By Sharon Atieno

Weevils are proving effective in curbing the spread of water lettuce (a floating aquatic plant with leaves that resemble lettuce) along the Naretoi Conservancy, part of the Maasai-Mara ecosystem in Kenya.

Water lettuce – also known locally as ‘yungi’ – can have a detrimental effect on fish nesting sites and mortality among fish and micro-invertebrates. It can also contribute to increased rates of siltation and reduced water flow.

Working in collaboration with staff from the Naretoi Conservancy, Kenya Plant Health Inspectorate Service (KEPHIS) and the Kenya Wildlife Service (KWS), Dr. Arne Witt, Invasives Coordinator, South, Centre for Agricultural Bioscience International (CABI introduced 850 tiny water lettuce weevils (Neohydronomus affinis) from South Africa into infested dams within the ecosystem to control the weeds.

Within just six months of the introduction of the water lettuce weevils into the ecosystem, they have eaten away at the dense mats of water lettuce – causing a dense mat in one dam alone to collapse and reduce its coverage by almost 100%, Dr. Witt reports.

According to him, the water lettuce weevil is paying dividends as a cost-effective biological control agent that could also be introduced to other invaded water bodies in Kenya.

“The water lettuce weevil is proving to be an effective ‘weapon’ against the scourge of invasive water lettuce and I’m convinced it can be utilised effectively elsewhere in the Maasai-Mara ecosystem, and Kenya in general,” Dr. Witt said.

“Not only that but the weevil is a very cost-effective solution to the problem with each release costing considerably less compared to trying to clear the dense mats using manual labour which is largely ineffective and can run into thousands of dollars annually. The latter of which also runs the risk of meeting hippos and crocodiles on the watercourses.”

Water lettuce Photo credits: CABI

A paper, published in Biocontrol Science and Technology, by Malueke et al, found for example, that the biological control of four aquatic weeds (Pista stratiotesSalvinia molestaAzolla filiculoides and Myriophyllum aquaticum) in South Africa was significantly cheaper than chemical control.

The estimated cost of the biological control of the four aquatic weeds was about ZAR7.8 million whereas the estimated cost of chemical control to achieve the same impact varied between ZAR150 million and ZAR1 billion.

Dr. Witt added that progress made by conservationists for the benefit of communities that are actively involved in managing eco-tourism sustainably can be severely hindered by invasive plants such as water lettuce as well as water hyacinth, salvinia and azolla.

CABI has now released or re-distributed at least 10 weed Classical Biological Control agents in Africa in the last 15 years or so.