Tiny weevils are waging battle on the invasive water lettuce plant choking South Africa’s Vaal River

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Water lettuce (Pistia stratiotes L.), also referred to as Nile cabbage, is a free-floating aquatic plant from the household Araceae, the identical household because the arum lily.

It’s discovered on each continent besides Antarctica and grows properly in tropical to sub-tropical climates. Analysis suggests it might have originated in South America as a result of it has pure enemies there which have co-evolved with the plant. Nevertheless, fossil data and historical Egyptian hieroglyphics depicting water lettuce point out that it might have been current in different areas for hundreds of thousands of years. It was possible unfold all over the world by early colonists as a decorative plant for ponds and aquatic gardens.

Right now, water lettuce is taken into account an invasive species in lots of elements of the world, together with African nations, the place it has brought on important unfavorable impacts on aquatic ecosystems and human actions.

Just lately, water lettuce has invaded one in all South Africa’s most essential rivers, the Vaal River, on the border of the Gauteng and Free State provinces. This has raised enormous issues for native communities, companies and stakeholders, in addition to Rand Water. Rand Water is the most important bulk water utility in Africa and is among the largest on the earth, offering bulk potable water to greater than 11 million folks.

An expanse of water is covered almost entirely with what looks like a mat of vividly green grass
Water lettuce blankets the floor of the Vaal River.
Julie Coetzee, Creator supplied (no reuse)

I’m the deputy director of the Centre for Organic Management at Rhodes College, the place I handle the organic management programme on aquatic weeds in South Africa. My crew and I are at the moment working with Rand Water on an built-in administration plan for water lettuce management within the Vaal River. This comes after good outcomes in controlling water lettuce in different elements of South Africa and in neighbouring nations resembling Zimbabwe and Mozambique since 1985 – because of a small species of weevil.

The injury

Water lettuce types dense mats on the water floor. This may cut back mild penetration and oxygen ranges within the water, negatively affecting all facets of aquatic life from microscopic plankton to massive fish. The mats may also impede water stream, resulting in stagnation and elevated mosquito breeding websites. Water lettuce can clog agricultural irrigation canals too. Its speedy progress may also intervene with fishing and boat navigation.

Administration of water lettuce can embrace guide removing and using herbicides to stop unfold. Herbicides had been routinely used to fight water lettuce in South Africa within the early Nineteen Eighties, and are nonetheless relied on closely within the US, significantly in Florida.

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Nevertheless, these strategies are labour-intensive and infrequently inadequate to regulate the plant’s speedy progress. They will additionally injury different vegetation.

That’s the place organic management is available in. This entails the introduction of pure enemies like bugs or pathogens, which can assist handle the plant’s inhabitants extra sustainably and successfully. By importing and releasing an acceptable organic management agent, such because the water lettuce weevil Neohydronomus affinis, the unfavorable results of water lettuce on the surroundings and native communities might be mitigated.

The weevil

This Brazilian weevil species was first launched to Africa in 1985 through Australia, following profitable management of water lettuce infestations there. The then Division of Agriculture gave permission to launch it in South Africa and 500 weevils had been launched right into a closely invaded pan within the northern Kruger Nationwide Park.

Since then, it has been used to regulate water lettuce infestations in Botswana, Benin, Ghana, Senegal, Zimbabwe, Zambia, Republic of Congo, Côte d’Ivoire, Kenya, Nigeria, Togo, Mozambique and Morocco. Invasions at websites in these nations, irrespective of how in depth, had been usually introduced beneath management inside a yr.

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The impression of N. affinis on water lettuce populations is critical: the mixed feeding actions of grownup and larval weevils trigger substantial injury to the crops, lowering their progress and reproductive potential. Grownup weevils chew small holes within the leaves, whereas larvae tunnel by means of the leaves, inflicting them to waterlog and sink.

The flexibility of N. affinis to supply a number of generations per yr allows it to rapidly construct up populations and preserve stress on water lettuce populations over time, making it an efficient organic management agent for managing water lettuce in affected areas.

As a host-specific insect, N. affinis poses little danger to non-target species or the surroundings. Organic management of water lettuce in Africa is taken into account one in all probably the most profitable programmes within the struggle towards invasive species.

So, how are these highly effective weevils getting used within the Vaal River?

The Vaal River

Water lettuce was first recognized on a tributary to the Vaal River in 2021, however native circumstances (floods and chilly winters) appeared to have restricted the unfold of this plant. Nevertheless, on the finish of 2023, a big infestation was observed on the Vaal River and was reported to related authorities.

Since then, the infestation has coated as much as 40km of the river within the Vaal Barrage space, across the city of Vanderbijlpark, and threatens to unfold downstream of the 1,200km lengthy Vaal River.

Rand Water is following an built-in technique to regulate and cut back the invasion. Organic management, utilizing the water lettuce weevil, is vital to the long run administration of the water lettuce invasion. The Centre for Organic Management at Rhodes College is working intently with Rand Water to make sure a continuing and plentiful provide of the weevils – to take action, the centre has established weevil rearing stations.

A group of ten people posing, smiling, alongside large pallets containing big leaves

The crew that rears weevils in Makhanda on the Centre for Organic Management.
Julie Coetzee, Creator supplied (no reuse)

1000’s of weevils have already been launched into the Vaal River since November 2023 from our mass rearing facility in Makhanda. Weevils are additionally being reared by companies and residents who reside close to the river, in addition to Rand Water. The weevils might be launched steadily and en masse, at essential instances, significantly after winter when the crops will germinate from seeds. That is termed inundative organic management.

Water lettuce is among the simpler invasive aquatic crops to regulate, biologically – quickly the infestation might be beneath management. What lurks alongside this invasion on the Vaal River, nonetheless, is the water hyacinth, which stays South Africa’s most problematic aquatic invasive plant. It’s a tremendous competitor, thriving within the nation’s nutrient wealthy waters. Efforts are underway by the Centre for Organic Management to focus on this menace. The standard of water upstream from the Vaal must be urgently remediated, as that is the final word reason behind each the water lettuce and water hyacinth invasions.


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