Potentially Misidentified Species:
A number of non-native Alternanthera species occur in the United States,
including sessile joyweed Alternanthera sessilis which is similar in appearance
to A. philoxeroides and co-occurs with it in the IRL region of Florida. Mis-identification of these two species has had negative ecological
ramifications elsewhere. A. philoxeroides was accidentally spread in Australia by members of
the Sri Lankan immigrant community because it was mistaken for its congener A.
sessilis which is the Indian herb mukunawanna (Hosking, et al 1996).
Several species of aquatic primrose (genus Ludwigia) and spiderwort
(genus Tradescantia) that occur in Florida also have appearances and/or
growth habits somewhat similar to those of A. philoxeroides.
II. HABITAT AND DISTRIBUTION
The native range of Alternanthera philoxeroides is reportedly the Parana
River region of South America (Maddox 1968, Vogt et al. 1979). It can now be
found elsewhere in South America, and also on the continents of North
America, Australia, and Asia, and on a number of adjacent islands.
In the United States, A. philoxeroides occurs throughout the southeast
from Virginia south to Florida and west to Texas. It has been reported as far
north as Illinois, and now also occurs in California (Virginia Cooperative
Extension undated, USDA).
Distribution records indicate Alternanthera philoxeroides occurs in the
northern IRL counties of Volusia and Brevard, and also at the far southern end
of the IRL region in Palm Beach County (Wunderlin and Hansen 2004).
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Alligator weed is a summer perennial that typically grows to around 1-2 m and whose horizontal stolons can reach 10 m
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). Biocontrol of A. philoxeroides is further discussed below.
Reproduction in Alternanthera philoxeroides is predominantly through
vegetative means; individuals rarely produce seeds, and when they do the seeds
are typically non-viable. Vegetative growth occurs at the apical stem buds and
axillary stem and root buds and the plant is spread through fragmentation
(Julien et al. 1992, Virginia Cooperative Extension undated).
IV. PHYSICAL TOLERANCES
Alligatorweed thrives in subtropical to cool climates; optimum growth
temperature of alligatorweed has been experimentally determined to occur
between approximately 15 and 20°C. The plant is susceptible to winter die-back
of exposed portions at higher latitudes (Julien et al. 1992, 1995).
Ensbey (2004) reports that Alternanthera philoxeroides can withstand
salinities of approximately 10% full strength seawater in static aquatic
environments and up to 30% full strength seawater in flowing brackish water.
The ability of this plant to grow in aquatic as well as terrestrial habitats allows it to persist in regions with pronounced wet and dry seasons, including Florida.
V. COMMUNITY ECOLOGY
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
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).
VI. INVASION INFORMATION
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
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
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
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).
Buckingham G.R. 2002. Alligatorweed. In: Van Driesche R., Lyon S., Blossey B.,
Hoddle M., and R. Reardon (eds.). Biological Control of Invasive Plants in the
Eastern United States, USDA Forest Service Publication FHTET-2002-04. 413 p.
Carley M., and S. Brown. 2006. Invasive plants; Established and potential
exotics, Gulf of Mexico Region. Gulf Coast Research laboratory, University of
Southern Mississippi. 8 p. Available online.
Cofrancesco A.F., Jr. 1988. Alligatorweed survey of ten southern states.
Miscellaneous Paper A-88-3. U.S. Army Engineer Waterways Experiment Station,
Vicksburg, Mississippi, USA.
Coulson J.R. 1977. Biological control of alligatorweed, 1959-1972. A review and
evaluation. USDA Technical Bulletin 1547. 98 p.
Ensbey R. 2004. Alligator Weed. Agfact P7.6.46, second edition. NSW
Environment Waikato undated. Alligator Weed fact sheet. Available online.
Galveston Bay Estuary Program and the Houston Advanced Research Center
(GBEP/HARC). 2006. The Quiet Invasion: A Guide to Invasive Plants of the
Galveston Bay Area. Available online.
Holm L., Doll J., Holm E., Pancho J., and J. Herberger. 1997. World Weeds:
Natural Histories and Distribution. John Wiley and Sons, New York. 1160 p.
Hosking J.R. Sainty G.R., and S.W.L. Jacobs. 1996. Certainty and uncertainty in
plant identification. Pp. 464-467 in: Shepherd R.C.H. (ed.). Proceedings of the
Eleventh Australian Weeds Conference, 30 September-3 October, 1996, Melbourne,
Julien M.H., and A.S. Bourne. 1988. Alligator weed is spreading in Australia.
Plant Protection Quarterly 3:91-96.
Julien M.H., and J.E. Broadbent. 1980. The biology of Australian weeds 3.
Alternanthera philoxeroides (Mart.) Griseb. The Journal for the
Australian Institute of Agricultural Science 46:150-155.
Julien M.H., Bourne, A.S. and V.H.K. Low. 1992. Growth of the weed
Alternanthera philoxeroides (Martius) Grisebach, (alligator weed) in aquatic
and terrestrial habitats in Australia. Aquatic Botany. 7: 102-108.
Julien M.H., Skarratt B., and G.F. Maywald. 1995. Potential geographical
distribution of alligator weed and its biological control by Agasicles
hygrophila Journal of Aquatic Plant Management 33:55-60.
Maddox D.M. 1968 Bionomics of an alligatorweed flea beetle, Agasicles
sp., in Argentina. nnals of the Entomological Society of America 61:1299-1305.
Quimby P.C., Jr., and S.H. Kay. 1976. Alligatorweed and water quality in two
oxbow lakes of the Yazoo River basin. Journal of the Mississippi Academy of
Science 21 (supplement):13.
UF/IFAS Center for Aquatic and Invasive Plants. Undated. Biological control
insects for aquatic and wetland weeds. University of Florida Aquatic and
Wetland Plant Information Retrieval System. Available online.
Virginia Cooperative Extension. Undated. Virginia Tech Weed Identification
Guide. Available online.
Vogt G.B., McGurie J.U., Jr., and A.D. Cushman. 1979. Probable evolution and
morphological variation in South American Disonychine flea beetles (Coleoptera:
Chrysomelidae) and their Amaranthaceous hosts. USDA Technical Bulletin 1593,
Wunderlin R.P. and B.F. Hansen. 2004. Atlas of Florida Vascular Plants.
Institute for Systematic Botany, University of South Florida, Tampa. Available
J. Masterson, Smithsonian Marine Station
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Page last updated: December 1, 2007