Dangerous daisy meets its weevil nightmare
One of South Africa’s most destructive weeds is wiping out farmlands, poisoning cattle and making people sick. It’s been doing this for more than a century. Now an army of weevils, fungus and beetles is being bred to fight back.
You’ve probably seen the plant, whose Latin name, Parthenium hysterophorus (Asteraceae), has been swapped out for famine weed because of its effect on food crops.
It grows to the height of an adult and spreads in vast clumps, with the weeds wrapping around each other. In spring and summer it sprouts deceptively beautiful white flowers. Those stick to everything, so tourists carry them across the country and workers take them back home. They love the mud underneath cars and use them to spread along motorways, and then into the veld.
A native of Central and South America, famine weed was first recorded here in the late 1800s.
Its evolutionary advantage is a chemical it leaks out that stops other plants from growing. This mechanism allows it to take over fields, farms and game reserves in a single summer. The animals and ecosystems that rely on indigenous plants and crops either starve or move away. It thrives in subtropical parts of the world and has invaded 48 countries.
Weevils, fungus and beetles are being bred to fight the Parthenium hysterophorus (Photo: Agricultural Research Council)
Much of what we know is thanks to a 40-year battle Australia has fought against the weed. There, it destroyed large tracts of land that cattle would graze on. Where famine weed took hold, food for cattle dropped by between 25% and 80%. The meat from cattle that eat the weed has to be destroyed.
In India and Ethiopia, famine weed has crippled food production in areas. During the past decade, Ethiopia’s yield from sorghum has dropped by between 45% and 80% in places where famine weed has invaded. Some 15 countries on the African continent are thought to have famine weed invasions. Many of them have no plan to deal with it.
The weed also targets humans. The small flowers cause hay fever and asthma, and touching the plant creates sores, rashes and skin lesions. These only get worse the longer people are exposed to them. In South Africa, that’s a problem because famine weed thrives in areas where the land is overused. Where farmers plough already damaged soil and the veld is overgrazed, the soil is vulnerable to a famine weed invasion. Large patches of farmland in Mpumalanga and KwaZulu-Natal have been worst hit.
Famine weed’s Afrikaans name, Demoina bossie, tips a hat to the first big invasion in 1984. Damage caused by Cyclone Demoina, and the excessive rainfall that came with it, created perfect growing conditions.
That’s when the Agricultural Research Council got funding to build a laboratory in KwaZulu-Natal to research famine weed, and other invasive plants. A quarantine facility was added in 2008. A cluster of access-controlled greenhouses act as the nerve centre for the fight to defeat this weed.
Lorraine Strathie works at the laboratory and focuses her full attention on famine weed. “All countries have different conditions but we have been really helped by all the lessons learned in Australia,” she says.
By 2008, the focus was on trying to stop famine weed from spreading.
This involved mostly manual methods such as spraying sections of game reserves and farms to act as barriers. Cars and people moving in and out of invaded areas were cleaned.
The way to control an invasive species is to get rid of its advantage. Famine weed — and other invasives — proliferate because they have no predators. Australia turned to biological control agents. Eleven non-indigenous natural predators were introduced to eat the plant. Each one goes for a different part of the weed, either the stem or its leaves.
In South Africa, the problem was that only one introduced predator had come across with the weed. Strathie and other scientists needed to bring in more predators. The worry is that the predators might then go on and become invasive species in their own right, doing damage that nobody had thought about.
Strathie says: “We only introduce biological control agents when we are 100% convinced that they won’t affect anything else.”
This has involved the researchers carrying out tests on fungus, weevils and other bugs that are in quarantine at the KwaZulu-Natal greenhouses. Each control agent has its own area where nothing is allowed to contaminate the research.
South Africa has a century-long record of not making errors with this type of experimentation, and the researchers have to shower and change every time they enter or leave their laboratory.
For between three and five years, the researchers watch whether the control agent eats anything besides famine weed. The agents tend to stick to the weed because they have evolved to do so.
When the control agents pass the test, they are bred in larger numbers and then released into the wild, after the national environment department signs off on the tests.
The first batch of famine weed control agents were released just before the drought that started in 2014. Many weevils and beetles died. They weren’t ready for such intense weather.
But some hardy variants thrived. About 4 000 have now been released at 330 sites around the country.
The mix of introduced species is important. Strathie says: “It’s unusual to find a silver bullet because plants have many natural predators.” And it takes an army of biological control agents to take down one of South Africa’s most dangerous invasive species.
That army is enjoying success. Australia has managed to shrink the famine weed invasion down to manageable levels — once a species invades it is almost impossible to kill off —and this is where South Africa is headed. By attacking each weed with a combination of predators, populations are being brought under control in KwaZulu-Natal and Mpumalanga.