Urgent action is needed to stop the rapid spread of the invasive alien plant, prosopis, that has already encroached 1,8 million hectares of South African rangeland.

At the present growth rate of eighteen percent per year, the size of the infested rangeland doubles every five to eight years. “The rapid spread of prosopis in South Africa is a major catastrophe,” says Ken Coetzee of Conservation Management Services.

“It is clear is that the problem is not being effectively addressed as it should be, and that many millions of hectares of grazing rangelands, rivers, wetlands and groundwaters are at risk of transforming rangelands to useless, environmentally damaged prosopis monocultures.” Ken believes the first step towards the effective control of the prosopis invasion is to improve awareness about the problem and to get landowners to see the need to control new infestations before an area is totally encroached and very difficult and expensive to clean up.

“Prosopis is notoriously difficult to control because if it is merely damaged through inadequate removal, it will simply resprout from the base again to form a dense multi-stemmed tree with two or three times as many flowers producing seeds for further invasion.”

Livestock and especially goats, as well as wildlife, love eating the pods of the prosopis. (Photo: Pixabay)

Mechanical control

The process involves the following:

Lightly infested areas

It is best to start by eliminating light infestations where single or only a few plants occur in small patches. This can be done by hand pulling seedlings and chopping off larger saplings just below the ground level with a spade or mattock or pick-axe. By doing so, partially invaded areas can be cleared and maintained. This will stop the encroachment from spreading.

Denser areas

Clearing small patches will isolate densely encroached areas, and clearing around the edges will help to shrink it. Over time, the denser encroachments can be cleared completely.

Cutting

The trunks must be cut down as close to the ground as possible. Using a paintbrush, every cut stem must be painted with a suitable, registered herbicide that will kill the stumps and roots and stop resprouting. Check with the herbicide suppliers or agricultural co-ops for the best herbicide to use for prosopis. Care must be taken not to spill herbicide on other indigenous plants, because these will also be killed.

Prosopis juliflora, also known as djembe, is native to Kenya. (Photo: Pixabay)

Maintenance

The cleared areas must be revisited repeatedly to ensure that any regrowth, either from resprouting stems or seeds that germinated, are removed. The initial control of denser infestations is labour intensive and expensive, but diligent maintenance will reduce the cost of every follow-up action. If follow-up is neglected, the money spent on the initial clearing will be money down the drain.

Biological control

Goats are used to help control the spread of prosopis in Texas. These animals eat more seedpods than any other livestock or even game species. Since few seeds survive the goat’s digestive system, it prevents them from germinating. Alien plants become naturalised or adapted to their environment, which leads to them to flourish in the absence of natural enemies in their new host region. “The biological control of invasive plants involves the deliberate use of these natural plant enemies that are specially collected from the area of origin of the invasive plant,” explains Ken. “By introducing these natural plant enemies, it is sometimes possible to decrease and even completely eradicate invasive alien plants.”

Biocontrol in Southern Africa that has been proved to be successful, includes the use of cochineal (Dactylopius austrinus) on the prickly pear (Opuntia ficus indica). The cochineal bug was introduced from Central America, where the prickly pear originated, to curb its spread. In some areas, prickly pear has been completely eradicated using this insect. Because the invasive plant’s nemesis also doesn’t have natural enemies in the host country, care must be taken to ensure that these biological control agents are specific to the host plant that you want to control because they may attack closely related indigenous plants and lead to their demise.

Plant Protection Research Institute

It is the task of the Plant Protection Research Institute (PPRI) of the Agricultural Research Council of South Africa to research biocontrol agents. Potential biocontrol agents are carefully screened in quarantine conditions. Exhaustive trials ensure that the agent is host-specific and therefore safe to use on problem plants.

“The procedure of screening a potential biocontrol agent is unfortunately a time-consuming and lengthy process that is rather expensive,” says Ken. “Once cleared and declared safe for introduction, successful biocontrol agents can become the most costeffective method of alien vegetation control and, in some cases, may result in the complete eradication of the problem plant.” Biocontrol is not a miracle quick-fix option, as the process takes time to develop. It cannot replace manual or mechanical control.

“Biocontrol should rather be introduced simultaneously to assist with the overall control strategy.” The PPRI can be contacted regarding the acquisition of approved biocontrol agents for release on invasive alien plants.

Biocontrol method

The seed-feeding beetles Algorobius prosopis and Neltumius arizonensis have been released on prosopis in South Africa. “Biocontrol of prosopis has been restricted to introduced seed-feeding insects due to the perceived value of prosopis as animal feed and as a source of wood,” says Ken. “As a result, the biocontrol released have not been aggressive enough to be effective for the widespread eradication of the invasion.”

Advantages of biocontrol

Biocontrol has numerous advantages.

“It is important to understand that the aim of biocontrol is not to eradicate the invasive alien plant, but to reduce its competitiveness with the indigenous plant species by reducing the density of infestation and its overall impact on the environment,” says Ken.

Advantages include:

  • No pollution, as no chemicals are used;
  • The biocontrol agents are host-specific and do not endanger indigenous plants or become a new pest;
  • The biocontrol agents are selfdispersing;
  • No follow-up action and investment are needed, as is the case with manual and mechanical control where chemicals must be bought to stop sprouting.

Disadvantages

The main disadvantages of biocontrol include:

  • Biocontrol agents take time before their results show;
  • The success of the method is not known beforehand. “There is, however, a very positive benefit to cost ratio – the benefits of biocontrol normally outweigh the drawbacks and it still represents a comparatively cheap and safe option for the control of alien plant invasions,” says Ken.

How do biocontrol agents work?

There are different types of biocontrol agents, including insects, mites, or fungi for alien trees. “To really damage an alien plant, flower and seed feeders, stem borers, leaf miners and fungal agents should be simultaneously introduced for maximum effect. Most often, the larvae of the agents do most of the damage,” says Ken. Introduced biocontrol insects, such as beetles, moths and flies, feed on the buds or heads of flowers. Others feed on the fruit or seeds, while yet others attack the leaves or the stem.

They will not destroy the plant, but will reduce its reproductive potential. Wasps and midges attack the growing tips of the host plant. They manipulate the host to divert valuable resources into gall production, rather than the production of flowers, seeds, stems and leaves. Insects use galls, a globular, woody swelling, as a site for laying eggs or as a source of food for the larvae and the galls severely disrupt the reproduction of the plant by replacing the seeds. The time these insects take to complete their lifecycle can be as little as ten days, while others may take up to ten months.

For more information, contact Ken Coetzee at 076-227-5056 or send an e-mail to consken@mweb.co.za.

References

Bromilow, C. (2018) Problem plants and alien weeds of Southern Africa. 4th Edition. Briza Publications, Pretoria.

Coetzee, K. (2005) Caring for natural rangelands. University of KwaZulu Natal Press, Scottsville.

Henderson, L. (2001) Alien weeds and invasive plants. Plant Protection Research Institute, Handbook No. 12, Agricultural Research Council.

Shackleton, R.T., Le Maitre, D.C., van Wilgen, B.W. & Richardson, D.M. (2015)The impact of invasive alien prosopis species (mesquite) on native plants in different environments in South Africa. S. African J. Botany, Vol. 97 (25-31).

Shackleton, R.T., le Maitre, D.C. & Richardson, D.M. (2015) Use of non-timber forest products from invasive alien prosopis sp. (mesquite) and native trees in South Africa: Implications for Management. Forest Ecosystems, Open Access, Springer Nature.

Strohbach, B.J., Ntesa, C., Kabajani, M.W., Shidolo, E.K. & DÁlton, C.D. (2015) Prosopis encroachment along the Fish River at Gibeon, Namibia. Dinteria, No. 35(53-73).

Van Wilgen, B.W. & Richardson, D.M. (2014) Challenges and tradeoffs in the management of invasive alien trees. Biol. Invasions. 16 (721-734). Springer Nature, Forest Ecosystems, open access on-line.

Zachariades, C. Hoffman, J.H. & Roberts, A.P. (2011) Biological control of mesquite (Prosopis species) (Fabaceae) in South Africa. African entomology,19 (2): 402-415).