Destructive aquatic weed reappears after 30-year absence in Botswana

The Botswana Department of Water Affairs (DWA) has appealed for emergency assistance to fight the resurgence of water lettuce (Pistia stratiotes), an invasive aquatic weed that displaces native plant species from water bodies.

Addressing the 2nd Botswana Biodiversity Symposium held in Maun recently, DWA invasive weed expert Dr. Naidu Kurungudla said the plant has reappeared along the Chobe River for the first time in 30 years.

The last outbreak in the Chobe River ecosystem was brought under control in 1986. Kurungudla said preliminary investigations showed that the latest plant invasion came from Namibia via recent flooding of the Chobe River.

“We are working with relevant water experts in Namibia and Zambia to find out how the weed got here (to Botswana) after such a long absence. At the moment, we want to raise alarm to everyone who can to help control the weed from spreading. It is dangerous to our waters,” he said.

Meanwhile, the department will embark on outreach exercises to teach communities living along the Chobe River basic control measures to stop the spread of the weed. Plans are also being made for the release of weevils, which eat the plant, into the Chobe ecosystem.

Among other impacts on waterways, overgrowths of water lettuce can block gas exchanges at the water interface, leading to fish deaths due to a reduction in oxygen. Large mats of overgrowth also block light, shading out native submerged plants. It also alters immersed plant communities by crushing them and mosquitoes lay their eggs under its thick leaves.

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