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Rhodes University’s Centre for Biological Control (CBC) recently released biological control agents to combat two invasive cacti species in Namibia.
On 23 August, the CBC joined the Namibian Chamber of Environment (NCE) and the Botanical Society of Namibia, to help them fight two non-indigenous cacti species that have been causing environmental and agricultural damage in Namibia.
“These cactus species are among the worst weeds in the country,” explained Dr Iain Paterson, Biocontrol Research Officer at the CBC. According to Dr Paterson, none of the cactus plants are indigenous to the area, and are thought to have originated from either North or South America.
“These plants are desirable garden plants, as they are easy to grow and don’t need much attention. However, these qualities have also led them to proliferate at an alarming rate and they can now be found growing in dense thickets all over Namibia,” he said.
This is a serious problem, said Dr Paterson, because indigenous plants and animals are excluded by the cactus and land devalues as the cacti reduce grazing capacity. “Wildlife and livestock are also injured by the spines of the cactus, adding to the problem,” he stated.
The cactus invaders the CBC is targeting in Windhoek and the surrounding areas are the Devil’s Rope Cactus and The Lady of the Night Cactus. “Both form dense and impenetrable thickets in much of the veld surrounding the capital city. NGOs, such as the Cactus Clearing Team, have done a fantastic job manually removing cacti so far, but today they gained a new weapon to help fight the invaders,” Dr Paterson explained.
The biological control agents that were introduced in Namibia are host-specific natural enemies of the cacti, and originate from the species’ source, the Americas. One of the agents is a cochineal insect and the other is a mealybug. “The way it works is these insects feed on the cactus plants, which slowly kill them. And since the insects are host-specific (they exclusively feed on a specific species of plant), the biocontrol agents will die too.” Both of these insect species have been extensively tested on Devil’s Rope Cactus and The Lady of the Night Cactus in South Africa and Australia, and the research proves their ability to successfully control both invasive cacti.
The two biological control agents were released at two sites near Windhoek, with the consent of the Ministry of Agriculture, Water and Forestry following an extensive Environmental Impact Assessment.
Dr Paterson released the agents in the presence of excited onlookers, including students from the Namibian University of Science and Technology, BotSoc and NCE members.
“We know that these agents are safe and will only kill the cactus, but this process takes a matter of years, not weeks or months, so we must be patient,” Dr Paterson addressed them.
Although biocontrol takes time (approximately 5 years for Devils Rope Cactus), the results are permanent and self-sustaining. “The cactus will never be completely eradicated – there will always be one or two plants left over. However, these will act as sites for the agents to live, and whenever the cacti populations start to increase again, the agents will get on top of them,” said Dr Paterson.
Biological control is an environmentally-safe, effective and sustainable method of controlling cactus weeds in Namibia. “These two agents are the first biological control agents to be released in Namibia for over 30 years, so it is a significant step,” said Dr Paterson.
Biological control and the Cactus Clearing Project will continue to work together to control cactus infestations around Windhoek, improving its ecosystems and allowing its indigenous biodiversity to thrive.