Tiny weevils are waging war on the invasive water lettuce plant choking South Africa’s Vaal River
Retrieved on:
Tuesday, April 9, 2024
Research suggests it may have originated in South America because it has natural enemies there which have co-evolved with the plant.
Key Points:
- Research suggests it may have originated in South America because it has natural enemies there which have co-evolved with the plant.
- However, fossil records and ancient Egyptian hieroglyphics depicting water lettuce indicate that it may have been present in other regions for millions of years.
- Recently, water lettuce has invaded one of South Africa’s most important rivers, the Vaal River, on the border of the Gauteng and Free State provinces.
- My team and I are currently working with Rand Water on an integrated management plan for water lettuce control in the Vaal River.
The damage
- This can reduce light penetration and oxygen levels in the water, negatively affecting all aspects of aquatic life from microscopic plankton to large fish.
- The mats can also impede water flow, leading to stagnation and increased mosquito breeding sites.
- They can also damage other vegetation.
The weevil
- This Brazilian weevil species was first introduced to Africa in 1985 via Australia, following successful control of water lettuce infestations there.
- Since then, it has been used to control water lettuce infestations in Botswana, Benin, Ghana, Senegal, Zimbabwe, Zambia, Republic of Congo, Côte d’Ivoire, Kenya, Nigeria, Togo, Mozambique and Morocco.
- Invasions at sites in these countries, no matter how extensive, were generally brought under control within a year.
The Vaal River
- However, at the end of 2023, a large infestation was noticed on the Vaal River and was reported to relevant authorities.
- Since then, the infestation has covered up to 40km of the river in the Vaal Barrage area, around the town of Vanderbijlpark, and threatens to spread downstream of the 1,200km long Vaal River.
- Thousands of weevils have already been released into the Vaal River since November 2023 from our mass rearing facility in Makhanda.
- What lurks alongside this invasion on the Vaal River, however, is the water hyacinth, which remains South Africa’s most problematic aquatic invasive plant.
Julie Coetzee receives funding from the National Research Foundation. She is affiliated with the South African Institute for Aquatic Biodiversity (SAIAB).