Alternanthera philoxeroides
(alligator weed)

Pictures

Picture	Title	Caption	Copyright
Title Leaves and inflorescence Caption A. philoxeroides is characterized by dark-green waxy leaves, lance-shaped, opposite, 12-14 x 1.5-2.5 cm. Inflorescence white, ball-shaped, papery, 1.5 cm in diameter. Copyright Bill Parsons	Leaves and inflorescence	A. philoxeroides is characterized by dark-green waxy leaves, lance-shaped, opposite, 12-14 x 1.5-2.5 cm. Inflorescence white, ball-shaped, papery, 1.5 cm in diameter.	Bill Parsons
Title Growth habit Caption A. philoxeroides is a perennial herb which grows as an emerged, aquatic plant, rooted in the soil or in the substrate below shallow water. Copyright Bill Parsons	Growth habit	A. philoxeroides is a perennial herb which grows as an emerged, aquatic plant, rooted in the soil or in the substrate below shallow water.	Bill Parsons
Title Growth and spread in water Caption The plant usually roots in a solid substrate and spreads in a tangled mat over the water surface. Copyright Bill Parsons	Growth and spread in water	The plant usually roots in a solid substrate and spreads in a tangled mat over the water surface.	Bill Parsons
Title Alligator weed infesting river Caption Excessive growth of A. philoxeroides covers waterways affecting navigation, preventing access, disrupting flow and adversely affecting the aquatic flora and fauna, as in this river at Baton Rouge, Louisiana, USA. Copyright Bill Parsons	Alligator weed infesting river	Excessive growth of A. philoxeroides covers waterways affecting navigation, preventing access, disrupting flow and adversely affecting the aquatic flora and fauna, as in this river at Baton Rouge, Louisiana, USA.	Bill Parsons

Identity

Preferred Scientific Name

• Alternanthera philoxeroides (Mart.) Griseb.

Preferred Common Name

• alligator weed

Other Scientific Names

• Achyrantes philoxeroides (Mart.)
• Achyranthes paludosa Bunbury
• Alternanthera philoxerina Suess.
• Bucholzia philoxeroides Mart.
• Celosia amphibia Salzm. ex Moq.
• Mogiphanes philorexoides D. Parodi
• Telanthera philoxeroides (Mart.)
• Telanthera philoxeroides (Mart.) Moq.

International Common Names

• English: alligator grass; pig weed
• Spanish: hierba caiman; hierba lagarto; lagunilla; yerba lagarto
• Chinese: xi han lian zi cao
• Portuguese: erva jacare; periquito-saracura; piriquito

Local Common Names

• Argentina: lagunilla
• Australia: mukuna-menna; pannankarni
• Brazil: erva de jacare
• Ecuador: hierba lagarto
• India: phackchet
• Mexico: hierba caimán; hierba del Caiman
• Sri Lanka: kimbul-wenna
• Uruguay: raiz colorado
• USA: alligatorweed

EPPO code

• ALRPH (Alternanthera philoxeroides)

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).

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Caryophyllales
•                         Family: Amaranthaceae
•                             Genus: Alternanthera
•                                 Species: Alternanthera philoxeroides

Notes on Taxonomy and Nomenclature

The family Amaranthaceae includes 174 genera and about 2500 species distributed worldwide especially in warm and dry temperate areas, the subtropics and saline habitats (Stevens, 2012). Alternanthera is a diverse genus (80–200 species) and the second largest in the subfamily Gomphrenoideae of Amaranthaceae.  The genus is largely restricted to the American tropics and its highest diversity occurs in South America, but many species also occur in the Caribbean, Central America and Mexico (Sanchez del Pino et al., 2012).

There are two biotypes of A. philoxeroides in Florida which differ morphologically: broad- and narrow-stemmed forms. Another two biotypes exist in Argentina which are morphologically similar but differ in chromosome number, the wild form being tetraploid (2n=68) and the weedy form being hexaploid (2n=102) (Parsons and Cuthbertson, 1992).

Description

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).

Plant Type

Distribution

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). 

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

In the USA, A. philoxeroides was probably introduced in contaminated ship ballast water, with the earliest herbarium specimen dating from South Carolina in 1885. In 1894 it arrived in Florida and in 1897 it was collected near Mobile, Alabama. By the early 1900s, it was recognized as a threat, but became a major threat after 1945 when chemical control of water hyacinth became effective and allowed A. philoxeroides to flourish (Coulson, 1977; Langeland et al., 2008). Now it is considered one of the worst aquatic weeds invading southern states (USDA-NRCS, 2016).

A. philoxeroides is present in Asia where it is widespread principally across warm temperate regions. In Sri Lanka A. philoxeroides was identified in 1998 and by 2004 it reached provinces at elevations > 2500 m (Jayasinghe, 2008). In China it is spreading across Beijing, Fujian, Guangxi, Hebei, Hubei, Hunan, Jiangsu, Jiangxi, Sichuan, Taiwan, and Zhejiang where it is causing serious impacts on aquatic habitats and famous scenic areas (Flora of China Editorial Committee, 2016). In India, A. philoxeroides is spreading across Assam, Bihar, West Bengal, Tripura, Manipur, Andhra Pradesh, Karnataka, Maharashtra, Delhi and Punjab. By 2008, A. philoxeroides was reported invading Wular Lake, the largest freshwater lake in India (Masoodin and Khan, 2012).

In Australia A. philoxeroides arrived during the 1940s, probably in ships' ballast (Julien and Broadbent, 1980). From initial sites at Botany Bay and Newcastle harbour, it had, by 1979, spread to the Lower Hunter River Valley and to near Albury (Julien et al., 1979). It was also found in backyard gardens of some Asian community groups in Australia who grew it as a leafy vegetable by mistake. It has been used since the 1960's and is now found in all Australian states and territories. The State of Victoria has the highest number of backyard plots (more than 800) of alligator weed in Australia (Gunasekera and Adair, 1999).

In New Zealand, it was first recorded in 1906 and now it can be found invading drains, streams, swamps and similar wet habitats across the country (Bassett et al., 2012; OEPP/EPPO, 2016).

In Europe, A. philoxeroides was first recorded in 1971 in France. Initially this species was confined to the southwest of France between the middle of the Gironde Estuary and the middle course of the River Garonne. However in the 2000s new populations were found on the Tarn River and in Sorgues (Provence) in 2013. In Italy A. philoxeroides was discovered in 2001 near Pisa, Tuscany. Currently, this species can also be found along the Arno River from Signa to Florence, in Lazio, and in Rome along the Tevere River (OEPP/EPPO, 2016). 

Risk of Introduction

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.

Habitat

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).   

Habitat List

Hosts/Species Affected

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

Host Plants and Other Plants Affected

Growth Stages

Biology and Ecology

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).

Climate

Air Temperature

Rainfall

Rainfall Regime

Soil Tolerances

Soil drainage

• free
• impeded
• seasonally waterlogged

Soil reaction

• acid
• alkaline
• neutral

Soil texture

• heavy
• light
• medium

Special soil tolerances

• infertile
• shallow
• sodic

Water Tolerances

Natural enemies

Notes on Natural Enemies

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).

Means of Movement and Dispersal

Natural Dispersal (Non-Biotic)

A. philoxeroides spreads by seeds and vegetatively by root and stem fragments. However, because seeds are generally not produced in areas outside its native distribution range, most reproduction is vegetative. Fragments are commonly spread downstream by waterways and floods and new plants develop rapidly from any piece of stem or root material containing a node (Langeland et al., 2008; ISSG, 2016; Weeds of Australia, 2016).

Accidental Introduction

A. philoxeroides has been accidentally introduced in the ballast of ships. Stem and root fragments, which have the ability to float, are easily dispersed by floods and water currents. Stem and root fragments can also be dispersed by boats, vehicles, in dumped garden waste, and by animals (ISSG, 2016; Weeds of Australia, 2016). A. philoxeroides seed has been found in Europe as a contaminant in bird seed originating from outside the EU, and seedlings have been found contaminating bonsai plants imported from China (OEPP/EPPO, 2016).

Intentional Introduction

A. philoxeroides has been intentionally introduced by humans to be used as an aquarium plant and ornamental aquatic plant (USDA-ARS, 2016). 

Pathway Causes

Pathway Vectors

Wood Packaging

Impact Summary

Economic Impact

A. philoxeroides is a problem in 30 countries. It is a serious weed in eight of these and a major weed in the others. It threatens the turf industry in the Sydney basin, Australia, and the vegetable industry in the Hawkesbury Nepean catchment. The plant can be a problem in rice paddies (Waterhouse, 1993) and is seen as a major threat to rice crops in southwestern New South Wales (Weeds of Australia, 2016). It has been estimated that the costs to agriculture in New South Wales could be as high as Aus$250 million per annum if the species was to reach its potential distribution in this state (Weeds of Australia, 2016).

On land, it invades and competes with pastures and this provides a source of further spread. Although it is grazed by cattle in Australia, it is not considered desirable in pastures (Julien and Chan, 1992) and is a declared noxious weed in all mainland states and territories (Parsons and Cuthbertson, 1992) as well as a weed of national significance in Australia (Thorpe, 1999).

A. philoxeroides infestations have been reported to reduce production of rice by 45%, wheat by 36%, maize 19%, sweet potato 63% and lettuce 47% (OEPP/EPPO, 2016). On average vegetable production is reduced by 5-15% (www.weeds.org.au/natsig.htm).

A. philoxeroides mats impede stream flow and lodge against structures thereby promoting sedimentation which contributes to flooding and structural damage. Infestations can disrupt recreational activities including boating, fishing and swimming.

Environmental Impact

A. philoxeroides is considered to be one of the worst aquatic weeds in the world. The aquatic form of the plant has the potential to become a serious threat to waterways, agricultural lands and the natural environment. The terrestrial form grows into a dense mat with a massive underground rhizomatous root system. This species has the potential to completely disrupt aquatic environments by blanketing the surface of the water impeding light penetration and gaseous exchange with adverse effects on flora and fauna (ISSG, 2016). A. philoxeroides also promotes sedimentation and flooding leading to a reduction in water quality (i.e. reduced oxygen levels in the water). When growing on land it also grows forming a dense mat of vegetation with a mass of creeping underground stems and is capable of out-competing all but the most robust plant species. It quickly displaces native plants and can be harmful to the native animals that rely on them (Weeds of Australia, 2016).

Impact on Biodiversity

In China, A. philoxeroides has been shown to decrease the stability of the plant community and, over time, permanently displace native species (Guo and Wang, 2009). In India, A. philoxeroides is reducing macrophyte species richness by up to 30% when the infestation was high. In New Zealand, an increasing cover of A. philoxeroides decreased the cover of native plant species, resulting in loss of native species (Bassett et al., 2012). In a study at different latitudes in China, small-scale invasion of A. philoxeroides was associated with higher species diversity, but community diversity was lower when A. philoxeroides species cover exceeded 36% (Wu et al., 2016). Zhang et al. (2010) also demonstrate reduced plant species diversity in severely invaded communities.

In Australia, A. philoxeroides is already an important environmental weed invading New South Wales, Victoria and Queensland. It is regarded as one of the worst weeds in Australia where it is currently having the greatest impact in New South Wales, where the total infested area is now estimated at 3,950 hectares. A. philoxeroides has also been found at several hundred sites in Victoria and is listed among the top 50 most invasive plant species in Queensland (Weeds of Australia, 2016). In New Zealand it is also noted to be harmful to native biodiversity and it is replacing most other herbaceous species on water and dry land. It also causes silt accumulation, obstructs water usage, and causes flooding. Rotting vegetation degrades habitats for native aquatic fauna and flora.

In the United States, A. philoxeroides is considered to be one of the worst aquatic weeds and is listed as a noxious weed in 15 states (USDA-NRCS, 2016). In southern states such as Florida and South Carolina it grows forming dense tangled mats that overtop native aquatic plants and out-compete them for sunlight. It eventually replaces desirable native species and can significantly alter the aquatic and riverine ecology of heavily infested areas. This species also invades drains, streams, swamps and similar wet habitats (USDA-NRCS, 2016). 

Social Impact

Thick mats of A. philoxeroides prevent access to and use of water, cause health problems by providing habitats for mosquitoes and degrade natural aesthetics. Also, the thick mats of the weed create a dangerous hazard for swimming, boating, rowing and other water sports. Excessive growth of A. philoxeroides affects irrigation and fisheries; it also covers waterways affecting navigation, preventing access, disrupting flow and adversely affecting the aquatic flora and fauna (Julien and Chan, 1992). Cultural services can be degraded by the infestation of scenic areas and waterbodies by A. philoxeroides (OEPP/EPPO, 2016).

The ability of A. philoxeroides to absorb heavy metals is a problem in countries such as Myanmar, Sri Lanka, Australia, and Philippines where it is used as food (Parsons and Cuthbertson, 1992).

Risk and Impact Factors

• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Highly adaptable to different environments
• Is a habitat generalist
• Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
• Pioneering in disturbed areas
• Highly mobile locally
• Long lived
• Fast growing
• Has high reproductive potential
• Has propagules that can remain viable for more than one year
• Reproduces asexually

• Altered trophic level
• Damaged ecosystem services
• Ecosystem change/ habitat alteration
• Infrastructure damage
• Modification of hydrology
• Modification of natural benthic communities
• Modification of nutrient regime
• Monoculture formation
• Negatively impacts agriculture
• Negatively impacts human health
• Negatively impacts animal health
• Negatively impacts livelihoods
• Negatively impacts aquaculture/fisheries
• Negatively impacts tourism
• Reduced amenity values
• Reduced native biodiversity
• Soil accretion
• Threat to/ loss of native species
• Transportation disruption

• Competition - monopolizing resources
• Competition - smothering
• Rapid growth
• Rooting

• Highly likely to be transported internationally accidentally
• Highly likely to be transported internationally deliberately
• Difficult to identify/detect as a commodity contaminant
• Difficult to identify/detect in the field
• Difficult/costly to control

Uses List

Animal feed, fodder, forage

• Fodder/animal feed

General

• Ornamental
• Pet/aquarium trade

Medicinal, pharmaceutical

• Traditional/folklore

Similarities to Other Species/Conditions

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).

Prevention and Control

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).
 

References

Abud JK, 1985. Effect of herbicide mixtures in the control of Echinochloa crus-galli (L.) Beauv. and Alternanthera philoxeroides (Mart.) Griseb. in irrigated rice. Lavoura Arrozeira, 38(358):12-15

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Contributors

11/11/16 Updated by:

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

Distribution Maps