Prosopis juliflora
(mesquite)

P. juliflora is a shrub or small tree native to Mexico, Central and northern South America. It has shown itself to be a very aggressive invader, especially in frost-free arid and semi-arid natural grasslands, both in its native range and in particular, where introduced. Prosopis as a genus is treated as one of the world’s worst invasive plant species, and P. juliflora is by far the most invasive species. This has led to the declaration of P. juliflora as an invasive and/or noxious weed in many African countries notably Kenya, Ethiopia and Sudan, Pakistan and other Asian countries, and also in Australia and South Africa. In more sub-tropical regions such as in Australia and southern Africa, however, it is much less common than other more invasive Prosopis species such as P. glandulosa and P. velutina (see datasheets in the Invasive Species Compendium, also P. pallida with which P. juliflora is often confused). P. juliflora was widely introduced and planted as a fuel and fodder species, particular during fuelwood programmes in the 1980s, and the seed are spread widely by grazing animals. It is a nitrogen-fixing and very drought and salt-tolerant species, which can rapidly out-compete other vegetation. The thorniness and bushy habit of P. juliflora enable it to quickly block paths and make whole areas impenetrable.

The distribution of P. juliflora is probably even more widespread than shown in the distribution table. P. juliflora is probably present in at least some frost-free arid or semi-arid regions of every country in Africa, and is possibly present but localised in many more Asian countries. It is mainly a tropical species, native to Mexico, Central and northern South America. For the most up to date information on the global distribution of Prosopis species at the country level, including P. pallida, see Shackleton et al. (2014).

There are records of P. juliflora in Ecuador and Peru (Burkart, 1976; Diaz Celis, 1995) however more recently these have been disputed and P. juliflora is absent from these countries (Palacios et al., 2011).

P. juliflora propagules are not traded and are only rarely introduced accidentally by any other means (e.g. naturally by water or inside live animal exports). However, there are unconfirmed reports of accidental international introduction in the Greater Horn of Africa with the movement of extensively ranging livestock flocks and herds, and even with donkeys carrying equipment for troops and militias. P. juliflora has been widely introduced around the world intentionally, due to its value as a fuel/fodder species and also an ornamental in some regions. It has repeatedly escaped from cultivation and become naturalized into natural areas. However, its infamy as an invasive species has lead to several governments banning further importation of planting stock, and the risk of further intentional introduction is perceived as low. However, EPPO conducted a weed risk assessment in 2017 based on identified threats to parts of some member countries around the Mediterranean, notably on its southern and eastern shores.

P. juliflora has a broad ecological amplitude and is adapted to a very wide range of soils and habitat types from sand dunes to cracking clays. It is generally found in areas where water and soil fertility are the principal agents limiting plant growth, and is able to survive, and even thrive, on some of the poorest land, unsuitable for any other tree species. P. juliflora dominates in dry, or seasonally dry, watercourses or depressions, and is often found in coastal flats and dunes. Importantly, however, P. juliflora is frost sensitive, thus in areas at its temperature limits, it will tend to inhabit more protected sites.

P. juliflora is principally a noxious weed of natural grasslands such as in Uganda where it is reported to be outcompeting and displacing native grass species (BioNET-EAFRINET, 2015). P. juliflora is also a common weed in many other habitats but no specific crops or plants are affected.

Genetics

The chromosome numbers of most recognised species of Prosopis are diploid with a haploid number of n=14 (2n=28) (Hunziker et al., 1975; Solbrig et al., 1977). These authors indicate that P. juliflora is exceptional in having at least some tetraploid forms (2n=56). Recent work by Harris et al. (2003) suggests that P. juliflora is entirely tetraploid, and ploidy can be used to separate this species from P. pallida, which is entirely diploid (see also Pasiecznik et al., 2004), confirmed by Trenchard et al. (2008) and Sherry et al. (2011). The karyotype morphology of all species investigated was similar, with somatic chromosomes, which are very small (0.8-1.3 µm), showing slight variations in size within the complement. Chromosomes are only slightly differentiated, with median to subterminal centromeres, one pair of which displays a terminal microsatellite in most species (Hunziker et al., 1975).

Reproductive Biology

Anthesis is protogynous (Burkart, 1976; Goel and Behl, 1996), occurring when the flowers fully open, simultaneously in all flowers of a single inflorescence (Díaz Celis, 1995). However, flower maturation often begins at the proximal end while flowers at the distal end are still immature. The stigma has a central depression into which pollen falls for fertilisation, assisted by a sugary secretion (Goel and Behl, 1996). Prosopis species are generally assumed to be self-incompatible (Solbrig and Cantino, 1975; Simpson, 1977), although some limited self-compatibility (4%) has been observed in P. juliflora following bagging and hand pollination (Sareen and Yadav, 1987). Anther glands in P. juliflora release a protein-carbohydrate exudate and the flower is pollinated while the insect eats the gland (Chaudhry and Vijayaraghavan, 1992). Anther glands also exude a sticky substance to attach the pollen to the body of the insect, to protect the anthers and ovaries, and may also exude an odorous chemical attractant. Percentage pollination in P. juliflora is always low, which is thought may be due to: poor pollen viability, short periods of pollen release or stigma receptivity, lack of synchronisation between pollen release and pollen reception, few pollinating insects (or too few at times of maximum receptivity), flower sterility or high rates of ovary abortion. Goel and Behl (1995) found P. juliflora pollen viability to be 79-96%. Although very large numbers of flowers are produced, not all are fertile and high rates of ovary abortion are found, and very few legumes are produced compared with the large numbers of flowers produced per tree. P. juliflora usually begins to flower and fruit after 2-3 years, but this is highly depended upon site conditions, as trees as young as 12 months old have been observed to flower in the Sahel, and trees 15 years or more old on poor exposed sites have never been seen to flower (Pasiecznik et al., 2001).

Physiology and Phenology

The seeds of P. juliflora possess an inherently high level of dormancy. The hard seed coats must be broken or weakened to allow water absorption by the seed and for germination to occur, though they will also degrade over time and older seed that is still viable tends to germinate without pre-treatment (Pasiecznik and Felker, 1992). Seeds in entire pods or endocarp shells exhibit decreased germination, thought to be due to impeded water uptake by the seeds, although an allelopathic chemical extract from pod pericarps decreased germination in P. juliflora (Warrag, 1994). The passage of seed through different animals has varying effects on germination, through the removal of the mesocarp or endocarp, or other mechanical or chemical factors. P. juliflora seeds showed no decrease in final germination with up to 30% added sea water, although the rate of germination was retarded (Khan et al., 1987). Increasing alkalinity markedly decreased the final germination and germination rate of P. juliflora seed above pH 9.0 (Srinivasu and Toky, 1996). The optimum temperature for germination of P. juliflora seeds is 30-35°C, with germination decreasing rapidly at temperatures below 20°C or above 40°C (Pasiecznik et al., 2001). The optimum sowing depth for seed is 10 mm for P. juliflora with germination falling markedly when sown below 20-30 mm deep (Mutha and Burman, 1998).

All Prosopis species are able to survive in areas with exceptionally low annual rainfall or very lengthy dry periods but only if the roots are able to tap ground water or another permanent water source within the first few years. Being adapted to arid and semi-arid climates, P. juliflora generally germinates and establishes during the brief rainy season and seedlings must be sufficiently well established to survive the first dry season. The existence of two root systems, a deep tap root to reach ground water and a mat of surface lateral roots to make use of infrequent rainfall events, puts Prosopis species firmly in the category of phreatophytes, but they show a variety of mesophytic and xerophytic characteristics depending on water availability. The need for rain or a high water table is reduced in coastal areas, where sufficient atmospheric moisture exists with persistent trade winds or seasonal fog.

In P. juliflora, the action of the pulvinus can cause the leaflets to fold, protecting the stomata on the upper leaf surfaces from water loss during periods of high evapotranspiration. Leaflets possess specialised adaptations promoting efficient utilisation and retention of water such as sunken stomata, more stomata on adaxial than abaxial surfaces, thick and waxy cuticles and the presence of mucilaginous cells. There are also metabolic changes within the leaf and the whole plant during periods of drought stress that better enable the plant to survive. Seasonal variations in P. juliflora leaf concentrations of proline, sugar and protein are assumed to be a response to drought. In natural stands of P. juliflora in Venezuela, osmotic adjustment was observed in the dry season, with an increase in leaf concentrations of all measured nutrients.

There were marked growth flushes of new leaves during the year in the more subtropical species such as P. glandulosa, although some smaller periods of leaf senescence and replenishment were observed in P. juliflora (Goel and Behl, 1996). The presence of chlorophyll in the green stems of P. juliflora is also a response to drought, allowing for leaf shedding during dry periods while still maintaining some photosynthetic potential (El Fadl, 1997). Diurnal changes occur in photosynthetic rate and stomatal conductance, with a marked depression in both during the high temperatures found at midday (Sinha et al., 1997).

Prosopis species exhibit high levels of variability in morphological characters. The reproductive self-incompatibility and obligate outcrossing observed tends to lead to large phenological variation, being a combination of both clinal (continuous) variation in response to broad climatic factors and ecotypic (discontinuous) variation in response to disjunct environmental factors. Differences in continuous climatic clines such as temperature, rainfall and day length, and discrete differences in site such as soil type, salinity or depth combine to create a variety of phenological responses. Variations are observed principally in native populations. In invading populations, clinal variations are obscured because of the rapid and widespread dispersal of diverse genetic material by humans and animals over a range of site and climatic conditions. Almost continuous year-round flowering of P. juliflora is seen in India (Goel and Behl, 1995) and Haiti (Timyan, 1996) but there is always a period of maximum fruit production. In parts of India, one or two fruiting periods occur, depending on site and the 'form' of P. juliflora present (Luna, 1996). With continuous flowering, periods of major fruit production may correspond to periods of increased pollinator activity and not necessarily to genetic controls, particularly with introduced material.

Associations

Mares et al. (1977) summarised the ecological associations of Prosopis tree species in their native American range as "representative large desert trees which provide protection from grazing animals, shade, a moist microhabitat, a substrate for climbing or perching, and a reliable supply of nutrients for parasitic and semi-parasitic plants. In providing these habitat components in their natural range, such desert scrub trees allow an increase in plant density and richness in the community as a whole. These species, which would be rare or absent without the presence of trees and shrubs such as Prosopis, in turn contribute to the support of other trophic levels by providing food sources (leaves, flowers and fruits) for desert scrub animals". In the native range of P. juliflora along the Pacific coast of Central America (Pasiecznik et al., 2001) several plant genera are common associates including Capparis spp., Cordia spp. and also the woody legume genera Acacia, Caesalpinia, Cercidium, Parkinsonia and Pithecellobium.

In areas where P. juliflora is native, it is thought that its removal would cause a significant decrease in the populations of small wild mammals, and P. juliflora introduced to severely degraded land or areas without much other native vegetation has also become an important habitat for some wild mammals and birds (e.g. Chavan, 1986). One main association is with domestic mammals, which have quickly developed a strong relationship with native Prosopis. Seed-feeding beetles, many of the family Bruchidae, have evolved alongside Prosopis and are very important in the ecology of American Prosopis species. Of the species of beetles found to feed on the pods of native American Prosopis, 93% were obligately restricted to Prosopis, showing a high degree of specialization. Pollinating insects are of great importance and several detailed reviews exist. P. juliflora has evolved a symbiotic relationship with Rhizobium and other nitrogen fixing bacteria and also mycorrhizal associations to varying degrees.

Environmental Requirements

A major limitation to the distribution of the truly tropical P. juliflora is mean minimum temperatures, and the frequency and duration of winter frosts. Light frosts cause dieback of the branches, harder frosts may cause complete stem mortality, and more severe or longer-lasting frosts can cause complete death of the plant (Felker et al., 1982). Frost damage is more severe on seedlings and younger trees of P. juliflora and on trees in inter-dunal or other low lying areas (Muthana, 1974). Hyde et al. (1990) found that P. juliflora seedlings were killed by a -2°C frost in Spain, whereas P. juliflora was noted to suffer frost damage but survive when temperatures fell below 0°C in India (Muthana, 1974). There is also considerable variation in frost tolerance exhibited by different provenances of the same species. Owing to the more subtropical native range of P. juliflora in Mexico, it can be assumed that it would be more frost tolerant than other land races of P. juliflora native to more equatorial regions of Peru, Ecuador and Colombia.

Soil nutrient status is rarely a limiting factor to distribution. Nitrogen is very rarely limiting and trees have been noted to fix nitrogen under conditions of high pH (Singh, 1996), high salinity (Felker et al., 1981) and high water deficits (Felker et al., 1982). Other macronutrients can occasionally be limiting to growth either directly or indirectly. A strong positive correlation was noted between soil nutrient status and leaf nutrient content in P. juliflora (Sharma, 1984). Saline and alkaline soils are often occupied by P. juliflora and it is known to tolerate saline sites in its native range such as lowland flats and coastal dunes and in such conditions, it can often dominate. P. juliflora has been successfully raised using saline irrigation water, with an electrical conductivity of 20 dS/m in India (Singh, 1996) and 6-21 dS/m in Pakistan (Khan et al., 1986). P. juliflora is particularly well able to tolerate alkaline soils, with marginal reduction in growth up to pH 9, and will survive and grow in soils of pH 11 (Singh, 1996). However, Prosopis species appear not to be well suited to acidic soils and the possibility that low pH is a limiting factor to the distribution has been suggested (Pasiecznik et al., 2001).

P. juliflora thrives in a wide range of rainfall zones, from 100 mm mean annual rainfall or less in dry coastal zones to 1500 mm at higher altitudes, and the ability to tolerate very low annual rainfall is well known. Mean annual air temperature in the shade where P. juliflora is found is generally above 20°C, with optimum temperatures for growth in the range 20-30°C. There appears to be no natural upper limit to temperature, with introduced P. juliflora known to tolerate day-time shade temperatures of over 50°C, and soil temperatures in full sunlight as high as 70°C in Africa and Asia (Pasiecznik et al., 2001).

P. juliflora can be found on all soil types from pure sands to heavy clays and stony soils, but deep free-draining soils are preferred. Soil depth is important, with a noted limitation to tree growth occurring where soils are thin, or have a calcareous or iron pan. Above ground growth is stunted if root system development is impeded for any reason and poor drainage or waterlogging can have similar effects on tree growth and survival, with poor oxygen content in the soil also thought to affect root growth. Altitude appears to have a limited effect on distribution. In its native range P. juliflora is abundant at altitudes below 200 m, but is increasingly less common as altitude increases.

For an extensive list of pests and pathogens known to attack P. juliflora and related species, refer to Pasiecznik et al. (2001). Defoliating insects vary in their severity of attack and twig girdlers (Oncideres spp.) are damaging in some areas, with adult beetles girdling small branches before ovipositing. Wood-boring beetles are rarely specific to Prosopis, attacking several taxa of woody perennials.

Recent work in Djibouti found massive infestations of seed-feeding beetles (assumed to be bruchids) on pods that had been in storage for several months, prior to milling for animal feed (Pasiecznik et al., 2010). It was estimated that at least 70% of the seed had been destroyed in some batches. They are also present in parts of Sudan. The beetle is present in Yemen, and was thought to have been introduced with pods introduced 40 years previously (Ali and Labrada, 2006). It could be assumed then, that the beetles were introduced accidentally to mainland Africa from Yemen. In any case, P. juliflora is rapidly invading areas in Yemen and Djibouti even with heavy infestations of bruchids, thus indicating that is not an effective means of control.

Animal feed, fodder, forage

• Fodder/animal feed
• Forage

Environmental

• Agroforestry
• Amenity
• Boundary, barrier or support
• Erosion control or dune stabilization
• Land reclamation
• Landscape improvement
• Revegetation
• Shade and shelter
• Soil conservation
• Soil improvement
• Wildlife habitat
• Windbreak

Fuels

• Biofuels
• Charcoal
• Fuelwood

General

• Ornamental

Genetic importance

• Gene source

Human food and beverage

• Beverage base
• Emergency (famine) food
• Flour/starch
• Food additive
• Gum/mucilage
• Honey/honey flora

Materials

• Alcohol
• Chemicals
• Dye/tanning
• Fibre
• Gum/resin
• Miscellaneous materials
• Poisonous to mammals
• Wood/timber

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore

Charcoal

Furniture

Roundwood

• Building poles
• Posts
• Roundwood structures
• Stakes

Sawn or hewn building timbers

• Carpentry/joinery (exterior/interior)
• Exterior fittings
• Fences
• Flooring
• For light construction
• Gates

Woodware

• Industrial and domestic woodware
• Marquetry
• Tool handles
• Turnery
• Wood carvings

P. juliflora is often mistaken for other Prosopis species, especially those from within section Algarobia, due to a wide variation in morphological characteristics. The most frequent confusions are with P. chilensis, P. glandulosa and P. pallida where introduced which is aided by poor records and a changing taxonomy. However, this long history of misidentification appears to have been resolved by the publication of a new field guide (Pasiecznik et al., 2004), which differentiates P. juliflora from seven other Prosopis species also found in tropical regions and includes a key to separate P. juliflora from P. pallida, the two most frequently confused species (see also Harris et al., 2003).

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.

Control

The following section on control methods is summarised from Pasiecznik et al. (2001) and comprises control methods that have been attempted on several closely related Prosopis species including P. juliflora. It is thought that most, if not all, control methods suitable for one species can be successfully applied to another. However, methods of eradication attempted for over half a century in the Americas have proved very expensive and largely unsuccessful in the long term. Total tree kill may be possible with some treatments, but adequate techniques for preventing the re-introduction of seeds and re-establishment of trees have yet to be developed. The potential environmental damage from the widespread use of herbicides must also be taken into consideration. It is now accepted that eradication is not possible using these techniques and, at best, only some form of control is feasible. Mixed mechanical and chemical methods have often proved more effective than either alone. Several integrated programmes that mix mechanical and chemical methods and fire have had reasonable success but are costly and require a high level of management input.

Cultural Control

Hand clearance is the first method used to deal with Prosopis as a weed. Work teams are sent into invaded pasture to fell the trees and uproot all stumps. Although very effective, the operation is labour-intensive and hand clearing remains practical only for small land holdings of high value, such as for agriculture or where labour is relatively cheap. Hand clearing can also be used in conjunction with some mechanical or chemical methods, such as chemical stump treatment. In Pakistan, hand grubbing was cheaper than chemical stump treatment (Khan, 1961). Grubbing is more cost effective in lighter infestations.

Fire, probably one of the original management tools used in American grasslands, has undergone limited assessment for controlling Prosopis. Young seedlings are sensitive to fire but older trees become increasingly protected by thick bark as they mature and will resprout rapidly after fire. However, fire can be used successfully as a management tool for preventing re-establishment of young Prosopis seedlings while also improving forage production. Fire has been used in conjunction with other methods in the development of integrated eradication programmes. For example, spraying with herbicides produces dead wood that will ignite and support a sustained fire with more likelihood of killing the remaining trees. New integrated systems are being assessed in Australia.

Studies on succession suggest the possibility of 'ecological control', by leaving succession to take its natural course. The invasion of Prosopis species into rangeland has been observed and studied for over a century in the USA (e.g. Archer, 1995) and for long periods in South America (e.g. D'Antoni and Solbrig, 1977) and India (e.g. Chinnimani, 1998). Long-term ecological observations and the use of models have indicated that dense thickets associated with the problems of invasion are only a temporary stage in the process of succession. The initial stages of invasion involve the introduction of small numbers of Prosopis trees, which eventually produce seed and act as centres of dissemination (Archer, 1995). Prosopis stand density increases if land-use systems allow the establishment of seedlings, leading to the formation of dense thickets where conditions allow. Chinnimani (1998) showed that Prosopis density eventually declines as other species become established and, if left to take a natural course, a new vegetation complex will occur with Prosopis as only a minor component. Felker et al. (1990) observed that self-thinning occurred in stands of P. glandulosa over time. The dense thickets identified as weedy invasions in many countries may only be indicative of the stage of invasion and, if left alone, ecological control may reduce Prosopis numbers.

Than (2011) reported that P. juliflora appeared to struggle to compete with the climber Combretum roxburghii [C. album] and the shrub Azima sarmentosa.

Mechanical Control

Mechanical site clearance involves tractor operations developed for removing trees, in which the roots are severed below ground level to ensure the tree is killed. These operations include root ploughing and chaining, which are often the most effective mechanical means, using a mouldboard plough pulled behind a Caterpillar tractor or a heavy chain pulled between two machines. For root ploughing, large trees must first be felled by hand, but this treatment has been used to remove stumps up to 50 cm in diameter without difficulty and has a treatment life of 20 years or more (Jacoby and Ansley, 1991). Other advantages are that only a single pass is required, and whole site cultivation is effected leading to improved soil water conservation, and there is a chance to reseed with improved forage species. However, this method is one of the most expensive control treatments and is recommended only on deep soils that have a high potential for subsequent increased forage production (Jacoby and Ansley, 1991).

The soil should be neither too wet nor too dry for effective root ploughing. Chaining involves pulling a heavy chain between two slow-moving Caterpillar tractors, with the effect of pulling over larger trees and uprooting them. A second pass in the opposite direction ensures that roots on all sides are severed to ease tree removal (Jacoby and Ansley, 1991). Soil moisture is again important, with soil that is dry on the surface and moist below giving the optimal conditions. If the soil is too dry, the stem breaks leading to coppicing, if too wet, the soil and understorey are damaged (Jacoby and Ansley, 1991). Smaller, unbroken trees have to be removed by other means. Although expensive, this treatment is effective where there are many mature trees. It is most widely used following herbicide application to remove dead standing trees. Clearance with a biomass harvester produces wood chips that can be sold for energy production offsetting the operational costs (e.g. Felker et al., 1999).

Biological Control

Several biological control programmes using species of seed-feeding bruchid beetles have been developed and implemented. The advantage with bruchids is their observed host specificity, with many species found to feed only on Prosopis, and some only on a single species. Other insect species known to have a deleterious effect on native and exotic Prosopis in the Americas, mainly twig girdlers and psyllids, have also been suggested as possible biological control agents. The twig girdler Oncideres limpida attacks P. pallida in Brazil (Lima, 1994), whereas Oncideres rhodostricta is seen as a serious pest of P. glandulosa in the USA (Polk and Ueckert, 1973). Psyllids are known to severely affect the growth of Prosopis (Hodkinson, 1991) and have been suggested for use in controlling invasions.

Most work on biological control of Prosopis to date has been carried out in South Africa, where several programmes are underway. The seed-feeding insects Mimosetes protractus and Neltumius arizonensis were introduced to eastern South Africa in conjunction with the bruchid beetles Algarobius prosopis and A. bottimeri for the control of invasive Prosopis species. N. arizonensis and A. prosopis were successful in establishing themselves in large numbers and having a significant effect on Prosopis spp., whereas the other species were only found in low numbers (Hoffmann et al., 1993). Maximum damage to seed occurred where grazing was controlled, as the multiplication and progress is hampered by livestock devouring pods before the insects destroy them.

The same two bruchid species were also introduced to Ascension Island in an attempt to control P. juliflora which is present on 80% of the island, often in dense thickets. Two other species, one a psyllid and the other a mirid, were identified as attacking P. juliflora on Ascension Island and were thought to have been introduced accidentally from the Caribbean. The mirid Rhinocloa sp. causes widespread damage and is thought to lead to substantial mortality of trees (Fowler, 1998). In Australia, Prosopis infestations are at a relatively early stage and extreme care is being employed in the selection of suitable biological control agents, following the long history of problems caused there by plant and animal introductions. Insect species continue to be tested for their efficacy and host specificity as possible biological control agents of Prosopis species in Australia (e.g. van Klinken, 1999; van Klinken et al., 2009). Besides the two Algarobius species, the sap-sucking psyllid Prosopidosylla flava and the leaf-tying moth, Evippe sp. have both been found to provide some control in Australia.

Prosopis species continue to spread widely in parts of their native ranges where many insect species including bruchids, twig girdlers, psyllids and other injurious pests are common components of the ecology. These regularly attack Prosopis but the trees have adapted to infestation by these pests and are still able to become invasive weeds over large tracts of land. Although there has been some success in the control of exotic Prosopis following the introduction of bruchid beetles and other insects, it appears that biological control alone may be insufficient. Also, increased utilisation of the pods as a food and/or fodder means that seed-feeding biological control agents are less likely to be acceptable. This was seen recently in Kenya, where A. prosopis had been cleared for release, but this was put on hold at the last minute (May 2006) for two years, awaiting the results of a new task force that continues to promote links between rural communities and livestock feed manufacturers (N. Pasiecznik, personal communication, 2006).

However, where identified as an invasive species in dry zone in northern Myanmar (e.g. Aung and Koike, 2015), there has been at least an initial focus on biological control agents for this forest invasive species (Than, 2011), with investigation for biological control agents conducted in Pyawbwe in January 2010. Damage was detected in the form of yellowing foliage and damage from pathogens around cuts during fuelwood harvesting, identified as Fusarium sp., Tubercularia sp. and Nectria sp., and small-scale trials have been initiated to examine the potential for these fungal pathogens to aid in biological control of P. juliflora.

Chemical Control

Chemical treatments involve the use of herbicides to kill trees, with the most effective being stem or aerial applications of systemic herbicides. Effectiveness is dependent upon chemical uptake, which in Prosopis is limited by the thick bark, woody stems and small leaves with a protective waxy outer layer. The formulation and application of chemicals for trees of mixed ages and sizes within a stand is difficult. Many herbicides and herbicide mixtures have been tested, mostly on P. glandulosa.). Although 2,4-D provided excellent suppression of top growth, few trees were actually killed and such chemical treatments had to be applied periodically to ensure that forage yields were maintained. Infested sites often needed spraying every 5-7 years. The most effective chemical for high tree kill in the USA is clopyralid, but dicamba, picloram and triclopyr have also been successfully used, either alone or in combination (Jacoby and Ansley, 1991). In India, ammonium sulfamate was successful in killing P. juliflora trees and as a stump treatment (Panchal and Shetty, 1977).

Integrated control

Fire has been used in conjunction with other methods in the development of integrated eradication programmes. For example, spraying with herbicides produces dead wood that will ignite and support a sustained fire with more likelihood of killing the remaining trees.

Control could also include the use of animals, other than insects, to eat and kill Prosopis seed. The factor common to most Prosopis invasions is over-grazing with cattle, which spreads Prosopis seed widely. Prosopis seed found in cattle faeces have much improved germination compared with uningested seed (Peinetti et al., 1993; Danthu et al., 1996). In contrast, the percentage of P. juliflora seed excreted after ingestion by sheep and goats was much lower (10-15%) (Harding, 1991; Danthu et al., 1996). Marked differences were noted in the germination of seed following passage through different animals (Mooney et al., 1977); germination was 82% with horses, 69% with cattle, but only 25% with sheep. P. flexuosa seed were killed completely followed ingestion by pigs (Peinetti et al., 1993). Replacing free-ranging cattle with other livestock, particularly sheep and pigs, possibly in conjunction with other control methods, could drastically reduce the spread of Prosopis species.

Weedy invasions of Prosopis can be successfully adapted to agroforestry systems by a conversion process developed by Felker et al. (1999) and adapted by Tewari et al. (2000) and Pasiecznik et al. (2001). This conversion requires three main management interventions: thinning, pruning and treatment of understorey. Weedy thickets with 1000-2500 trees/ha and dense infestations with over 2500 trees/ha need to be thinned to 100-625 trees/ha. This thinning operation is the most problematic and costly aspect of conversion and limits the uptake of such a system. The use of a tractor-mounted flail-mower to cut rows through the stand is the most economical means of initial thinning. The harvested biomass is sold to offset some of the cost of the operation (Felker et al., 1999). The aim is to leave rows of undisturbed Prosopis at least 1 m wide and at 5-10 m intervals across the site. Clearing by hand is a laborious and unpleasant task owing to thorns and difficult access, and rows of thorny brash must be left on site. However, most of the nutrient-rich foliage is retained on site with this method, adding to soil fertility. All tree stumps need to be killed, both of Prosopis and other woody weed species.

Following initial systematic thinning to a stand density of approximately 500-1000 trees/ha, a secondary selective thinning is required to create the desired final density of 100-625 trees/ha or less. Although trees do not necessarily have to be equally spaced, leaving open rows 5-10 m apart will facilitate access and increase the number of understorey management options possible with tractor operations. Trees with desirable characteristics and at defined spacings should be marked, and all others removed and the stumps treated. Trees should be selected on the basis of their large size, erect form, straight trunk, pod production, lack of thorns and good tree health. Selected trees are then pruned to improve form, by removing any basal shoots and side branches to at least one-half of tree height for timber production, leaving a clear bole preferably over 2 m. A shorter bole and broader crown is preferred for pod production. Treatments can be applied to reduce resprouting from the tree base and wounds (Pasiecznik et al., 2001). Pods can be browsed or collected as a source of food or fodder. Incorporating bees into the system will produce honey and wax and increase pod production, and the trees could be a minor source of other raw materials. Further thinning of trees in later years can also be carried out, or trees can be removed entirely from the site and the land returned to agriculture (e.g. Bhojvaid et al., 1996).

19/02/2017 Updated by:

Nick Pasiecznik, Consultant, France

04/11/11 Updated by:

Nick Pasiecznik, Consultant, France

28/11/2007 Updated by:

Nick Pasiecznik, Consultant, France