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Alligator Weed

Alternanthera philoxeroides

Summary 7

Alternanthera philoxeroides, commonly known as Alligator weed, is an emergent aquatic plant. It originated in South America, but has spread to many parts of the world and is considered an invasive species in Australia, China, New Zealand, Thailand and the United States.

Population biology 8

Alternanthera philoxeroides was substantially more abundant in the southeastern U.S. four decades ago than it is now. In 1963, over 65,000 ha of land in 8 southern states were overrun with invasive alligatorweed. Within two decades, however, the infested land in those states was brought down to only about 1% of that total, primarily through successful biological control (Coulson 1977). During this same period, however, the amount of infested land in Texas and Louisiana increased (Cofrancesco 1988).

Associations 9

Closely associated with Alternanthera philoxeroides is the alligatorweed flea beetle, Agasicles hygrophila, an insect not native to the U.S. but intentionally introduced here in the mid-1960s as potential biological control agent of A. philoxeroides (Buckingham 2002). A. hygrophila has the distinction of being the first biocontrol insect released in the U.S. in order to combat an invasive plant. Overall, management impacts on alligatorweed have been excellent as indicated by the dramatic decrease in the amount of infested aquatic habitat since the the insect was first released. However, it is not considered to be effective against plants occurring in terrestrial habitats (UF/IFAS CAIP undated).Additional alligatorweed-associated insects purposely introduced to the U.S. as potential biocontrol agents are the alligatorweed thrips, Amynothrips andersonii, the alligatorweed stem borer moth, Arcola (=Vogtia) malloi and another species of moth, Arcola malloi. Several additional insects are being studied to determine their suitability as A. philoxeroides control agents, primarily in Australia (Buckingham 2002).Invasion History: Alternanthera philoxeroides is native to South America. It was first reported in the U.S. in 1897 near Mobile, AL. The introduction of this aquatic /terrestrial plant is interesting in that it is believed to have been the result of accidental release from ship ballast (Carley and Brown 2006). Ballast release has also been implicated as the vector for introduction to Australia.Alligatorweed now occurs as an invasive exotic in subtropical to temperate regions of the Americas, Asia, Australia, New Zealand and a number of Pacific island nations. In the U.S. A. philoxeroides occurs throughout the southeast as far north as Illinois, and west to Texas, and coastal California.The ability of A. philoxeroides to persist in terrestrial, semi-aquatic, and aquatic environments, the ability to rapidly take root along waterway banks, and the ability to propagate via vegetative fragmentation and waterborne dispersal of vegetative propagules all contribute to its success as an invasive species. Potential to Compete With Natives: Dense mats of aquatic alligator weed displace native aquatic and shoreline vegetation and also alter aquatic systems by decreasing water flow, increasing sedimentation, shading submersed plants, reducing oxygen levels, and filling in and choking off formerly open water column habitat (Quimby and Kay 1976, Holm et al. 1997, Carley and Brown 2006). Possible Economic Consequences of Invasion: Alternanthera philoxeroides has adverse economic impacts as an invasive species in several ways. It is capable of choking waterways and thereby impacting boating and sportfishing activities. Aquatic alligatorweed mats provide breeding habitat for mosquitoes. The terrestrial form of the plant can also invade agricultural and pasture lands, and drainage and irrigation may be impacted as well (Coulson 1977, Julien and Bourne 1988, Julien and Broadbent 1980).Mechanical removal of A. philoxeroides mats is costly, and often results in the dispersal of large numbers of vegetative fragments that can exacerbate the infestation (GBEP/HARC 2006). Although biocontrol by means of the aligatorweed flea beetle (Agasicles hygrophila) and other control agents has greatly attenuated the threat of this plant, the cost associated with carefully studying, planning and managing the release of biocontrol agents is substantial.Alligatorweed is a federal noxious weed and a prohibited or noxious plant in Arizona, California, Florida, and South Carolina (USDA/NRCS).

Sources and Credits

  1. (c) eyeweed, some rights reserved (CC BY-NC-ND), http://www.flickr.com/photos/12626159@N05/3861901414
  2. (c) Doug Beckers, some rights reserved (CC BY-SA), https://www.flickr.com/photos/dougbeckers/6260512054/
  3. (c) Doug Beckers, some rights reserved (CC BY-SA), https://www.flickr.com/photos/dougbeckers/6259983791/
  4. (c) eyeweed, some rights reserved (CC BY-NC-ND), http://www.flickr.com/photos/12626159@N05/3861119183
  5. (c) anonymous, some rights reserved (CC BY), https://upload.wikimedia.org/wikipedia/commons/2/2b/CSIRO_ScienceImage_783_Alligator_weed.jpg
  6. (c) anonymous, some rights reserved (CC BY), https://upload.wikimedia.org/wikipedia/commons/a/ae/CSIRO_ScienceImage_784_Alligator_weed.jpg
  7. Adapted by Kate Wagner from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Alternanthera_philoxeroides
  8. Adapted by Kate Wagner from a work by (c) Smithsonian Marine Station at Fort Pierce, some rights reserved (CC BY-NC-SA), http://eol.org/data_objects/11525529
  9. (c) Smithsonian Marine Station at Fort Pierce, some rights reserved (CC BY-NC-SA), http://eol.org/data_objects/11525525

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