FEATURE
strategy to release a variety of control agents for one particular alien plant species . Most agents are either insects , mites or fungi . Insects ( often beetles , moths and flies ) are usually flower head or flower bud feeders , seed feeders , leaf miners or stem borers . It is most often the larvae of the insects that do the damage . Flower , fruit or seed-feeding insects will not destroy the plant , but will reduce its reproductive potential . To really damage an alien plant , flower and seed feeders , stem borers , leaf miners and fungal agents should be simultaneously introduced for maximum effect .
Some insect agents ( certain wasps and midges ) attack the growing tips of the host plant and manipulate the host to divert valuable resources into gall production rather than the production of flowers , seeds , stems and leaves . A gall is a globular , woody swelling , sometimes used by the insect agent as a site for egg-laying or as a food source for larvae . Some fungi also cause growing tips to gall . Irrespective of how the galls are formed , the result is the same – the reproduction of the host plant is severely disrupted .
Some insect bio-control agents can complete their life cycle in as little as 10 days from egg to new adult , while others may take as long as 10 months to complete their cycle .
The Plant Protection Research Institute ( PPRI ) of the Agricultural Research Council of South Africa can be contacted regarding the acquisition of approved biocontrol agents for release on invasive alien plants .
The two biocontrol agents that have been released in South Africa are both seed-feeding beetles Algorobius prosopis and Neltumius arizonensis .
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 , and as a result , the biocontrols released have not been aggressive enough to be effective for the widespread eradication of Prosopis . The introduction of other more effective biocontrol agents has been frustrated by the potential that these bio-controls may affect noninvasive , indigenous and useful Prosopis in North African regions . In South Africa the perceived value of Prosopis hampers the “ stepping up ” of effective biological control , of which there are an additional 9 beetles , 4 moths and a gall midge , all known to be highly effective in the control of Prosopis elsewhere .
An extensive study in South Africa revealed that local rural communities preferred indigenous firewood sources to Prosopis even though Prosopis was freely available . Prosopis coals do not last as long as those of indigenous wood , the thorns make it more difficult to harvest , wet Prosopis gives off unpleasantly smelly smoke and dried Prosopis is more quickly reduced to powder by wood-boring beetles . The solution to the Prosopis problem is clearly not firewood .
The use of Prosopis for timber in South Africa is relatively low-key compared to its use elsewhere , such as in India , so silviculture with Prosopis does not appear to be a solution to the problem in South Africa either .
So , in a nutshell , a more effective biocontrol solution to the rampant spread of Prosopis is really urgent .
Summary Clearly , the rapid spread of Prosopis in South Africa is a major catastrophe . Equally 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 .
A fresh , new awareness is needed , landowners need to be advised , helped and encouraged to take action with new and spreading Prosopis invasions . Farmers ' associations , farmers ' unions and the formal agricultural sector all have a great responsibility here for creating greater awareness about the Prosopis invasion threat . We need videos about the problem , the threat and the control actions required .
The future implications of doing nothing must be clearly and visually expressed in the media and to those that have the responsibility of guiding agriculture , nature conservation and running the country .
Research and implementation of additional , more aggressive biocontrol agents are urgently required as this is probably the most practical way in which to effectively address the areas that are already densely invaded .
The use of Prosopis for the manufacture of charcoal , biochar , dry animal feed and activated charcoal should be investigated , researched and implemented . Attaching a value to the Prosopis product will certainly help with the control of the invasion .
The above are some of the options that could be explored because without any dedicated human intervention , much of our valuable rangelands will be forever transformed into useless , environmentally damaged Prosopis monocultures .
References Bromilow , C . 2018 . Problem plants and alien weeds of Southern Africa . 4 th Edition . Briza Publications , Pretoria . Coetzee , K . 2005 . Caring for natural rangelands . University of Kwa-Zulu 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 online .
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 ).
25 Grassroots Vol 22 No 2 July 2022