Biocontrol agents manage alien plants

April 15, 2021

Invasive alien plants have negative impacts to biodiversity and are damaging to business practices on farms and private game reserves. In the Mountain Zebra Camdeboo Protected Environment (MZCPE), some of the most problematic invasive alien plants are cacti. Cacti are indigenous to either North or South America and were introduced as alien species to South Africa. If alien plants are not controlled, they become more abundant and spread over larger areas thereby increasing negative impacts to biodiversity, agriculture and livelihoods. 


Dense stand of Imbricate cactus on a farm in the MZCPE. (Image: Maryke Stern)

The MZCPE and Rhodes University’s Centre for Biological Control (CBC) have collaborated to control invasive alien plant species in the protected environment and areas surrounding the Camdeboo and Mountain Zebra National Park as part of the MZCPE Alien and Invasive Species Management Programme. 

Methods used to control alien plants include mechanical, chemical and biological control and, in some instances require an integrated control strategy that uses a combination of these methods. Biological control is a long-term, but environmentally friendly method, where the alien plant’s natural enemy is used to control the targeted invasive alien plants. In the case of the cactus plant species it is usually cochineal insects. 


“Gogga” box with biocontrol insects on plants. (Image: Maryke Stern)
Gogga” box with biocontrol insects on plants. (Image: Maryke Stern)

Zeze Mngeta, a PhD student involved in biocontrol research at Rhodes University, explained that biological control is the most effective and environmentally safe method used for the control of invasive alien cactus plants. “This method reduces the negative impacts of cactus weeds which include threatening native biodiversity, reducing agricultural productivity and harmful effects to livestock, wildlife and humans due to the plants’ sharp spines,” she said. 


Biocontrol cochineal on invasive alien, the creeping prickly pear cactus. (Image: Maryke Stern)

One of the main reasons why alien plants become problematic is due to the lack of herbivorous insects that would have naturally controlled their populations in their native environment. Cochineal insects are natural herbivores of cactus species in the Americas, but they do not naturally occur in South Africa. This has been rectified in South Africa by manually releasing cochineal insects as biological control agents. All biocontrol agents are thoroughly tested to ensure that they will not cause any damage to indigenous or useful plant species.  


CBC worker releasing biocontrol on Imbricate cactus. (Image: Maryke Stern)

The CBC has released biological control agents against the Imbricate Cactus (Cylindropuntia imbricata), Creeping Prickly Pear (Opuntia humifusa), Jointed Cactus (Opuntia aurantiaca) and Silver-leaf Bitter-apple (Solanum elaeagnifolium) on several farms and nature reserves that are part of the MZCPE. For every release, certain data is recorded and follow up visits are implemented to monitor the effectiveness. 


Monitoring the impact of biocontrol agents on Imbricate cactus. (Image: Maryke Stern)

A current post-graduate study by a student of the CBC is analysing land user’s perceptions about the efficacy and safety of biological control on cacti. The data collected to date indicates that there has been a decline in the amount of cactus in areas where the agents have been released and that this has resulted in the land becoming more useful to farmers, conservationists and other land-users. 

During lockdown this work didn’t stop. CBC couriered the biocontrol agents to MZCPE landowners allowing them to release it in affected areas on their properties. Some of the farms and reserves within MZCPE now even act as small source populations where other landowners can collect the biocontrol agents to release them on their own farms. Under the MZCPE a database is set up that identifies which farmers have source populations of these species that others could collect from. People are now able to connect over the full MZCPE network beyond just their farmers association and friendship circles.


MZCPE landowner Trenly Spence who farms on Kriegerskraal in the Camdeboo, releasing biocontrol on creeping prickly pear. (Image: Maryke Stern)

Although biological control is slow, it is an effective and environmentally safe method of controlling invasive alien species like the cacti. This assists the protection of our natural biodiversity and agricultural productivity. The use of biocontrol agents in areas surrounding national parks will reduce the spread of problematic invasive alien species into the parks. 


Maryke Stern, the MZCPE Ecologist releasing biocontrol on Imbricate cactus on a farm near Graaff-Reinet. (Image supplied)

James Kingwill, a MZCPE landowner farms cattle and sheep on Coloniesplaats in the Sneeuberg. He remembers when Maryke Stern, the MZCPE Environmental Ecologist and the Rhodes biological control group became involved in the containment and eradication of an infestation of Imbricate cactus. The invasive alien covered an area of approximately 1 hectare on his property. “The biocontrol team came in 2019 and released biocontrol on the plants. To date a significant amount of these plants have died, conservatively I would say in excess of 50%. On the balance of the plants the biocontrol is noticeable, with some outlying plants that still need to be addressed. It is clearly a success and the most efficient way to control the plant, Imbricate cactus,” he said.


The effect biocontrol agents have on Imbricate cactus. (Image: Maryke Stern)

For more information about Biological Control, please contact Dr. Iain Paterson at