Have you ever heard of a fungus that helps control invasive alien plants? Meet Cylindrobasidium torrendii, a naturally occurring fungus that could revolutionise black wattle treatment. This tiny organism is helping manage areas of invasive species like Acacia mearnsii (black wattle). Black wattle is a fast-growing, aggressive plant that spreads rapidly and outcompetes indigenous vegetation. Black wattle disrupts ecosystems by consuming excess water and nutrients, displacing the indigenous flora.
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Recently, the Grootvadersbosch Conservancy had the privilege of hosting Dr Alan Wood from the Agricultural Research Council, an expert in plant pathogen behaviour and biocontrol development. Dr Wood presented his biocontrol solution as an alternative to herbicide to control black wattle on cut stumps. The Conservancy is working with Dr Wood to test this approach in real world conditions.  

During his visit, Dr Wood demonstrated to the Grootvadersbosch team and a group of alien clearing workers how the biocontrol works. He explained how to mix and apply the fungus on stumps immediately after cutting down black wattle trees, ensuring it is absorbed through the stump and into the roots. This method targets the tree’s internal systems without the potential environmental and human health risks associated with herbicides.
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Unlike herbicides that can harm the environment and workers who appply, without the correct protective equipment, Cylindrobasidium torrendii offers a natural, water-based solution. The biocontrol is applied directly onto the stump after cutting the tree at ankle height with either a chainsaw or silky saw. From there, the fungus colonises the tree’s roots and slowly decomposes them, preventing regrowth. It’s like pressing a natural shutdown button for unwanted black wattle trees. Best of all, the fungus is species-specific, meaning it won’t spread to surrounding indigenous plants, making it an eco-friendly alternative to herbicides. Dr Wood is trialing a new, and more cost effective, formulation of this fungus. 

Although Cylindrobasidium torrendii isn’t a fast-acting solution, it is possibly a more sustainable and less harmful method for combating black wattle. It takes time for the fungus to spread and for the decomposition process to fully take effect, but the long-term benefits could make it worthwhile and this approach allows indigenous plants to recover naturally. 

​The Grootvadersbosch Conservancy has worked on the removal of Pereskia aculeata over the past few years and knows how problematic an alien invader it is. The conservancy used to burn off every stem of the plant which was quite an expensive process. The Grootvadersbosch Conservancy has been working with the Swellendam municipality to manage alien invasives and noticed an invasion in the centre of town. We urgently required an easier way of eradicating the pesky Pereskia, so we contacted Dr. Iain Paterson to aid us with some biocontrol. He posted the biocontrol to us where we could release it at two sites – the first site is at a Swellendam municipality near the mall and the second site is at the Buffeljags Dam.

​Pereskia aculeata originates from Brazil in South America and thus it has no natural predators in South Africa. It is a creeper plant that can easily outcompete and overgrow indigenous plants especially in forest and plantation areas, leaving them with no access to sunlight, natural pollinators, and water (Invasives SA, 2014). The spread of Pereskia aculeata is problematic as the areas with the infestation will become degraded and will have little growth of indigenous plant diversity over time. 

​Biological control (Biocontrol) is the use of a natural enemy, usually an insect or pathogen, to control and/or eradicate an alien invasive plant species (ARC, 2014). Biocontrol is an alternative method to control and eradicate alien invasive plant species and is very effective and often used in combination with mechanical control

Luckily for this Pereskia invasion, both biocontrol agents were delivered to us, Pereskia Flea Beetle (Phenrica guérini) and Pereskia Stem-wilter (Catorhintha schaffneri). We released both at the Swellendam municipality site and only the Pereskia Stem-wilter at the Buffelsjag Dam site. Both agents can only survive by feeding on Pereskia. The Pereskia Flea Beetle feeds on the leaves, forming big holes while leaving a trail of brown faeces. The Pereskia Stem-wilter feeds on the plant shoot tips, splitting them open and/or rotting them. 
 
We are very grateful for the incredible scientists who work so hard in developing these agents and so we decided to interview Dr. Iain Paterson who shares his experience in developing a biocontrol agent for Pereskia aculeata.
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​Dr. Paterson has always had a passion for nature, he loved plants and animals since childhood. Once he started university, he decided to study both Zoology and Botany. While studying, he discovered an interest in insects, he was fascinated by the idea of using insects as a biocontrol agent, consequently, he decided to pursue Entomology and he then did his Ph.D. on Pereskia aculeata.

​Dr. Paterson explained that they experimented with potential Biocontrol insects, which are all indigenous to South America. Five insects were used in the experiment and only two were found successful as a biocontrol. It took him 2 years to prove that the Pereskia stem-wilter beetle (Catorhintha schaffneri) will only feed on Pereskia aculeata and not on any other indigenous plants of Africa. 

​Pereskia aculeata is an alien-invasive vine plant species. Vine plants are almost impossible to remove successfully using chemical or mechanical methods without damaging the surrounding indigenous vegetation. Pereskia aculeata also has a nasty habit of resprouting where it has broken off and any present stem fractions will eventually develop into a new plant. Dr. Paterson once brought a Pereskia stem fraction from overseas within a clip file and, with no soil or water, the fraction grew leaves, displaying its remarkable survivability and durability.

​In general, insects are selective feeders, they always seek and select the plants with the most nutrients that their bodies can absorb. In their natural habitat, the Pereskia stem-wilter evolved by only feeding on Pereskia. Here in Africa none of our insects are familiar with Pereskia aculeata and will not feed on them. Without natural pests, Pereskia will outcompete our indigenous plant species and spread rapidly. 

​The public is advised to learn how to identify Pereskia aculeata and not to plant and grow it within their gardens. In the past, Pereskia was used as a hedge plant in the garden because it grows so effortlessly. If the Pereskia aculeata is removed by hand, all the biomass that is removed and disposed of will resprout. Wherever it ends up, it will create a new infestation so it is advised to let the biomass dry out, or even better, spray it with herbicide and once it turns brown, it should be burned.

​Dr. Paterson states that the Pereskia biocontrol will not immediately eradicate the whole alien plant, the beetles must first establish their population. Once established, the Pereskia infestation will slowly deteriorate over time as the beetles feed on the shoots and roots. This will decrease the spread of the plant and reduce the Pereskia infestation.

​The biocontrol is funded by the Department of Fisheries, Forestry, and the Environment (DFFE) and is provided by the Centre for Biological Control (CBC) of Rhodes University.
 
The Grootvadersbosch Conservancy can also be contacted if Pereskia is observed so we can track its spread and the success of the agent in its control.

​With the help of Dr. Paterson and the biocontrol agents, we are doing all we can to remove Pereskia aculeata!
 
We thank Dr. Paterson for his time and knowledge regarding Pereskia aculeata and the tremendous journey in creating a biocontrol agent!

Last week, some team members went to the mountains above Suurbraak to apply a biocontrol agent to invasive hakea trees. Silky hakea (Hakea sericea) is an evergreen tree with very prickly leaves. It invades mountain fynbos as well as coastal grassland. A native of Australian, it was once cultivated for dune cultivation and hedging. Hakea trees usually grow fast and produce a lot of seeds. They form large stands and seeds can be transported by the wind for long distances, and new infestation develop quickly in neighbouring valleys. 

There are large infestations in the coastal mountains of the Cape floral region in the Western Cape. Many infestations occur in remote or inaccessible areas and so invasions can become extensive before they are noticed. A further challenge is that fire increases the prevelance of the population. For remote mountainous areas, bio control is most effective. Regular follow up is also always essential to keep the aliens in check. 

​Biocontrol makes use of a plant's naturally occurring enemies to reduce the invasive impact of a species. There are many different bio control agents that can control hakea but we used an indigenous fungus that has successfully been used to control hakea. The biocontrol was applied to the adult trees by spraying it on to an exposed wound which was made with a simple, custom-made tool (a plank with exposed nails). 

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We are combining our approach to alien clearing in the high mountain areas with the release of biocontrol agents. Biocontrol makes use of an invasive plant's naturally occurring enemies to reduce the invasive impact of the species. The approach reunites weeds with their naturally occuring enemies, thereby achieving more sustainable weed control. Biological control is best combined with other mechanisms (mechanical and chemical) to achieve a multi-pronged approach.

We are specifically concerned with the prevalence of silky hakea in the upper mountain areas. In October 2020, we received training on the applicability of different biological control on silky hakea. The training was completed by Dr Wood from the Agricultural Research Institute. We have now ordered the appropriate hakea fungus which will assist with control across the conservancy.​