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Alternanthera philoxeroides (Amaranthaceae)

Alligatorweed

 

Alternanthera philoxeroides was added to the EPPO Alert List in 2007 and transferred to the List of Invasive Alien Plants in 2012.

 

Why
Alternanthera philoxeroides (Amaranthaceae) is a perennial herb found both in aquatic and terrestrial habitats, and originating from South America. Pathways for introduction are unvoluntary introduction through ship ballast waters (the most probable way of introduction in Australia), and through plant mulch. In France it is thought that the possible origin or pathway of entry of the plant may be a voluntary introduction as an ornamental plant for ponds and aquaria. The common name of this species is “alligatorweed” in English, and the plant is considered as one of the worst weeds in the world. Within the EPPO region, the species only occurs in France and in Italy. Because its distribution is still very limited, this plant can be considered a new emerging invader in Europe.
Alternanthera philoxeroides

Pictures: Chris Evans
Bugwood.org


Geographical distribution
EPPO region: France, Italy.
North America: Mexico, USA (invasive) (Arkansas, California, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Texas, Virginia).
Central America: Honduras.
South America: Argentina (native), Bolivia (native), Brazil (native), Colombia, Paraguay (native), Peru (native), Uruguay (native), Venezuela (native).
Asia: China (invasive - Fujian, Hunan, Jiangsu, Sichuan, Yunnan), India (invasive - Delhi, Maharashtra, Rajasthan), Myanmar (Burma), Singapore (invasive), Sri Lanka (invasive), Taiwan, Thailand (invasive), Vietnam.
Caribbean: Puero Rico (invasive).
Oceania: Australia (Invasive - New South Whales, Northern Territory, Queensland, South Australia, Tasmania, Victoria, Western Australia), Indonesia (invasive - Java), New Zealand (invasive - Auckland, Canterbury, Waikato), Papua New Guinea (invasive).
Note: In France, the plant seems only present in the Gironde estuary and on the river Garonne and does not seem to show there the same invasiveness as in other places of the world. In Italy, the species is only known in Pisa (Toscana).

Morphology
This perennial aquatic or semi-terrestrial stoloniferous herb with branched, thick and hollow stems can grow till 10 m long. Leaves are shiny, opposite, entire and about 2-7 cm long and 1-2 cm wide. White flowers are born in heads of 8-10 cm in diameter. The plant can both colonize aquatic and terrestrial habitats. Over water, roots are adventicious and stems grow up to 60 cm high and have large, hollow internodes. On land, adventicious roots and thickened taproots occur, stems are shorter, and internodes smaller and much less hollow.

Biology and ecology
Frost and ice kill exposed stems and leaves, however, protected stems survive. It grows on a wide range of substrate, from sand to heavy clay. The plant grows best on eutrophic conditions. The plant is salt tolerant and can adapt to low light conditions (up to 12% of full light). The plant does not produce viable seeds, reproduction is entirely vegetative and relies on the production of nodes. The fragments are dispersed by water. The plant is a serious problem in waterways in tropical and warm-temperate regions of the world.

HabitatsAlternanthera philoxeroides
The plant grows best in aquatic sites but may establish as a terrestrial species in wet and poor pastures and on irrigated lands, estuaries, lakes, riparian zones, water courses, wetlands, ponds, and irrigation canals.

Impacts
The aquatic form can become a serious threat to waterways, agriculture and the environment. While invading agricultural systems, such as pastures, horticulture areas and irrigation areas, it can significantly reduce production. Livestock sometimes suffer from a skin condition, with increased sensitivity to sunlight after contact with A. philoxeroides. In Australia, it is known to blanket the surface of the water, impeding penetration of light and gaseous exchanges, resulting in adverse impacts on native flora and fauna. Moreover, mats impede flow and lead to flooding, they prevent access to and the use of water, and promote health problems by providing habitats for mosquitoes. The plant also negatively affects recreational water use.

Control
Control of this species has proven expensive and complicated wherever it has established. A. philoxeroides has rarely, if ever, been successfully eradicated once it has invaded a water body, despite numerous costly attempts.
Mechanical control: mechanical harvesting can be useful, but all fragments have to be collected.
Chemical control: the species is resistant to many common herbicides, although dicamba, triclopyr, and bentazone are used to control this plant.
Biological control: Clinodiplosis alternantherae (Diptera: Cecidomyiidae) forms galls on branch tips and is a likely candidate to control A. philoxeroides. The two fungi Nimbya alternantherae and Cercospora alternantherae (Mycosphaerellaceae: Mycosphaerella) have also proven to have pathogenic effects on the pest. Three South American species were released in North-America: Agasicles hygophila (Coleoptera: Chrysomelidae), Amynothrips andersoni (Thysanoptera: Phlaeothripidae), Vogtia malloi (Lepidoptera: Pyralidae), which seemed to heavily damage the plant. Moreover, a disease caused by Fusarium sp. that occurs on natural populations may be a good biocontrol agent.

Sources
Global Invasive Species Database. http://www.invasivespecies.net/database/species/ecology.asp?si=763&fr=1&sts=sss
Invasive species website. http://www.invasive.org/browse/subject.cfm?sub=2779
Hawaiian Ecosystems at Risk Project – Althernanthera philoxeroides. http://www.hear.org/pier/species/alternanthera_philoxeroides.htm
Weber E (2003) Invasive plant species of the world – a reference guided to environmental weeds. CABI Publishing. Wallingford, UK, 548 p. p. 40.

EPPO RS 2007/206
Entry date 2007-10 / 2012-05

 

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