Humble fly takes on Brazilian invader
● A submerged weed choking dams and rivers in four provinces met its match this week with the release of its natural enemy: a tiny fly from Argentina.
Brazilian waterweed is taking over rivers and dams in the Eastern Cape, Western Cape, KwaZulu-Natal and Gauteng, where the fast-growing plant threatens indigenous species, water quality and water sports.
About half of the Midmar Dam in the KZN midlands, where the world’s biggest openwater swimming event takes place, will disappear under dense mats of the weed by 2030 without intervention, say scientists.
This is the first time in the world an insect (Hydrellia egeriae) has been used to control Brazilian waterweed (Egeria densa), said entomologist Professor Julie Coetzee, deputy director of the Centre for Biological Control at Rhodes University.
On Friday, tanks containing about 10,000 hand-reared larvae were tipped into the Nahoon River in East London by Ahmed Kahn, a director of natural resources in the department of environmental affairs.
The Kouga River and dam, which feed drought-stricken Port Elizabeth, the Bushmans River near Estcourt and the Liesbeek River in Cape Town are among the water sources where the fly will soon be released.
Invading submerged weeds are spreading after success in controlling four of the big five invasive “floating weeds” in SA using biological agents, with hyacinth the exception. Bugs such as weevils have reduced hyacinth at some dams, but high levels of human and agricultural pollution feed the fast-spreading surface weed.
The Brazilian waterweed was probably imported for aquariums, said Rosali Smith, the scientist who has studied the plant and its enemy for three years — testing them and getting permission for the fly’s release into the wild.
The plant, which can grow from a small fragment, was often used in fish tanks and school biology experiments and could spread easily by “hitchhiking”. Smith said: “Fishermen catch it on their hooks or boats or boots, and, without knowing, spread it to the next dam.”
Pulling out the plant does not work because every piece can become a new problem. Biological control takes less effort, costs less, lasts longer and is less toxic than chemicals. The only registered herbicide against this weed would kill other indigenous species.
Before a living agent can be released it must go through stringent tests to make sure it attacks only the target plant and not, for example, an indigenous submerged weed or a waterblommetjie.
Philip Ivey, chairman of the Biological Control Release Application Review Committee for SA, said: “We cannot release an insect if there is a remote chance it will attack an indigenous plant or agriculturally important plant or crop.”
Brazilian waterweed is a huge problem in places such as California and is one of New Zealand’s top three aquatic invaders. New Zealand scientists have been liaising with the centre about the fly’s release and will be monitoring its impact.
“This is not a short-term solution. It will take at least eight months to show results and can take years to be successful,” said Smith.
The researchers also released 200 sap-sucking bugs against the water hyacinth in the Nahoon River on Friday.