Alligatorweed (Alternanthera philoxeroides) is native to southern South America (Brazil-Argentina). There is an unsubstantiated record in FL in 1894, but the first confirmed record, in 1897, was on ship's ballast in Mobile AL. By 1901, it was recognized as a potentially troublesome weed in wet soils and shallow waterways in the southeast U.S. (Coulson 1977). Alternanthera philoxeroides invaded reservoirs of the Tennessee Valley authority in 1935, possibly spread by shipping from the Ohio and Mississippi Rivers. By the 1930's, it was present from Florida north to North Carolina and west to Louisiana and Texas, By 1963, 162,400 acres of water were considered infested, and small populations were observed in Virginia, Arizona and California. A biological control program is considered to have successfully controlled A. philoxeroides in much of the southeast However, the insects used, primarily the flea beetle Agasicles hygrophila, have not been established in North Carolina (Buckingham 1996) and little or no control has occurred there (and probably also in Virginia). Alligatorweed is considered likely to expand northward and inland with greenhouse warming. Alternanthera philoxerorides has been introduced to Puerto Rico, Asia (India-China), Australia, and New Zealand.
In 2017, patches of Alligatorweed were discovered in the Sacramento-San Joaquin Delta, and by 2018, it was found extensively in the Delta between Sacramento and Pittsfield. This plant is considered to have negative effect on habitat, foodwebs, and navigation in the Delta. It is a competitor with the native floating plant, Water Pennywort (Hydrocotyle umbellata), and has replaced it in some areas. The release of biocontrol insects, such as Agasicles hygrophila, is being considered. However, winter temperatures in the Delta, may prevent establishment. Scientists are studying more southern populations of this beetle in Argentina, hoping to find greater cold-tolerance.