In South Africa, the common names mesquite or prosopis refer to a group of decidious, leguminous thorn tree species and hybrids, belonging to the genus Prosopis, which is native to South and North America. They were introduced into many countries worldwide and promoted as useful trees in arid areas. Prosopis was actively planted in South Africa over vast areas in the Northern Cape, Western Cape, Free state and North-West Provinces, from the late 1800's, untill as recently as the 1960's. It was introduced as a beneficial fodder, fuel and shade tree in the arid regions lacking in indigenous trees. However, the appearance of dense stands of this major water user, in these particularly water scarce areas, turned mesquite into a highly undesirable invasive alien weed.
The most prominent invasive forms in South Africa are P. glandulosa var. torreyana (honey mesquite) and P. velutina (velvet mesquite) as well as their hybrids. These species and hybrids have been listed as invasive species in terms of the Alien and Invasive Species Regulations (AIS), National Environmental Management: Biodiversity Act (Act No 10 of 2004). In particular, they were listed as category 1b species in the Eastern and Western Cape, Free State and North-West Provinces, which means that they have to be controlled, or to be removed and destroyed if possible. In the Northern Cape Province they have category 3 status, which means that they may remain where they are already present, except for riparian areas, where they will be regarded as category 1b species. No trade, propagation or planting is allowed in any of the listed provinces. The regulations do not apply to any of the provinces that are not listed here.
Interbreeding of Prosopis species often gives rise to less desirable growth forms. In dense populations, these form bushy or shrub-like plants, whose shade is inaccessible, and which hardly produce flowers or pods for nectar or fodder, thus laying the land waste. Seed is dispersed by animals that browse on the pods, or they are washed away by water during flooding events and then deposited en masse in floodplains. This results in expansive, impenetrable thickets of this weed. Prosopis species have an extensive tap root system (depths of 58m have been recorded). This enables them to effectively source shallow as well as deep soil water, thus lowering the ground water table and reducing the amount of water that is available to desirable grazing plants and indigenous tree species. A more detailed description of the plant and the problems it causes can be found in an earlier pamphlet [pdf] while control options for this species are discussed in another pamphlet. [pdf]
The initial focus of the biological control programme of mesquite in South Africa was to reduce the viable seed load in an attempt to retain some of the beneficial properties of this tree, whilst retarding further invasion. Seed feeding weevils that develop only on ripe prosopis seeds were therefore studied for their safety and released in South Africa. Two species, Algarobius prosopis and Neltumius arizonensis, were effectively established in the field. Further information on these first weevils can be found in a short pamphlet [pdf] or a more detailed pamphlet [pdf] and in a more recent review (2011) [pdf] that summarizes the biological control research on this weed.
Coelocephalapion gandolfoi, another weevil species that develops on green prosopis pods, was studied for its potential and safety as a seed reducer, and may be introduced into South Africa in the near future to help reduce the invasive potential of this weed.
Several studies in South Africa, and in other countries where mesquite has become invasive, have established that the negative effects of this invasive tree far outweigh its positive attributes. The focus of the biocontrol programme against mesquite has therefore now shifted to investigating biocontrol candidates that affect vegetative plant parts. The rationale is to reduce the vast stands of this invasive shrub, and to turn it into sparser, useful, single-stemmed trees, reminiscent of the way in which they grow in their native home, where they are kept in check by several natural enemies that develop on all plant parts.