University of the Witwatersrand – PhD
On the savannas and grasslands of South Africa, a biological war is raging. A South American invader indiscriminately slaughters local vegetation, scattering its seed across the landscape like tiny land mines. In parts of the country, it has won the battle: famine weed, as it is commonly known, dominates the landscape in fields of leafy green and floral white. These plants stand triumphant, densely packed in battalions throughout afflicted parts of South Africa. And the invading army marches on.
Viewed as one of South Africa’s most economically and ecologically damaging weeds, famine weed, scientifically known as Parthenium hysterophorus, has invaded swathes of KwaZulu-Natal, Mpumalanga, Limpopo and the North West. If it is not stopped, famine weed will invade all but the driest parts of South Africa, with catastrophic consequences for national food security, biodiversity, and animal and human health.
Similar in appearance to a bunch of carrot leaves, this inconspicuous daisy-like shrub survives for only about seven months during spring and summer – but this is more than enough time for it to lay siege to an area. A short lifespan and rather feeble appearance mean nothing when a single plant can produce 25 000 seeds, enough to fill half of Ellis Park stadium with famine weed. These seeds are the real scourge, and like mines spread across the landscape, they remain active for up to 10 years, ready to detonate at the first sign of rain.
Equipped with a chemical arsenal, famine weed uses self-made herbicides to prevent other plants from growing near it. It poisons the ground and pillages its resources, all while the plant recruits more of its kind.
However, the onslaught doesn’t stop there, famine weed – as the name suggests – has the potential to wreak havoc in agricultural settings. Despite its seemingly edible appearance, the entire plant is toxic to wild game and livestock, and is fatal if ingested in high quantities. For commercial and subsistence farmers, a famine weed invasion spells disaster, severely reducing crop yields, contaminating produce and making entire harvests inedible and unusable.
For people unfortunate enough to come into contact with the weed, an encounter can lead to burning rashes, skin lesions, blisters, and asthma. These conditions may increase in severity and duration with prolonged exposure to the weed.
Realising the multitude of threats posed by famine weed, South Africa initiated the continent’s first control programme in 2003. Rather than trying to eradicate this invader using costly herbicides or hazardous manual clearing, the country opted to use biocontrol. Biocontrol imports the weed’s natural enemies, often insects, from other countries where they’re endemic, to prey upon the weed. Like expertly trained soldiers, these agents are safe and can identify and attack their target in the crowd, without collateral damage.
Since 2013, South Africa has released three agents, two weevils and a beetle, to control famine weed. Each of these specialised insects attacks a different part of the plant – one has a taste for the leaves, another the stems, and the final one, its flowers and seeds.
The relatively recent release of these insects means biocontrol efforts against famine weed have not yet reached their full potential. But our research into the voracious leaf-feeding Zygo beetle shows promise in reducing the spread of the weed. Similar in size and shape to a large ladybird, these black and beige beetles, have in some areas, overwhelmed famine weed and left nothing more than the weeds’ skeletons in their wake.
Researchers have high hopes that with time all three control insects will establish themselves throughout the most problematic regions of South Africa, steadily increasing in their numbers and in their damage, to the famine weed scourge.
But in the meanwhile, researchers are not resting on their laurels. My research at Wits University, in conjunction with the Agricultural Research Council’s Plant Protection Research Institute, aims to understand the weed’s invasion and the impact of the biocontrol insects that we release to stop it. Even though biocontrol insects are battling famine weed in the wild, my research also explores other ways to thwart the spread of this destructive weed.
Directly attacking the plants is only one front in our war against famine weed. Our studies suggest that good land management is paramount. Well-managed lands may act as strongholds, warding off famine weed invasions. By contrast, poorly managed areas allow the weed to invade uninhibited – in severe cases, we have discovered as many as 100 plants and 35 000 seeds in a single square metre.
We have devised two strategies to combat these vast famine weed seed banks: forcing seeds to compete for survival as soon as they germinate or, more directly, destroying them where they lie buried.
Fighting famine weed with indigenous grasses holds promise in curbing the weed’s rapid spread and recruitment. Many of our grasses grow at the same time as and in a similar way to famine weed, which makes them useful natural competitors. On the ground, native grasses are heavily outnumbered by the weed, but if we intentionally overseed an area with local grass seeds we may offer “our guys” a fighting chance.
Our preliminary studies show that by increasing an area’s grass cover through seed sowing, we can slow the spread of famine weed. The use of native grasses could also help us to restore natural vegetation previously lost to the weed.
Fire is another option we are investigating. Much like a strategic airstrike, the passage of fire should kill off any famine weed seeds lurking on or beneath the soil surface. However, researchers worry that fire might hinder ongoing biological control efforts.
Many of the biocontrol insects we use hibernate just a few centimetres below the soil surface during winter, so if fires are too intense they may become unwanted casualties. But if our fires are not intense enough, they will fail to kill off famine weed’s extensive seed banks. This means we will have to carefully time and control our fires to get the best results.
Although famine weed is not yet under complete control, our research is well on the way to showing this relentless invader that it is unwelcome here.