Eurasian watermilfoil has slender stems up to 3 m long. The submerged leaves (usually between 15-35 mm long) are borne in whorls of four, pinnate, with the numerous leaflets thread-like, 4–13 mm long. Plants are monoecious with flowers produced in the leaf axils (male above, female below) on a spike 5–15 cm long held vertically above the water surface, each flower inconspicuous, orange-red, 4–6 mm long. Eurasian water milfoil has 12- 21 pairs of leaflets while northern watermilfoil M. sibiricum only has 5-9 pairs. The two can hybridize and the resulting hybrid plants can cause taxonomic confusion as leaf characters are intermediate and can overlap with parent species.
This plant is distributed by watercraft or other objects that have come in contact with Eurasian Watermilfoil or carries fragments. Watermilfoil can reproduce from the smallest fragment of plant.
Myriophyllum spicatum was likely first introduced to North America in the 1940s where it has become an invasive species in some areas. As of 2003 Eurasian watermilfoil was found in every state of the contiguous US save for Wyoming and Montana, with Rhode Island, New York, New Jersey, Michigan, Minnesota, and Washington having the largest areas of infestation.
In lakes or other aquatic areas where native aquatic plants are not well established, the Eurasian plant can quickly spread. It has been known to crowd out native plants and create dense mats that interfere with recreational activity. Eurasian watermilfoil can grow from broken off stems which increases the rate in which the plant can spread and grow. In some areas, the Eurasian Watermilfoil is an Aquatic Nuisance Species. Eurasian watermilfoil is known to hybridize with the native northern watermilfoil (M. sibiricum) and the hybrid taxon has also becomes invasive in North America. It is known from across the USA upper midwest (Minnesota, Michigan, Wisconsin) and in the Northwest (Idaho, Washington).
The aquatic moth Acentria ephemerella, the water veneer moth, feeds upon and damages this water milfoil. It has been used as an agent of biological pest control against the plant in North America. The milfoil weevil (Euhrychiopsis lecontei) has also been used as biocontrol. Another method for biocontrol is Grass Carp, (one of the Asian Carp species) which have been bred as sterility, is sometimes released into affected areas, since these fish primarily feed on aquatic plants and have proven effective at controlling the spread. However, the carp prefer many native species to the milfoil and will usually decimate preferred species before eating the milfoil.
Since roughly 2000, hand-harvesting of invasive milfoils has shown much success as a management technique. Several organizations in the New England states have undertaken large scale, lake-wide hand-harvesting management programs with extremely successful results. Acknowledgment had to be made that it is impossible to completely eradicate the species once it is established. As a result, maintenance must be done once an infestation has been reduced to afford-ably controlled levels. Well trained divers with proper techniques have been able to effectively control and then maintain many lakes, especially in the Adirondack Park in Northern New York where chemicals, mechanical harvesters, and other disruptive and largely unsuccessful management techniques are banned.
- Moody, M. L.; Les, D. H. (2007). Biological Invasions 9: 559–570.
- Couch, R.; Nelson, E. (1985). "Myriophyllum spicatum in North America". Proceeding of the first international symposium on watermilfoil (Myriophyllum spicatum) and related Haloragaceae species: 8–18.
- Moody, M. L.; Les, D. H. (2002). Proceedings of the National Academy of Sciences, USA 99: 14867–14871.
- Myriophyllum spicatum-released allelopathic polyphenols inhibiting growth of blue-greenalgaeMicrocystis aeruginosa. Satoshi Nakai, Yutaka Inoue, Masaaki Hosomi and Akihiko Murakami, Water Research, Volume 34, Issue 11, 1 August 2000, Pages 3026–3032, doi:10.1016/S0043-1354(00)00039-7