Invasive Exotic Plants Fact Sheet No. 10
Eurasian Watermilfoil - spiked water-milfoil; myriophylle en épi
Myriophyllum spicatum L.
Watermilfoil family - Haloragaceae
Field Recognition
Eurasian watermilfoil is a perennial, submersed, rooted aquatic herb with smooth stems that branch near the water surface. Leaves are whorled, feather-like in outline, and usually in 4s, with each consisting of 14-24 pairs of filiform divisions. The lowest division is about half the length of the leaf. At time of flowering, the upper part of the stem, below the terminal spike, is rigid, twice as thick as elsewhere, and lies parallel to the surface of the water. The spike is often pink, 5-20 cm long, and is held erect above the water during flowering but becomes horizontal as fruits ripen. The tiny unisexual flowers are grouped in whorls of 4s with adjacent whorls rotated 45 degrees. The lower whorls are female flowers and the upper male. Bisexual flowers may, on occasion, develop in the region between the unisexual flowers. The lower flowers are surrounded by floral bracts that are often longer than the flowers and have pectinate (comb- like, fringed) margins. These bracts become shorter than the flowers, broader than long, and have entire margins upwards on the spike. The female flowers have no sepals or petals and consist of a 4-lobed pistil. Male flowers have 4 pink petals and 8 stamens. The fruit is globular, indehiscent, 2-3 mm long and contains four seeds.
Differentiating this alien species from the native watermilfoils requires considerable experience and access to technical descriptions such as are given by Aiken et al. (1979). Vegetative features are variable. Specimens of watermilfoil without flowering spikes can be difficult to identify precisely to species.
Habitat
E urasian watermilfoil is an aquatic weed of ponds, lakes and rivers. Although most frequently found in quiet bodies of water, it has also shown an ability to grow luxuriantly in rapidly flowing water (Newroth 1985). In clear waters, such as in Okanagan Lake, British Columbia, with good light penetration, it has been recorded as deep as 8 m (Newroth 1975). In Ontario, most plants tend to grow in waters 0.5-3.5 m deep with less than 25% of the plants in waters below 0.5 or above 3.5 m deep (Aiken et al. 1979). Eurasian watermilfoil occurs mainly in nutrient-rich (euthrophic) lakes and waterways. Substrates vary from sand to acidic peat and waters can be highly alkaline, with pH around 9 to 10, or saline (Aiken et al. 1979). Plants thrive in water with a salinity of up to 10 parts per thousand but show reduced growth at 15 parts per thousand (Beaven 1960). It has even been documented as withstanding 1 m tides in Maryland (Steenis and Stotts 1961).
History of Introduction and Spread
Eurasian watermilfoil is a widespread species of Europe, Asia and North Africa. Confusion over the distinction between native North American watermilfoils and Eurasian watermilfoil has led to uncertainty as to when the alien plant was first officially recorded in North America. Specimens not available for verification by Couch and Nelson (1985) had been used as evidence that Eurasian watermilfoil had been introduced in the late 1800s (Reed 1977). The oldest verified North American record, however, is a collection made in 1942 at Belch Spring Pond in the District of Columbia (Couch and Nelson 1985). As Aiken et al.(1979) have indicated, the species had already been recognized as an aquatic weed, at least locally along the Susquehanna Flats at the north end of Chesapeake Bay, by the late 1930s. Its introduction may have been through ship ballast (Aiken et al. 1979). Chesapeake Bay subsequently developed a major infestation of this aquatic weed. Couch and Nelson (1985) speculated that early introductions could have been made by federal authorities in the Washington, D.C., area since another alien watermilfoil (M. aquaticum) was known to have been introduced locally.
Distribution of Eurasian watermilfoil in North America. The range is based primarily on published accounts by Aiken et al. (1979) and Couch and Nelson (1985).
Another probable source of introduction and spread to western states in the mid- to late-1940s was the aquarium trade. Couch and Nelson (1985) provide good documentation for the likely spread of Eurasian watermilfoil in Oklahoma through its use as fresh packing material for worms sold to fishermen in the state. Bates et al. (1985) make reference to a suspected introduction of Eurasian watermilfoil in Tennessee around 1953 by a commercial dock operator, presumably to try to improve fish habitat. Infestations of this species reached a peak of about 8,900 ha in the state by 1968-69 prior to herbicide treatment. The historic spread of Eurasian watermilfoil in North America has been documented by Couch and Nelson (1985); generalized range maps for North America were also compiled by White et al.(1993). Data available on the internet site of the US Department of Agriculture's PLANTS database indicate that Eurasian watermilfoil occurs in 23 states of the contiguous United States.
In Canada, the first record may have been specimens collected in Lake Erie at Rondeau Provincial Park, in 1961. Subsequent records from the 1960s came from the St. Lawrence Seaway in Ontario and Quebec and, later in the 1970s, it was clearly established as a nuisance plant within the recreational lakes of the Kawartha Region of southcentral Ontario and in southwestern Quebec. The first records of its presence in western Canada was indicated by a collection made in 1970 from Vernon Arm, Okanagan Lake, British Columbia (Aiken et al.1979). The plant was already a nuisance at that time around Vernon and subsequenntly spread rapidly downstream throughout all of the main valley lakes and into the Columbia River system in the United States. It has also spread to other watersheds in British Columbia on boating equipment (Newroth 1985). The possible spread of Eurasian watermilfoil in North America by migrating waterfowl has also been suggested (Couch and Nelson 1985).
In greenhouse culture, all Eurasian watermilfoil samples from across North America, including three provinces in Canada and five states, exhibited the same growth form and differed somewhat from European materials grown under the same conditions (Aiken et al. 1979). These results seemed to indicate that the North American material may have had a common clonal origin. One might wonder whether the widespread occurrences documented in the 1940s in the United States may be due to deliberate or casual releases of clonal material obtained from a primary supply source for the aquarium trade. A study of isozyme variation in Minnesota populations of Eurasian watermilfoil identified two genotypes suggesting that at least two introductions occurred in the lakes sampled (Furnier and Mustaphi 1992).
The entire North American distribution of Eurasian watermilfoil exhibits a bimodal distribution. Its greatest geographical spread is throughout eastern North America, reaching its northern limits in southern Ontario and parts of the St. Lawrence Valley in Quebec. It is presently absent in the Canadian Maritimes. It is also generally absent in the central interior of North America but then occurs again along the west coast in an arc from New Mexico northward, mainly along the coastal regions of the Pacific states, reaching its limits in southern Vancouver Island and southcentral British Columbia.
Biology
Variability
The relative similarity of the native North American watermilfoil, Myriophyllum sibiricum (=M. exalbescens), to the alien M. spicatum has resulted in some authors treating the native species as a variant of the alien (M. spicatum var. exalbescens). Studies by Aiken (1981), Aiken et al. (1979) and Ceska and Ceska (1985) have maintained the two species as distinct entities based on differences in morphology, ecology and flavonol compounds.
Under natural conditions, leaf form in Eurasian watermilfoil can vary depending on conditions of growth. Plants that become exposed due to the slow lowering of the water level caused by evaporation may develop leaves characteristic of a land form. These leaves are smaller and stiffer with fewer divisions than normal. Renewed growth by such plants when water levels rise results in the new leaves having few divisions initially and only assuming their normal appearance after a period of growth. Young plants and floating fragments can also develop leaves with fewer than 14 divisions. The application of low doses of the herbicide 2,4-D results in the development of thicker distorted leaves with a wider midrib and fewer divisions (Aiken et al. 1979).
A study of water chemistry characteristics in British Columbia provided some indications that Eurasian watermilfoil preferred softer water and had a higher tolerance to ammonia levels than the two common native species, M. sibiricum and M. verticillatum (Warrington 1985). Studies of sediment composition have shown that growth decreases as the organic matter content increases up to 20% and that growth is relatively low at higher values of organic matter in the substrate (Barko and Smart 1985).
Growth cycle
The overwintering root crowns begin growth early in spring and develop leafy stems that branch profusely near the surface (Aiken et al.1979). A horizontal floating canopy of shoots forms that shades the lower branches and leaves causing them to slough. Asexual reproduction occurs through the release of small root crown buds in the spring that may establish new plants early in the season and through the release of numerous stem fragments during the summer through a process of natural fragmentation. These fragments may develop roots even before they are released from the parent plant. Sections of shoots broken off by wave action or other mechanical means are also viable. Such fragments are commonly the source of material for colonizing downstream waterbodies.
Plants flower, in Canada, mainly from late July to early August. The lower female flowers mature before the stamens of the male flowers on the same spike. Wind is the primary agent of cross-pollination (Patten 1956). Insect pollination may also play a significant role. Spikes recline on the surface of the water after flowering. A maximum of four seeds develop in each fruit and, when ripe, the fruits detach and float for a few hours or a day, allowing time for some dispersal by flowing water. Ripe fruits do not split open and the seeds are considered by some to have a long dormant period and to germinate erratically (Sculthorpe 1967). Reports have also been made, however, of seeds from ripe fruits germinating readily in the laboratory soon after picking (Aiken et al. 1979). In areas of high infestation, as many as four million seeds per hectare have been estimated to be produced, as in Tennessee and in Currituck Sound, North Carolina and Okanagan Lake, British Columbia (Aiken et al. 1979). Artificial cross-pollination between M. sibiricum and M. spicatum resulted in fruit with ripe seeds, over 30% of which germinated (Aiken et al. 1979). In spite of the abundance of seed produced in some water bodies, vegetative propagation is still considered the primary means of reproduction. Seedlings have generally not been reported, even from areas of high infestation (Aiken et al. 1979). Seed germination studies in the laboratory found that temperatures over 15øC were necessary for germination (Hartleb et al. 1993). The same authors found that in situ studies indicated that germination was significantly reduced when seeds were buried deeper than 2 cm within the sediments. Light, by itself, was not a limiting factor based on tests of seed germination at various depths of water from one to five metres.
Physiological activity
Studies of mechanical harvesting of Eurasian watermilfoil in the Kawartha Lakes region of southern Ontario indicated that the 3 million kg of plant material that was removed from Southern Chemung Lake contained 560 kg of phosphorus. This represented about 92% of the net annual phosphorus loading into the lake (Wile 1978). The removal of this quantity of Eurasian watermilfoil, representing the loss of a substanial quantity of phosphorus nutrients, coincided with a natural reduction in the algal biomass of the lake. Other similar impacts are reported by Aiken et al. (1979). More recent growth chamber studies have shown Eurasian watermilfoil, as well as three other aquatic macrophytes, absorb phosphorus from both the sediments as well as the water column (Waisel et al. 1990). Leaves are more efficient than roots in this regard. Eurasian watermilfoil accumulates mineral crusts comprising up to 80% of the plant's dry weight. Such crusts contain calcite, quartz, apatite and aragonite.
Studies of cadmium accumulation by Eurasian watermilfoil have shown that this species may be useful for absorbing cadmium from nutrient-rich waters if the concentration falls within the range 0.04- 7.63 æg Cd. The highest levels of cadmium, however, retard plant growth (Sajwan and Ornes 1996).
Flurprimidol has been shown to inhibit growth in Eurasian watermilfoil with increased time of exposure. Under conditions that suppress shoot length, starch accumulates in both shoots and roots (Nelson 1996). Dry biomass were unaffected by treatment with flurprimidol.
Diseases and pests
Lake Venice disease was discovered in 1962 in Maryland populations of Eurasian watermilfoil (Aiken et al. 1979). This condition results in the formation of a thick brownish coating of various microorganism that eventually cause the death of the plants. No specific causal organism has been demonstrated. Northeast disease was observed in 1964. Its appearance is described by Aiken et al. (1979). The primary pathogen was thought to be a virus but transmission of the disease could not be effected through laboratory studies (Bayley 1970).
As many as 25 insect herbivores have been described as feeding on Eurasian watermilfoil in the former Yugoslavia and in Pakistan (Spencer and Lekic 1974). Some of these have been subjected to testing as potential control agents (Aiken et al. 1979).
Environmental and Economic Impacts
Many of the common problems associated with Eurasian watermilfoil infestations are reviewed by (Newroth 1985). One of the most common complaints of adverse effects is that due to its nuisance in the recreational use of waters. Dense mats have an impact on sailing boats with keels, motorized fishing boats and on water skiing activities. Shore-based angling as well as trolling are negatively affected. Dense mats are considered to affect the esthetic appearance of infested bodies of waters and have an impact on the recreational use of beaches through increased risk to swimmers and problems of "swimmers itch". Infestations also clog water intakes for industrial plants and electric power plants, depress reality values, result in lowered dissolved oxygen levels and cause significant increases in mosquitoes such as Anopheles quadrimaculatus (Bates et al. 1985). Temperature profiles of lakes may be altered by as much as 10øC per metre in shallow water (Dale and Gillespie 1977).
Eurasian watermilfoil infestations also have interfered with accurate measurement of water discharges in some waterways and have been implicated in minor flooding of Vaseux Lake. The aggressive growth has resulted in extensive infestations in portions of its North American range. In British Columbia it has been shown to occupy areas of native aquatic vegetation as well as habitats not previously occupied by aquatic macrophytes (Newroth 1985). Its aggressiveness is such that it even suppresses other alien macrophytes such as curly-leaved pondweed (Potamogeton crispus). In the southern United States, Hydrilla verticillata, another alien aquatic, is the only North American submerged aquatic that can successfully compete with Eurasian watermilfoil due to its low light tolerances (Bowes et al. 1977).
At Cultus Lake, British Columbia, infestations of Eurasian watermilfoil had begun to have a negative impact by 1985 on gravel spawning beds used by salmon. Water quality may also be impacted. Eurasian watermilfoil concentrates large quantities of sediment phosphorus in its tissues. The seasonal breakdown of the mats results in a high level of phosphorus release into the water column. It is estimated that in some Okanagan Valley lakes, the release of phosphorus from the decaying mats is greater than that attributed to the combined influx from storm sewers, industrial and agricultural sources (Newroth 1985).
In addition to the biological and recreational impacts mentioned, the considerable expense related to its control is another important impact that affects management authorities. A comparison of the various costs of selected treatments used in British Columbia are summarized by Newroth (1985). A multimillion dollar program aimed at eradicating Eurasian watermilfoil from the Okanagan Valley was initiated in 1977 (Newroth 1977).
Some beneficial attributes have been identified. The beds of Eurasian watermilfoil serve as protective habitat for young fishes and may serve as prime location for bass fishing (Aiken et al. 1979). Aquatic macrophytes also utilize water nutrients thereby reducing algal blooms (Davis et al. 1973). Harvested mats have also been tested, with limited success, as fertilizer (Anderson et al. 1965), animal feed (Muztar 1976; Muztar et al. 1976) and as a soil conditioner (Wile et al. 1978).
Control Measures
Chemical control
The most effective and commonly used herbicide is 2,4-D (2,4-dichlorophenoxy acetic acid). It is effective in concentrations as low as 1 ppm for an exposure period of 48 h when there is no water movement. A concentration of 5 ppm for only 1 h will kill all plants (Steward and Nelson 1972). Application of 2,4-D at high concentrations was reported to have no direct impact on aquatic fauna or water quality in the Tennessee Valley Authority reservoirs. Indirect results can include fish kills resuling from oxygen depletion caused by the decay of a large biomass of watermilfoil (Brooker and Edwards 1975). Various other factors affecting the use of 2, 4-D and aspects of the use of broadspectrum contact herbicides are reviewed by Aiken et al.(1979).
Mechanical control
A variety of methods have been used in attempts to physically control infestations of Eurasian watermilfoil. Among these methods are the use of mechanical harvesters, underwater cultivators, diver- operated dredges, bottom barriers and the lowering of water levels to expose plants to freezing temperatures or to allow dessication.
Mechanical harvesters and underwater cultivators, for example, are effective at reducing a large biomass in a short period of time (Aiken et al. 1979; Bates et al. 1985; Maxnuk 1985; Newroth 1985; Truelson 1985). One of the main problems associated with such methods, however, is the production of large numbers of fragments that promote the spread of this plant. Such methods potentially exacerbate the problem downstream. The use of fragment barriers can reduce the extent of this problem. Mechanical harvesting is a costly operation and may need to be done several times a year to adequately control this fast-growing aquatic. Careful timing of two harvests per season can provide adequate short-term control (Painter and Waltho 1985). The slower rototilling techniques seemingly provide greater control over mechanical harvesting of plants (Newroth 1985). The costs of the various mechanical control techniques have been documented by Newroth (1985).
The use of high intensity ultrasound, which kills plants in situ, has been tested but no recent data are available (Soar 1985). Establishing quarantine programmes in an attempt to minimize transportation of plants by boaters and fishermen has limited effect and only delays the inevitable spread (Newroth 1985).
The manipulation of water levels has proved effective in Tennessee Valley Authority reservoirs for controlling Eurasian watermilfoil (Bates et al. 1985). Water drawdowns in the reservoirs reduced infestations after a short period of cold exposure. This procedure did not increase infestations at deeper depths (Goldsby et al. 1978). It has been shown that exposure of plants during winter to a period of freezing temperatures for only 96 hours is sufficient to kill Eurasian watermilfoil (Stanley 1976). Drawdowns in test lakes within the Kawartha Lakes region of Ontario were not found to be of value. Control proved to be limited, re-infestation from adjacent areas was rapid and there was the added potential for damage to lakeside properties and fish kills (Wile and Hitchin 1977).
Biological control
Several biocontrol agents have been considered for possible use. In British Columbia, a native chironomid larva (Cricotopus myriophylli) was found to suppress Eurasian watermilfoil colonies. Large populations of this insect are required, however, in order to serve as an effective control (Newroth 1985).
The pathogen Mycoleptodiscus terrestris has been tested as a potential microbial herbicide for Eurasian watermillfoil (Verma and Charudattan 1993). Additional testing was indicated due to the development of disease symptoms on a variety of forage plant seedlings.
The herbivorous grass carp (Ctenopharyngodon idella) has been studied for its potential use as a biocontrol agent in Florida and Tennessee (Kobylinski et al. 1980; Bates et al. 1985). The development of sterile, triploid grass carp, in particular, holds promise for the control of aquatic weeds.
A native North American freshwater weevil, Euhrychiopsis lecontei, has been reported as causing a decline in the population of Eurasian watermilfoil in Brownington Pond, Vermont (Creed and Sheldon 1993, 1995). Experiments indicated that the adult weevils, which are fully submersed aquatic herbivores, feed preferentially on Eurasian watermilfoil stems and leaves when given a choice between the alien plant and the native watermilfoil, Myriophyllum sibiricum. The authors suggested that this weevil has expanded its diet or has undergone a host shift in favour of the alien M. spicatum. Other studies by Creed and Sheldon (1994) showed that the late-instar larvae significantly reduced Eurasian watermilfoil growth in experiments where the aquatic weed exhibited a fast growth rate. The larvae burrowed through the stems at a rate of 6-8 mm per day. These authors also studied the impact of a moth larva (Acentria nivea) on Eurasian watermilfoil, alone or in combination with the weevil. The moth larvae significantly reduced Eurasian watermilfoil growth by cutting the stems and feeding on leaves. Both herbivores are considered as potential biological control agents. The potential value of the native weevil in controlling Eurasian watermilfoil was further emphasized in studies using weevil enclosures (Sheldon and Creed (1995). Plants within enclosures exhibited 50% less biomass and plants did not form a floating canopy as compared to control enclosures. The life history of this weevil has also been documented recently (Sheldon and O'Bryan 1996).
Creed and Sheldon (1995) have also shown that stem fragments damaged by weevils reduce their viability and that weevils suppressed the production of new watermilfoil biomass.
Tests for oviposition preferences of E. lecontei indicate that this aquatic herbivore is a watermilfoil specialist (Solarz and Newman 1996). Studies of oviposition preferences show that weevils reared on Eurasian watermilfoil preferred this species over the native watermilfoil, M. sibiricum. Weevils reared on M. sibiricum did not exhibit a preference for one or the other species but did prefer watermilfoils over other macrophytes. These results support the contention that E. lecontei may be a good biocontrol agent.
Interesting Facts
The generic name comes from the Greek myrios, meaning numberless, and phyllon, for leaf, in reference to the numerous leaf segments (Fernald 1950). The descriptive epithet spicatum refers to the flower cluster that consists of a short spike of whorls of small stalkless