Alternanthera philoxeroides
(alligator weed)

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).

A. philoxeroides is native to South America, principally the Parana River region (Julien et al., 1995), from Guyana to Brazil and northern Argentina (USDA-ARS, 2016). It has been introduced into Europe, North and Central America, the Caribbean, tropical Asia, and Oceania (DAISIE, 2016; ISSG, 2016 OEPP/EPPO, 2016; USDA-ARS, 2016). 

The risk of introduction of A. philoxeroides is very high. Because this species is able to grow in both aquatic and terrestrial habitats, grows vigorously and spreads from floating fragments, it has a great potential to increase its present distribution into new areas. According to Julien et al. (1995) much of Africa, Asia and southern Europe provide a suitable habitat for this weed.

Liu et al. (2017) modelled the potential for further spread of invasive aquatic weeds following China’s South to North Water Diversion project, and predict that A. philoxeroides has high potential for northward range expansion in China.

A. philoxeroides grows as a weed in both aquatic and terrestrial habitats, and often grows at the interface between these two environments (OEPP/EPPO, 2016). In natural and semi-natural habitats it is prone to become invasive principally in forests, riverbanks and wetlands. It can be found growing along canals, rivers, swamps, lakes, dams, ditches, and wetlands, being rooted to the ground and emerging above the water surface. However, it can also be found in riparian habitats free-floating in dense mats on the water surface. A. philoxeroides is also an important weed of wetter pastures and irrigated crops (ISSG, 2016; USDA-NRCS, 2016; Weeds of Australia, 2016).   

A. philoxeroides primarily affects floating aquatic plants and pastures but submerged and emerged aquatic plants are also affected.

Genetics

It has been suggested that the populations of A. philoxeroides within and outside its native distribution range are composed of a complex of hybrids. Consequently, the chromosome number for A. philoxeroides differs among populations with reports varying from 2n=66 to 2n=100 (Xu et al., 1992; Sosa et al., 2008).

Reproductive Biology

A. philoxeroides reproduces both sexually and asexually within its native distribution range, but propagates primarily through vegetative means in its introduced range. In this species, traits associated with sexual reproduction become degraded for sexual dysfunction, with flowers possessing either pistillate stamens or male-sterile anthers. Degradations of sexual characters for loss of sexuality commonly take place in clonal plants; such is the case of A. philoxeroides populations spreading mainly by vegetative (clonal) propagules (Zhu et al., 2015).

Physiology and Phenology

A. philoxeroides is a perennial, fast-growing, amphibious herb (ISSG, 2016.) Maximum growth of A. philoxeroides occurs during the warmer summer months with growth initiating from parent stock, usually rooting in a solid substrate and spreading in a tangled mat over the water surface. In early winter, emergent stems lose many leaves and become prostrate forming part of the mat that supports the next season's growth. Flowering occurs from mid to late summer (Julien and Broadbent, 1980). The plant does not always set viable seed under natural conditions; but reproduces vegetatively from axillary buds at each node (Julien and Broadbent, 1980).

Activity Patterns

A. philoxeroides grows as an emerged, aquatic plant, rooted in the soil or in the substrate below shallow water. Roots are short and filamentous in water, rising mainly from the nodes. It also grows in terrestrial habitats where its high growth-rates allow it to displace native vegetation and easily become the dominant species. This plant has an amazing ability to grow vigorously forming a massive underground rhizomatous root system that is difficult to control. When growing in terrestrial conditions, this species can survive without any water for several months (Gunasekera and Adair, 2000).

Environmental Requirements

A. philoxeroides prefers to grow at temperatures around 30°C, and growth is suppressed at temperatures below 7°C. However, the species can tolerate mean annual temperatures ranging from 10 to 20°C (OEPP/EPPO, 2016). The photosynthetic optimum for this species occurs between 30°C and 37°C and light saturation at 1000 μmol photons m^-2 s^-1 (OEPP/EPPO, 2016), but it can adapt to low light conditions (Weber, 2003). It can tolerate cold winters, but cannot survive prolonged freezing temperatures (Langeland et al., 2008). It has been observed growing in water with pH ranging from 4.8 and 7.7 and it is fairly salt tolerant and can survive in upper tidal beaches and other saline conditions (10-30% that of sea water). A. philoxeroides grows well in high-nutrient (eutrophic) conditions, but can survive in areas with low nutrient availability  (Weber, 2003; Langeland et al., 2008).

During South American explorations for biological control agents, over 40 arthropod species were found to feed on A. philoxeroides (Coulson, 1977). The flea beetle Agasicles hygrophila can cause considerable damage to the aquatic form of A. philoxeroides by eating the leaves and boring into the stem, where it pupates. The thrip species Amymothrips andersoni produces limited damage to the stands by attacking and deforming the apical leaves. The stem borer Arcola malloi (formerly Vogtia malloi)) is a small moth that lays its eggs on the apical leaves. The larvae bore into the stem and work their way down the stem, resulting in wilting and drooping of the plant (DiTomaso and Kyser, 2013).

Animal feed, fodder, forage

• Fodder/animal feed

General

• Ornamental
• Pet/aquarium trade

Medicinal, pharmaceutical

• Traditional/folklore

A. philoxeroides is superficially quite similar to A. sessilis, but the latter is only an annual species and the clusters of flowers are sessile in the leaf axils, not on peduncles. Members of the genus Ludwigia spp. may be confused with A. philoxeroides due to a similar growth habit (Flanagan, 1991; Julien and Broadbent, 1980). A. philoxeroides has a similar appearance to Persicaria decipiens (smartweed) and Tradescantia albiflora (wandering jew) in Australia (Gunasekera, 1999). Weeds of Australia (2016) lists differences between A. philoxeroides and several native and introduced species of Alternanthera. The native Australian joyweeds (e.g. Alternanthera denticulata and Alternanthera nana) can be easily distinguished from terrestrial A. philoxeroides plants by the fact that their whitish flower clusters are stalkless.

The water primroses (Ludwigia adscendens and Ludwigia peploides subsp. montevidensis) often form similar dense mats of vegetation out over the water surface, but can be distinguished by their alternately arranged leaves, larger four-petalled flowers (about 25 mm across) and elongated fruit (Weeds of Australia, 2016).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

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).

Eradication

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).

Control

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).
 

11/11/16 Updated by:

Julissa Rojas-Sandoval, Department of Botany-Smithsonian NMNH, Washington DC, USA