Water lettuce, or Nile cabbage, is a floating aquatic plant that has spread across various continents, and has invaded the Vaal – one of South Africa’s most important rivers.
Originally considered an ornamental plant for ponds, the alien water lettuce has turned into an invasive plant species. This has had significant negative impacts on aquatic ecosystems and human activities.
The invasion of water lettuce in the Vaal River has raised concerns among local communities, businesses, and stakeholders. This includes Rand Water – the largest bulk water utility in Africa serving over 11 million people.
The effects of invasive plants like water lettuce
Water lettuce forms dense mats on the surface of the water, reducing light penetration and oxygen levels in the water.
This affects all aspects of aquatic life, from microorganisms to large fish. The mats can also restrict water flow, and create perfect breeding sites for mosquitos.
Moreover, the invasive plant clogs agricultural irrigation canals, and interferes with fishing and boat navigation.
Traditionally, control methods for water lettuce involved manual removal of the plant, and herbicides. However these techniques have proved to be labour-intensive and damaging for the environment.
Say hello to the weevil
A weevil is a type of beetle belonging to the superfamily Curculionoidea. They are often associated with damaging crops and stored food products, but can also be beneficial and used as a biological control agent to manage invasive plant species.
The introduction of the water lettuce weevil (Neohydronomus affinis) has shown promising results in managing water lettuce infestations. It also does this in a sustainable and effective manner.
Native to Brazil, the weevil species was introduced to Africa in 1985. Water lettuce invasions in several African countries and other sites (including the Kruger National Park) were brought under control within a year, thanks to this tiny beetle.
The weevil has a notable impact on water lettuce populations. Adult weevils chew small holes in the leaves, while the larvae tunnel through the leaves. This causes the leaves to get waterlogged and sink.
“The ability of N. affinis to produce multiple generations per year enables it to quickly build up populations,” Julie Coetzee, deputy director of the Centre for Biological Control at Rhodes University writes in The Conversation.
“This maintains pressure on water lettuce populations over time, making it an effective biological control agent for managing water lettuce in affected areas.”
The Vaal release
Water lettuce was initially limited to a tributary of the Vaal River. However, it later exploded along the river, prompting an integrated control strategy by Rand Water using the water lettuce weevil.
Thousands of weevils have been released into the river since November 2023 from a mass rearing a facility in Makhanda to combat the infestation, with plans for ongoing releases. Weevils are also being reared by Rand Water, as well as by residents and businesses who live near the river.
The presence of the water hyacinth, a highly competitive invasive plant, remains a concern on the Vaal river. Efforts are underway to address water quality issues upstream, which are the root cause of both invasions.
