvertebrates not be separated using only individual specimens or than native macrophyte communities (including ones without flowers. The date of introduction of M. benthic invertebrates) and have a reduced abundance spicatum to North America is debated and some au- of native fish species (Keast, 1984). Fish spawning thorities consider reports before 1940 as taxonomic areas and fish growth are reduced in lakes with large misidentifications of M. sibiricum (= M. exalbescens infestations of Eurasian watermilfoil. During certain Fern.) (Johnson et al., 1998). Myriophyllum spicatum times in winter, waterfowl forage extensively in mil- is variable in appearance with long stems, and usu- foil beds in Alabama (McKnight and Hepp, 1998; ally 12 to 21 leaflet pairs, which are limp when out of Benedict and Hepp, 2000), although the same spe- the water. In contrast, the very similar M. sibiricum cies largely avoid Eurasian watermilfoil in the Great usually has five to 10 leaflet pairs with leaflets that Lakes (Knapton and Pauls, 1994). stay rigid when out of the water. Leaf morphology Extent of losses. Direct losses are difficult to may be used to separate these two very similar spe- quantify due to lack of data from long-term moni- cies successfully (Gerber and Les, 1994). Plants of- toring programs. ten branch at the water surface (or in response to her- bivore damage to apical meristems) and flowers are Geographical Distribution arranged on emersed spikes (associated with a dra- From the initial points of introduction in the North- matic shift in plant morphology). The flower spike east, M. spicatum has spread to 44 states and at least bears whorls of female flowers basally and whorls of three Canadian provinces (Creed, 1998) and is now male flowers apically. Each female flower produces considered a major nuisance species throughout the four small nutlike fruits (2 to 3 mm). Northeast, northern Midwest and Pacific Northwest Biology of the United States (Couch and Nelson, 1985, White et al., 1993). The mode of dispersal is not completely Eurasian watermilfoil occurs in ponds, lakes, and understood, but M. spicatum can be spread short dis- pools that vary from rather deep to very shallow tances as fragments tangled on boats and trailers (from more than 100 m to a few cm), and may be (Nichols and Shaw, 1986). Also, human activities, stagnant or slowly moving fresh to slightly brackish such as motor boating and mechanical weed harvest- water (Spencer and Lekic, 1974). Plants overwinter ing, produce and distribute stem fragments allowing rooted in the sediment and grow rapidly once favor- increased propagation (Nichols and Shaw, 1986). able temperatures are reached. Flowering can occur Long distance dispersal has been linked to the in early summer and can continue for several months aquarium and aquatic nursery trade (Reed, 1977), and (Spencer and Lekic, 1974). Eurasian watermilfoil re- the species continues to expand its range in North produces by seed, but fragmentation is the most likely America. mode of spread in the northern parts of the range in 80 Eurasian Watermilfoil North America. Sexual reproduction appears unim- species in these chapters). The closest relative to M. portant in shaping population structure of Eurasian spicatum is northern milfoil M. sibiricum and the watermilfoil in Minnesota (Furnier and Mustaphi, ranges of these species overlap widely in the temper- 1992); however, significant germination is observed ate regions of North America. in Lake George in New York State (Hartleb et al., 1993). Seeds require high temperatures (above 14°C) for germination. Light is not considered a limiting HISTORY OF BIOLOGICAL CONTROL factor, but increased sedimentation can greatly sup- EFFORTS IN THE EASTERN press germination (Hartleb et al., 1993). Under unfa- UNITED STATES vorable conditions or when plants are attacked by herbivores, plants may not reach the water surface For almost 30 years, overseas and domestic research and do not flower. Fragmentation still allows popu- has evaluated potential agents (insects and pathogens) lations to expand and colonize adjacent areas or reach for the biological control of Eurasian watermilfoil more distant areas through the aquarium trade, trans- (Buckingham et al., 1981; Creed et al., 1992; Creed port in currents, or through recreational activities. and Sheldon, 1993, 1995; Shearer, 1994; Sheldon and Creed 1995; Cofrancesco, 1998; Creed, 1998; Analysis of Related Native Plants in the Eastern Johnson, et al., 1998; Mazzei et al., 1999; Gross et al., United States 2001). Several species of insects have been identified The genus Myriophyllum belongs to the taxonomi- feeding on and damaging Eurasian watermilfoil in cally isolated watermilfoil family, Haloragaceae, in North America (Batra, 1977; Buckingham and the order Haloragales. The genus Myriophyllum is Bennett, 1981; MacRae et al., 1990; Creed and representative of this order; more distantly related Sheldon, 1993). Some of these species appear to be species occur in the southern hemisphere. There has native to North America and to have switched from been much confusion about taxonomic status and their original hosts; others may have been acciden- identity of Myriophyllum species. Muenscher (1944) tally introduced from Europe along with M. spicatum lists approximately 20 species of Myriophyllum in (Buckingham et al., 1981). Since 1963, the grass carp, North America; Gleason and Cronquist (1991) list Ctenopharyngodon idella (Cuvier and Valenciennes), 10 species in the eastern United States. Three of the has been released to suppress Eurasian watermilfoil 10 Myriophyllum species mentioned by Gleason and and other nuisance aquatic plants in numerous sites Cronquist (1991) are species introduced to North within
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