Alternanthera philoxeroides, commonly known as Alligator weed, is an immersed 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.
Alligator weed can grow in a variety of habitats, including dry land, but is usually found in water. It may form large interwoven mats over the water or along shorelines. Alligator weed stems are long, branched, and hollow. Leaves are simple, elliptic, and have smooth margins. Alligator weed flowers during the warm months of the year and has whitish, papery ball-shaped flowers that grow on stalks.
When alligator weed invades waterways it can reduce water flow and quality by preventing light penetration and oxygenation of the water. It can also reduce water bird and fish activity and cause the death of fish and native plants. Alligator weed mats create a favorable habitat for breeding mosquitoes. Alligator weed is also difficult to control.
Insects have been released for the biological control of alligator weed. The most successful and widely used is Agasicles hygrophila commonly called the alligator weed flea beetle; it has been released for biocontrol in Australia, China, Thailand, New Zealand and the United States. Amynothrips andersoni, the alligator weed thrips, and Vogtia malloi, the alligator weed stem borer, have also been released in the United States.