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Plantwise Technical Factsheet

alligator weed (Alternanthera philoxeroides)

Host plants / species affected
Ipomoea batatas (sweet potato)
Oryza sativa (rice)
Zea mays (maize)

Decumbent or ascending glabrate aquatic perennials, the simple or branched, often fistulose stems to 100 cm. long. Leaves glabrous or glabrate, lanceolate to narrowly obovate, apically rounded to acute, basally cuneate, rarely denticulate, 2-10 cm. long, 0.5-2 cm. broad; petioles 1-3 mm. long. Inflorescences of terminal and occasionally axillary white glomes, 10-18 mm. long, 10-18 mm. broad, the usually unbranched peduncles 1-5 cm. long. Flowers perfect, bracts and bracteoles subequal, ovate, acuminate, 1-2 mm. long; sepals 5, subequal, oblong, apically acute and occasionally denticulate, neither indurate nor ribbed, 5-6 mm. long, 1.5-2.5 mm. broad; stamens 5, united below into a tube, the pseudostaminodia lacerate and exceeding the anthers; ovary reniform, the style about twice as long as the globose capitate stigma. Fruit an indehiscent reniform utricle 1 mm. long, 1-1.5 mm. broad (Flora of Panama, 2016).

Prevention and control


In New Zealand, A. philoxeroides is included in the National Pest Plant Accord list, which bans the sale, propagation and distribution of the plant throughout New Zealand. In Australia, it is a prohibited species whose propagation and supply is prohibited, and legislation requires the species to be controlled and/or eradicated. In the USA, the species has varying classifications at federal or state levels (OEPP/EPPO, 2016).


In 1992, A. philoxeroides was recorded and eradicated from Brisbane and Queensland (Parsons and Cuthbertson, 1992). An infestation in Canberra's Lake Ginninderra was also found and eradicated (Julien et al., 1995).


Mechanical control

Mechanical control methods such as using a cookie cutter, flail chopper, hand removal, harvesting, hand cutter, or rotovation are good for clearing water ways, but unless all fragments of the stems are collected these management practices could exacerbate the problem. Since A. philoxeroides reproduces vegetatively, if any fragments move downstream they can develop into another colony ((DiTomaso and Kyser, 2013).

Mechanical harvesting and ploughing are not suitable control methods for A. philoxeroides because the weed is able to spread from cut stems and roots (Julien and Broadbent, 1980).

Biological control

A. philoxeroides has been the subject of successful biological control in the USA, Australia, New Zealand and Thailand. There is an extensive biocontrol programme in China. Partial control of the species has been achieved in New Zealand by biocontrol methods (Hayes et al., 2013).

Amynothrips andersoni, a biocontrol agent originally from Argentina, has been introduced into the USA; it is established in Florida, Georgia and South Carolina. It has been released in Alabama, California, Mississippi and Texas, but Julien (1992) could not confirm establishment in these states. Thayer and Pfingsten (2017) report that while biocontrol agents have been successful in managing A. philoxeroides in the USA, the effectiveness of A. andersoni is questionable as the insect is flightless and rarely seen on wild populations.

Agasicles hygrophila, another biocontrol agent originally from Argentina, has been introduced into other countries. In Australia, it is established and successfully controls A. philoxeroides in aquatic habitats. In New Zealand, it destroys most foliage of the weed annually. In Thailand, it has spread around Bangkok and the lower central plain area producing excellent control of A. philoxeroides. In the USA, this biocontrol agent is generally successful in controlling the weed in Florida, Louisiana and Texas; it is also well established in South Carolina, Georgia, Alabama and Mississippi (Julien, 1992). Thayer and Pfingsten (2017) say that this beetle along with other introduced insects has provided “exceptional control” of A. philoxeroides in the USA, but that the northern spread of the weed is beyond the range of A. hydrophila’s ability to overwinter. The beetle is, however, collected annually in St. Johns River in Florida to ship to areas of the country where the biocontrol agent does not overwinter and A. philoxeroides persists.

Tests of specificity of A. hygrophila in China have confirmed that the beetle cannot complete its life cycle on plants other than A. philoxeroides and Alternanthera sessilis (Lu et al., 2012; Zhao et al., 2013). Li and Ye (2006) suggest that Agiscles hygrophila has been successful in limiting growth of A. philoxeroides in water but not on land. Ma et al. (2013) report that since the first introduction of A. hygrophila from Florida to China in 1987, the genetic diversity of the control agent has decreased. It is suggested that genetic diversity should be considered in planning introduction and long-term maintenance of populations.

Arcola malloi (formerly Vogtia malloi) is also from Argentina and was introduced into Australia in 1977 where it has become established and spread through the aquatic habitat. It was released unsuccessfully in New Zealand in 1984, and again in 1987; it is now well established and reducing the spread of A. philoxeroides at three sites (Julien, 1992). In the USA, it is established in Arkansas, Florida, Louisiana, South Carolina and Texas. In Mississippi, it reduces floating mats by 50-90%, infestations are, however, uneven and may cycle over several years. Thayer and Pfintsten (2017) quote a 1992 publication by Vogt et al. suggesting that the stem borer A. malloi has produced more damage to A. philoxeroides in the interior regions of the weed’s adventive range than has Agasicles hygrophila in the southern and coastal regions. This insect is capable of migrating considerable distances and is the most cold tolerant of the insects used for biocontrol of A. philoxeroides.

Hymenia recurvalis removed between 25-50% of the leaf material of A. philoxeroides in the mid to late summer of 1976/1977, in the Sydney area of Australia. This was of little consequence as it was after most regrowth had occurred (Julien and Broadbent, 1980).

Candezea palmerstoni killed most of the stem tips of A. philoxeroides in several areas near Williamstown, New South Wales, Australia, in the summer of 1977/1978; the damage was not widespread and did not occur in succeeding seasons (Julien and Broadbent, 1980).

Three species (Hymenia recurvalis, Nanophyes sp. and Junonia lemonias), found feeding on A. philoxeroides in Thailand, were not sufficiently damaging to be considered useful as biological control agents (Napompeth, 1991).

Research in China has investigated the pathogenicity of fungal agents against A. philoxeroides, including Alternaria alternata (Zhou et al., 2016). Use of competing plants has also been studied. Cao et al. (2014) found that Humulus scandens strongly inhibited growth of A. philoxeroides, and suggest that as an annual herb H. scandens can then be eliminated by harvesting before its seeds mature.

Chemical control

A. philoxeroides is more resistant to herbicides than other aquatic macrophytes (Julien and Broadbent, 1980). Parsons and Cuthbertson (1992) reported control, but not eradication, of the weed in rice fields with herbicides including bentazone, bifenox, dicamba, fenoprop, pendimethalin, propanil and triclopyr, without causing serious damage to the crop; 2,4-D has only a temporary effect. Bowmer (1992) reported the following two treatments as effective against the weed: one application of dichlobenil followed 9 months later by metsulfuron; and three sprays over 18 months with metsulfuron or a metsulfuron/glyphosate mixture. However, certain treatments cannot be used close to waterbodies where there is the possibility of water being contaminated.

At Griffith, New South Wales, Australia, glyphosate was used on all aquatic areas, metsulfuron on terrestrial areas and dichlobenil in selected areas where terrestrial plants were growing in shallow ponded water (Milvain, 1995). The herbicides, metsulfuron methyl, glyphosate, dichlobenil and a mixture of glyphosate and metsulfuron methyl have been used to control  A. philoxeroides infestations in Australia. All naturalized sites associated with water were treated with glyphosate at three 2 monthly intervals (Gunasekera and Bonila, 2001). Dugdale et al. (2010) caution that herbicide treatment can leave viable stem fragments which are capable of colonization. Clements et al. (2014) report control of early invasion stages of A. philoxeroides in Australia using glyphosate or metsulfuron-methyl, followed by physical removal after initial treatment. Use of herbicides to control A. philoxeroides was reviewed by Dugdale and Champion (2012).

Summary of invasiveness

A. philoxeroides is one of the worst weeds in the world because it invades both terrestrial and aquatic habitats. The aquatic form of the plant has the potential to become a serious threat to rivers, waterways, wetlands and irrigation systems. The terrestrial form grows forming dense mats with a massive underground rhizomatous root system (ISSG, 2016). This weed is extremely difficult to control, is able to reproduce from plant fragments and grows in a wide range of climates and habitats, including terrestrial areas. In aquatic habitats it has deleterious effects on other plants and animals, water quality, aesthetics, vector populations, water flow, flooding and sedimentation. In terrestrial situations, it degrades riverbanks, pastures, and agricultural lands producing massive underground lignified root systems penetrating up to 50-60 cm deep. Currently, A. philoxeroides is listed as invasive in the United States, Puerto Rico, France, Italy, India, Sri Lanka, China, Taiwan, Indonesia, Myanmar, Singapore, Australia and New Zealand (Weber et al., 2008; Chandra, 2012; Rojas-Sandoval and Acevedo-Rodriguez, 2015; DAISIE, 2016; USDA-ARS, 2016; USDA-NRCS, 2016; Weeds of Australia, 2016).  Once established, it behaves as an aggressive invader with the capability to totally disrupt natural aquatic ecosystems, shoreline vegetation and terrestrial and semi-aquatic environments (ISSG, 2016; USDA-NRCS, 2016).

Related treatment support
External factsheets
AgPest Fact Sheets, AgResearch, English language
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