Mimosa pigra (giant sensitive plant)

M. pigra is a small prickly shrub that infests wetlands and is also an agricultural weed in rice fields in many parts of the old world tropics. In natural wetlands the shrub alters open grasslands into dense thorny thickets and negatively impacts on native biodiversity. It is regarded as one of the worst alien invasive weeds of wetlands of tropical Africa, Asia and Australia, and the cost of control is often high.

The nomenclature of M. pigra is rather confusing as this name has been widely applied to collections of Mimosa pellita. In 1991, Barneby corrected this mistake by clearly differentiating the two species. Accordingly, the name M. pigra corresponds with an endemic species restricted to the lower Paraná basin of Paraguay and Argentina. M. pellita on the other hand corresponded with the widespread weedy species which has been misidentified as M. pigra in numerous botanical and technical publications. Given the pervasive misapplication of M. pigra, this name was conserved with the type of M. pellita and therefore, the name to be used for the widespread species is M. pigra. In addition to the typical variety, M. pigra contains the narrow endemic M. pigra var. dehiscens restricted to parts of Venezuela.

M. pigra is a spreading, multi-stemmed, thorny shrub usually up to 2 m tall, but occasionally up to 6 m, with a maximum lifespan of about 5 years.

The plant is evergreen and bears bipinnate, sensitive leaves, up to 18 cm in length. Recurved spines (to 7 mm long) are located on the undersides of the petioles, petioules and stems.

The inflorescences, containing up to 100 flowers, are spherical (about 1 cm across) and pink. The species is androdioecious with both male and hermaphrodite flowers bearing eight short and long stamens. These flowers exhibit an intra-specific pollen polymorphism (El Ghazali et al., 1997). The flat pods of M. pigra are hairy and up to 15 cm long and clustered (up to seven pods) at the stem tips. They contain between 8 and 24 seeds. Each seed is about 5 x 2.4 mm and weighs 0.09 mg. The fruits ripen in about 3 months and, when mature, fragment into indehiscent one-seeded segments. The pods are covered with bristles which facilitate floating and enhance dispersal along river systems.

M. pigra has until recently been under-reported both in the native and invaded ranges. Furthermore, taxonomic uncertainties throw doubt as to the actual native range of the species in the neotropics. Rejmánek (2002) has stated that M. pigra is not native to Central America, while USDA-ARS (2013) gives it a broad native range in Africa and the Americas.

The species is now widely distributed in Africa and Asia but it is unclear how the weed was transported from tropical America. Although the species is thought to be introduced to Africa, Sheded and Hassan (1999) described it as 'endangered shrub' in Egypt, presumably considering it as a native species. Seburanga et al. (2013) report that M. pigra probably spread from Egypt through Uganda to Rwanda, where it first infested the country through the Akagera-Nyabarongon river system before the mid nineteenth century.

It was introduced, as an ornamental or seed contaminant, to the Darwin Botanic Gardens of Australia's Northern Territory before 1891. It remained an occasional nuisance around Darwin until the late 1950s. When it reached the open, treeless floodplains in the 1970s, M. pigra spread considerably to form monotypic stands. Vitelli et al. (2006) reported its presence near Proserpine in central coastal Queensland, suggesting that this was the only known infestation to have established in Australia outside the Northern Territory.

It was introduced to Thailand in 1947 as green manure and as a cover crop. It was thought that the prickliness of the weed would restrict access to the banks of water bodies and reduce erosion. It has now spread extensively and covers large areas of standing waters and the banks of water bodies. Based on population genetics, Pramaul et al. (2011) suggest that there were multiple introductions in Thailand.

M. pigra is also spreading in Indonesia, Peninsular Malaysia and Papua New Guinea. In Malaysia it was first noted by the Peninsular state of Kelantan by farmers, who claimed that it had been introduced from Thailand to cure snake bites. The Department of Agriculture only recorded it in 1980 (Anwar, 2001), but Mansor and Crawley (2011) report its presence at 55 out of 106 sites of six main habitat types. Distribution in Vietnam is described by Nguyen Thi Lan Thi et al. (2011).

In Sri Lanka the weed was first noted in 1997 and now forms dense thickets along a 30- to 35-km strip of the Mahaweli River in the Central province (Kandy District) (Marambe et al., 2001).

M. pigra is probably now more common in Costa Rica than it was before European colonization.

There is a high risk of infestation for many wetland habitats in tropical countries where the shrub is absent.

M. pigra has been declared a noxious weed in Florida and Hawaii, USA, northern Australia, Thailand and South Africa. The plant must either be eradicated or its spread controlled in these areas. In Western Australia and Queensland, legislation exists to prohibit the introduction of the plant. In Malaysia, the shrub was gazetted in as an A2 pest in the 4th Schedule of the Agriculture Pest and Noxious Plants (Import/Export) Regulation (Anwar, 2001).

In other parts of the tropics M. pigra still appears to be planted outside its native range despite its invasive tendencies but some caution appears to be shown by seed suppliers. For instance, Richardson (1998) reported that "ICRAF does not routinely supply M. pigra unless it appears that strict procedures will be implemented" although he does not indicate what these 'strict procedures' entail and how they can be successfully implemented.

Natural Dispersal (Non-Biotic)

The bristles covering the pods facilitate floating and enhance dispersal along river systems.

Agricultural Practices

Cattle transportation traffic is an effective means of long-distance dispersal.

Accidental Introduction

The seeds of M. pigra are spread by road construction equipment and the plant is thus typical of roadsides. In Vietnam, Nguyen Thi Lan Thi et al. (2011) report that transportation of sand for construction purposes is an important means of seed dispersal in the region.

Intentional Introduction

The species is still viewed to be beneficial in parts of the tropics and germplasm distributed when circumstances are believed not to be conducive to its weediness.

The principal crop attacked by M. pigra is rice (Waterhouse, 1993).

M. pigra grows with a number of other Mimosa species and is difficult for untrained personnel to identify (Kuniata, 1994). In Australia, it has been misidentified as other Mimosa species such as M. pudica (Lonsdale et al., 1989). These species can be distinguished by the number of pairs of pinnae per leaf; M. pigra has 6-14 pairs and M. pudica has 1-2 pairs (Lonsdale et al., 1989). M. pudica also differs in being very much less robust, rarely over 0.5-1 m high. M. invisa is also a densely spiny shrub, much larger than M. pudica, differing from M. pigra in having narrow pods up to 5 mm wide, compared with at least 1 cm in M. pigra.

M. pigra may also be confused with Leucaena leucocephala, Aeschynomene spp., Sesbania spp. and juveniles of Acacia pachyphloia, but is readily distinguished from these species by its sensitive leaves (Lonsdale et al., 1989). Confusion with the sensitive species, Neptunia dimorphantha, is also possible, but this species lacks stem prickles and a leaf rachis.
 

In Costa Rica, M. pigra is found on the banks of large rivers, lake shores, marsh edges and roadsides. In Australia, it is spreading into sedgeland and grassland communities on open floodplains and Melaleuca forest fringing these floodplains.

M. pigra can spread into pasture land, fallow rice paddies, immature oil palm plantations and fruit orchards. In Malaysia, Mansor and Crawley (2011) report the impact on natural habitats is relatively low, with distribution principally in disturbed areas. Construction sites were the habitat most likely to have infestations of M. pigra.

Genetics

Seijo (1999) reported the chromosome number of M. pigra var. dehiscens as 2n = 26 and that of M. pigra var. pigra as 2n = 52.

Physiology and Phenology

Flowering may start within a year of germination. Anthesis takes place about 8 days after bud formation. The spherical inflorescences, containing up to 100 flowers, last one day. One inflorescence is produced daily on main branches for 5 months during the rainy season. In evergreen forests, a few flowers and fruits are found throughout the year. Flowering occurs all year round in open and permanently moist sites. The fruits ripen in about 3 months and when mature, they fragment into indehiscent 8 to 24 one-seeded segments.

In Australia, Vitelli et al. (2006) reported that flowering and podding in Queensland occurs all year round, whilst flowering in the Northern Territory occurs from February to May and podding from March to July, though flowering can occur whenever sufficient water is available.

Reproductive Biology

In Australasia, on average, <5% of flower buds produce seeds; most of the seeds are produced by autogamy, although wind pollination may also occur.

In northern Australia, the soil seed banks can reach up to 12,000 seeds per square metre and the seeds remain viable for more than 2 years (Lonsdale et al., 1988). The seeds here generally germinate when they are first wetted and the rate of germination is high. More recent studies on a floodplain in northern Australia ten years after stand removal found that some viable seeds were still present, indicating that M. pigra seeds can remain viable under grass cover for over a decade unless work is performed to break seed dormancy (Lukitsch and Elliott, 2012). In Queensland, seed bank declined by 90% over a three year period at one study site (Vitelli et al., 2006).

Some workers have suggested that scarification is needed for high germination and Dillon and Forcella (1985) showed that the scarification effect was produced by alternating temperatures, an amplitude of 20°C having a much greater effect than 10°C. In Sri Lanka, 100% of seeds remained viable after storage at room temperature (28°C) and at 8°C, and 99% of the seeds germinated after sand scarification (Marambe et al., 2001).  

Although M. pigra is adapted to seasonally flooded habitats, where fibrous adventitious roots are formed around the base of the multiple stems, it can also regenerate under some degree of canopy cover. The plants resprout freely after natural fires but M. pigra does not naturally reproduce vegetatively.

Once established as monotypic stands, M. pigra can regenerate under its own canopy. In these stands, the half life of plants taller than 20 cm varies between 13 and 22 months, depending on soil type.

For further information, see Janzen (1983) and Lonsdale et al. (1989).

Environmental Requirements

M. pigra is found in tropical regions with >750 mm annual rainfall but is not found in tropical rain forest areas with a rainfall of >2250 mm. In areas of <750 mm annual rainfall, it may grow around dams and watercourses. M. pigra does not have any soil type preferences (Lonsdale et al., 1989). In Sri Lanka the species is currently found at an altitude of around 500 m above sea level (Marambe et al., 2001).

Associations

Mycorrhizae have sometimes been found associated with a few strains of Rhizobium, although the importance of these associations to the nitrogen budget is not known.

A number of chrysomelid beetle species feed on the leaflets of M. pigra, but the plant is avoided by cattle and horses. In Mexico, many insects feed on new growth and inside the reproductive structures.

In Costa Rica, the seeds are heavily predated by the larvae of a number of beetle species, including Acanthoscelides zebrata, A. pigrae and A. pigricola (Janzen, 1983). In Honduras, Habeck and Passoa (1983) collected 60 species of phytophagous insects. Adults of Chalcodermus serripes were common and their larvae fed on the seeds of M. pigra. An uncommon Coreid, Pachylis laticornis, also caused significant damage to the seeds.

In northern Australia, apart from some post-dispersal seed predation, insect herbivory is limited and large ungulates have little impact on stands of M. pigra.

Natural enemies of M. pigra in Thailand and Indonesia are listed in Napompeth (1983).

In Thailand, M. pigra interferes with irrigation systems by causing the accumulation of sediment, affects access to electric power lines and is a safety hazard along roads. It also spreads readily into fallow rice paddies increasing reclamation efforts and costs.

In Malaysia it encroaches into immature oil palm plantations and fruit orchards and it is feared that the shrub will spread to the rice bowl states of Kedah/Perlis with serious repercussions (Anwar, 2001). In Vietnam, local farmers report negative impacts on agricultural activities, their fishery catch and cultivation and their crop productivity. In addition, M. pigra has reduced the appearance of the natural environment, and had a considerable negative impact on travel, sightseeing and research activities in Cat Tien National Park particularly in Bau Chim and Bau Sau Ponds. The invasion of mimosa in rice paddies has increased cultivation expenses for soil preparation, and the labour required to remove mimosa before cultivation. In some areas, this expense is so much greater than the income of agricultural products that local people fallowed (ceased to crop) part of or all of their land (Nguyen Thi Lan Thi et al., 2011).

In northern Australia, M. pigra poses a threat to the cattle industry as it is spreading into buffalo pasture. The spread of M. pigra into pasture land reduces herbaceous vegetation and greatly reduces the grazing capacity of the land.

The shrub completely alters floodplain and swamp forest. The main impact of the weed is to reduce the number of birds and lizards, and the level of herbaceous vegetation; it also hinders tree regeneration. The occurrence of M. pigra along irrigation systems increases sediment accumulation and restricts water flow.

For further information, see Janzen (1983), Lonsdale et al. (1989), Wilson et al. (1990) and Braithwaite et al. (1989).

It restricts access to waterways, particularly to fishermen. If the spread of M. pigra is not halted, it may affect the touristic value of the Kakadu National Park in Northern Territory, Australia, as many visitors come to see the wetland's birdlife. Tourism has been reported as affected in Cat Tien National Park, Vietnam, due to effects on the landscape of M. pigra infestations ((Nguyen Thi Lan Thi et al., 2011).

The species is used in a various herbal remedies and magic rites in Africa (Burkill, 1995). In Malaysia it is reported to be used to cure snake bites in traditional medicine (Anwar, 2001). It has also been used as a green manure, a cover crop, beanpoles, and for hedges and fuel wood. Silivong et al. (2013) report that methane production from rumen incubation is lower from M. pigra leaves than from Gliricidia sepium.

Survey techniques for the detection of M. pigra are discussed by Pitt and Miller (1988). For discussion of identification and how to distinguish M. pigra from similar species, see the section 'Similarity to other species/conditions'.

Introduction

A guide to the management of M. pigra is provided by Harley (1992); this guide covers all aspects of management, especially the control options.

Cultural Control

In Malaysia, recommended control include involves slashing and brushing the stem with herbicide to be repeated every 6 months to prevent regeneration (Anwar, 2001).

In Australia, viable seeds have been found a decade after stand removal. The dormant seed bank should be be taken into consideration when devising management strategies including type of control, stocking rates and duration of follow-up control of seedlings (Lukitsch and Elliott, 2012).

Mechanical Control

In Malaysia, recommended control includes digging and uprooting plants to remove stands (Anwar, 2001). Schatz (2001) investigated the impact of cutting height on mortality. Cutting plants off ca. 10 cm below ground level killed all plants whereas cutting off at ground level or 15 cm above ground level resulted in resprouting in most plants. Thus slashing and chaining is not effective in controlling the weed whereas blade ploughing, a method which cuts the plant below ground level, can be an efficient physical control method.

Chemical Control

Total control of M. pigra was achieved within 12 months using a range of herbicides in foliar, basal bark and soil applications, and stem injections in field trials in Thailand (Thamasara et al., 1991). Of 15 herbicides tested, nine killed all 6-8-week-old plants grown under greenhouse conditions (Creager, 1992); the most effective herbicides were picloram, tebuthiuron, hexazinone, sulfometuron, dicamba, triclopyr, linuron and glyphosate.

Chemicals are used to contain the spread of M. pigra in Australia and to eradicate new infestations. Aerial spraying of gelled gasoline, followed by fire, kills stands of M. pigra and soil surface seeds, but enhances buried seed germination (Lonsdale and Miller, 1993). Lane et al. (1997) tested tebuthiuron against seedlings. It proved to be ineffective, with at best, 43% of seedlings surviving. Effective control of M. pigra is difficult to achieve because of the large soil seed bank.

Biological Control

A number of biological control agents have been investigated for the control of M. pigra in Australia and Thailand (Napompeth, 1983; Wilson et al., 1990). The potential of seed-feeding bruchid species has been studied following field investigations of insects associated with M. pigra in the Americas (Kassulke et al., 1990). Heard et al. (2012) report studies on Nesaecrepida infuscata (Coleoptera: Chrysomelidae), a common insect on M. pigra in tropical America. The larvae develop on the roots while the adults feed on the leaves. In host specificity tests, larvae did not develop on any of the 65 test plant species other than M. pigra. Adult feeding on test plant species other than M. pigra was minimal. Based on these results, this insect has been released in Australia. 

Fungal pathogens which may be useful in controlling this weed have also been identified (Evans et al., 1995). Sacdalan et al. (2012) investigated a site where sporadic dieback of M. pigra has been reported in order to look for potential control agents, and found ten isolates pathogenic towards mimosa seedlings in a laboratory trial. Five of these ten isolates were identified as Lasiodiplodia theobromae by DNA sequencing. Burrows et al. (2012) report that the neotropical rust Diabole cubensis, introduced as a biological control agent against M. pigra in the Northern Territory during the period 1996-1999 but thought not to have established, has recently been detected on M. pigra plants in several locations.

In Malaysia, four agents (Acanthoscelides puniceus, A. quadridentatus, Carmenta mimosa and Coelocephalapion pigrae) were introduced in the 1990s with limited success (Anwar, 2001).

Harley and Forno (1992) provide a valuable source of information on the biological control of weeds, and practical advice on undertaking a biological control programme. For further information on the potential for biological control of M. pigra, see Habeck and Passoa (1983). A more recent paper by Ostermeyer and Grace (2007) discusses the establishment, distribution and abundance of M. pigra biological control agents in northern Australia. It is suggested that: (1) seed and flower feeders must be capable of surviving periods of low food availability; (2) some climate matching may be beneficial before fungal biocontrol agents are released and (3) even in well studied systems such as M. pigra, the failure of an agent to establish cannot always be explained

Integrated Control

In Australia, Finlayson et al. (2001) reported that US$12 million had been spent on research and control of mimosa. Their recommended strategy for controlling M. pigra is to prevent initial invasion, eradicate small infestations by physical or chemical means and, for large infestations, to adopt an integrated approach involving biological control, herbicide application, mechanical removal, fire and pasture management. Finlayson et al. (2001) also stressed that some level of training and logistical support is required to implement such a management programme and identified key difficulties such as the lack of awareness of the problems that could occur if the weed is not effectively controlled, and discontinuity in control.

More specifically, work is being carried out to determine the optimal timing of herbicide application in order to optimize the effectiveness of biocontrol agents. Paynter (2003) found that treating regenerating M. pigra seedlings with herbicide at an optimal time can minimize the impact of the herbicide on the population of Neurostrota gunniella, a biocontrol agent that stunts plants.