It’s a David and Goliath story, an everyday bug vs a deadly queen. In this version, the galling mealybug conquers the queen of the night cactus, succeeding where people armed with pesticides could not.
In 1940, the queen of the night cactus reached South Africa from the American desert. Over half a century later, the treelike cactus had invaded all nine provinces.
The infestations were restricting grazing and throttling indigenous flora in game reserves, posing a danger to wildlife. “The fruit is tempting for wildlife such as giraffes and monkeys but its spines can cause harm,” says entomologist Pippa Muskett, from the Centre for Biological Control at Rhodes University.
Enter the galling mealybug, its natural enemy. In 1990, this “biological control agent” was released onto queen of the night infestations where spraying had made little impact, and its numbers plummeted. Now it is no longer a threat to wildlife and livestock.
More than 120 biological controls have been released in South Africa since 1913 — about 450 in the world — and this case study shows their potential to control alien invasive plants and protect its flora. Effective and safe biological agents are more environmentally friendly, cost-effective and sustainable than the alternatives, such as pesticides.
Muskett presented a new study conducted across three provinces on the killing of the queen of the night at the recent National Symposium on Biological Invasions, organised by Rhodes University’s Centre for Biological Control (CBC) and Sol Plaatje University in Kimberley.
“This is a success story for biological control. We released a safe insect and it works the way it was meant to and relatively quickly. We do not need much follow-up besides moving it to areas where the plants are not infested,” she says.
Invasive species have become one of the five horsemen of the biodiversity apocalypse that is riding down harder and faster upon the world.
— Inger Andersen, executive director, United Nations Environment Programme
Prof Iain Paterson, chair of the symposium’s scientific committee and study co-author, says invasive aliens are a huge problem across the world, from the Kruger National Park to Antarctica. “They disrupt ecosystem functions like freshwater supply, agriculture and human health,” says the CBC deputy director.
The WWF’s 2024 Living Planet Report: A System in Peril released on Thursday identified habitat loss as the most reported threat to monitored wildlife populations – in Africa their average size has declined by 76% in the past 50 years – and invasive species as another major threat. The other perils listed are overexploitation, disease, pollution and climate change
South Africa has 773 invasive alien plant species, 378 of them regulated by government, according to the CBC. “The best way to manage them is to stop them becoming invasive in the first place and to eradicate new invasions before they become established,” says Paterson.
Last chance on earth
Even Antarctica is not safe from invasives, says the symposium's keynote speaker, Peter Convey, from the British Antarctica Survey. “The Antarctica is not all white ice and penguins. There are endemic species which have been there, often for tens of millions of years, in isolation. Now there are a small number of invasive species, about 18, starting to get into Antarctica, and it is the last continent where we still have the potential to control this.”
Education and improved biosecurity could stop the accidental importation of invasive species — which hop aboard with cargo, scientists and tourists — onto the White Continent, Convey says. “About 250 invasive species have been identified across all of the sub-Antarctic and 99% were introduced by us.” He says controlling them needs engagement by the countries which govern the sub-Antarctic islands.
These islands, such as Marion and Prince Edward islands in the Southern Ocean, have problems too with invasive species. On the South African territory of Marion Island, for example, mice introduced about 200 years ago are preying on birds like the wandering albatross.
But birds are not always the prey of invasive species; they can be invaders themselves, as the smart house crow has shown, from South Africa to Kenya. The second keynote speaker, Prof Colleen Downs from the University of KwaZulu-Natal, who edited the book Invasive Birds: Global Trends and Impacts, says there are 31 bird species globally considered invasive.
“We need to try to remove them or try to stop them breeding, but there is no one solution,” she says, suggesting that several strategies are needed to protect native birds.
In South Africa the house crow, common starling and rose-ringed parakeets are among the most common invasive birds. People regard the house crow as ugly and a nuisance, but when it comes to a bird like the parakeet they oppose eradication, says Downs. “People don’t want to get rid of the parakeets because they are pretty but they are also having an impact.”
The public can play a role in protecting birds by using apps like iNaturalist to report locations and by reporting sightings of alien species. “We want to involve citizen scientists,” says Downs.
Invasive birds take over the nesting sites and harm the breeding of native birds, which play a key role in ecosystems such as seed dispersal.
When indigenous plants, animal and birds are threatened, habitats suffer. Mlungele Nsikane, a senior scientist at the South African National Biodiversity Institute (Sanbi), says about 700-million hectares of land in Africa — nearly seven times the size of South Africa — is “already degraded in one form or another”.
“That’s about 25% of the continent,” says Nsikane, noting that South Africa is experiencing high levels of degradation. “People need to clear land for farming, mining and development, but we can help it recover.”
Good research is being done on restoration but there is a lot of work to be done in implementing this on the ground, he says, calling for more funding.
“Restoration is a tool not only to protect nature but livelihoods,” he says, adding that South Africa has examples of successful restoration projects across provinces.
There is a desire for a silver-bullet treatment (for the polyphagous shot hole borer beetle) but this does not exist
— Luke Potgieter from Stellenbosch University's Centre for Invasion Biology
Restoration is necessary in urban as well as rural areas when the invasives come to town. The polyphagous shot hole borer beetle (PSHB) from Southeast Asia — which has been decimating trees countrywide since it was first detected in KwaZulu-Natal — is a famous example of how much damage they can do.
Post-doctoral fellow Luke Potgieter, at Stellenbosch University’s Centre for Invasion Biology, says that certain lists being circulated — of what trees to plant to replace those felled by the PSHB beetle — are unscientific and could make the problem worse. “They contain species that we known are hosts of the shot hole borer,” he says.
Trees to avoid are: species on the Nemba (National Environmental Management Biodiversity Act) alien and invasive species list; the known hosts of the PSHB beetle; and trees that are invasives in places with similar climates to South Africa, Potgieter says.
Some trees in South Africa, such as the English oak, will succumb fast to the beetle, but not every tree will die, he says. “We have seen fever trees which are highly infested seeming unscathed.”
Once the trees are weakened, however, they are more vulnerable to climate change extremes; for example, Stellenbosch lost almost 200 trees during an intense storm in April.
“There is a desire for a silver-bullet treatment but this does not exist,” says Potgieter, warning against snake oil products. The only effective way to get rid of the PSHB beetle is to cut down the highly infected reproductive trees; spraying widely has proven ineffective he says.
Scientists are drawing up a pool of trees to replace the fallen ones, from about 800 species in the tree-planting data set published in the South African Journal of Botany.
“Indigenous trees will be a substantial proportion, but we can’t advocate only for them. We need non-invasive, non-native trees which provide services [like shade in cities] which indigenous trees cannot,” says Potgieter. “We have to reforest what we are losing in a safe manner.”
Oceans also need protection
Trees play a major role in absorbing the carbon emissions that are driving the climate crisis, as do the kelp forests in the oceans — another world where marine invasions are making the worst kind of waves.
“Marine ecosystems are more diverse than terrestrial ecosystems and complex . .and vulnerable to invasions,” says conservation scientist Sebataolo Rahloa, who was a co-ordinating lead author on the 2019 IPBES Global Assessment Report on Biodiversity and Ecosystem Services.
In SA we have had huge success controlling invasive alien trees, invasive alien water weeds and invasive alien cacti
— Prof Iain Paterson, deputy director of Rhodes University's Centre for Biological Control
Climate change, pollution, coastal zone development and invasive alien species are among the major threats he identified. “Almost every plastic we use and every effluent ends in the ocean,” he says.
“Now there are many invasive alien species from different continents in marine ecosystems which are detrimental, [like] the zebra and Mediterranean mussels,” says Rahloa, urging greater protection for the marine life which underpins the blue economy.
He appeals to people living along the coast and estuaries to help reduce pollution and to report any new species they spot. “If they see a species they don’t know, they should alert the authorities so that they can make an identification,” he says.
Biological controls to fight invaders
Stopping new invasions and finding ways to manage the new arrivals was high on the agenda for delegates from universities, Sanbi and the government at the symposium, and Sanbi is leading the way on this, says Paterson.
South Africa’s biological control programme has reduced the threat of more than 45 alien invasive plant species, according to the CBC.
The US, UK and other countries are funding world-class South African researchers to find biological controls to combat invasive alien species in their countries originating here. “South Africa is a greater donor, for example, of invasive grasses than we receive. We give more than we receive,” Paterson says.
Before biological controls are released, they go through rigorous testing in quarantine — much like developing a human vaccine where testing for safety and efficacy is paramount — which takes time and funding.
• 35%: percentage reduction in the proportion of queen of the night cacti (Cereus jamacaru) producing fruit or flowers on average after the release of the galling mealybug (Hypogeococcus sp.), a study on large monitoring sites set up in Groot Marico, North West, Makhanda and uMkhuze from 1990 to 1999 found.
* Source: Study by Pillippa Claire Muskett, Guy Frederick Sutton and Iain Douglas Paterson, Centre for Biological Control, Rhodes University
— NATURAL BORN CONTROLLERS
But government funding for this work has been terminated and this came under discussion at the symposium. Lack of funding puts the work of the biological control research and implementation facilities, such as those in Kariega or Makhanda in the Eastern Cape, and other provinces, at risk.
Asked about funding, Peter Mbelengwa, communications chief director for forestry, fisheries and the environment, replies: “The department is currently exploring co-funding opportunities with other stakeholders to continue the biological control service.”
Biocontrol is important to South Africa’s integrated pest management programme, which aims to reduce the overall risk from pesticides and highly hazardous chemicals “by at least half”, he says.
Paterson says biological control is essential to managing well-established invasive species. “In South Africa we have had huge success controlling invasive alien trees, invasive alien water weeds and invasive alien cacti,” he says.
“Biological controls have changed the trajectory [of plant invasions] so that instead of their numbers getting worse they have got better and better. We are already benefiting, though it might take a couple of generations to see all the benefits.
“This is a long-term game and what we want is to leave a better world for future generations.”
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