Saving a Cameroonian Lake From an Invasive Plant

As floating weeds, salvinia form dense mats on the surface of water bodies, disrupting aquatic flora and fauna underneath and thus hindering water flow, fishing, swimming, boating, water sports, and navigation. (AMMCO)

LAKE OSSA, Cameroon—A Cameroonian lake choked with invasive weeds may be a science project for biologists, but for Charles Elingua, it means starvation.

“I have been fishing in this lake for more than 30 years,” said Elinga, the 56-year-old leader of fishermen in Lake Ossa and father of eight.

“The salvinia weed has disrupted fishing considerably. I once was able to save up to FCFA$10,000 (US$17.99) from fishing daily. But today, it is pretty difficult to even fetch FCFA$1,000 (US$1.80) from the activity, which can hardly afford three square meals for my family.”

Since appearing in the lake in 2016, Lake Ossa, one of the largest natural lakes in Cameroon, has been invaded by a fast-spreading waterweed known as salvinia molesta.

Nearly 70 percent of the lake’s 4,000-hectare surface is now engulfed by this floating aquatic fern, which thrives in slow-moving, nutrient-rich, warm freshwater.

Fishermen have noticed a dramatic decrease in fish stocks. This has a direct bearing on the business of fish sellers. (Nalova Akua/Epoch Times)

But the government of Cameroon has initiated a nature-based solution to control the obnoxious weed. 

More than 6,000 individual black, subaquatic insects measuring 2 to 3.5 millimeters (.07 to .13 inches) long, known as the Salvinia weevil, have been mass-reared for eventual release into the lake.

On July 27, a small quantity of the weevils was released in a portion of the lake, marking an experimental phase.

“They were brought in from the Louisiana State University in the United States with the authorization of the Cameroon government,” said Dr. Aristide Takoukam Kamla, founder of the African Marine Mammal Conservation Organization (AMMCO) in Cameroon.

The insects feed on the bud of the salvinia weed, which, in turn, is likely to lead to the death of the plant.

“The weevils are effective because when the bud is damaged, it will most likely cause the remainder of the plant to die and sink,” said Matthew Purcell, director of the Australian Biological Control Laboratory, a facility run by the U.S. Department of Agriculture and the Commonwealth Scientific and Industrial Research Organisation.

The weed itself is native to south-eastern Brazil and northern Argentina, but has been widely spread throughout the world during the past 50 years. Water currents, floods, animals, and humans have been identified as the main vectors spreading it.

Removing the invasive salvinia manually has proven to be tedious and unrealistic because it regenerates very quickly. (AMMCO)

Human factors are partly to blame for its spread in Cameroon.

“Being the economic capital, the Littoral Region and especially Douala is the most industrialized municipality in Cameroon with about 60 percent of the country’s industries’ discharges often released in the open spaces,” Kenfack Voukeng Sonia Nadège, a Cameroonian weed scientist working with Green Connection, a local environmental conservation non-governmental organization, told The Epoch Times.

“Houses built without proper flushing systems contribute to the increase of the nutrients in the environment.”

Lake Ossa and the Sanaga River—the largest River in Cameroon—are connected by a less than two-mile-long channel, and this has also facilitated its spread, according to Kamla.

“Two main nutrients needed for invasive weeds to grow are nitrogen and phosphorous coming from upstream. Once this river gets polluted, the lake also gets polluted,” he said. 

More than 80 percent of the locals used to depend on fishing for their livelihoods, according to the environmental group Global Water Partnership. But today, doing so is no longer possible.

The African Marine Mammal Organisation (AMMCO) and partners have been at the forefront of mass-rearing the weevils to be released into Lake Ossa.

Biological Control

The water-dwelling salvinia weevil feeds exclusively on the salvinia plant and will die without their host, according to scientists.

This method—known as biological control—has been outstandingly successful in suppressing populations of the invasive plant and restoring water bodies back to ecological balance.

The weevils were first collected by Australian researchers from the native range of salvinia molesta in southern Brazil in 1980.

The water-dwelling salvinia weevil feeds exclusively on the salvinia plant and will die without their host. (AMMCO)

Since its first release at Lake Moondarra, Mount Isa, Australia, in 1980, the salvinia weevil has successfully been used to control salvinia molesta in many Asian, Pacific, and African countries.

“Lake Moondarra is mostly clear of salvinia today,” Purcell said.

“The larvae initially feed on roots then move to the buds, finally tunneling into the Rhizome which can kill the plant; adults feed on all plant parts externally.”

Arnold Pieterse, a senior staff member of the Royal Tropical Institute who has coordinated tropical ecology and weed control projects in various parts of the world, agrees.

“The insect cyrtobagous salviniae [the scientific name of the weed-eating insects] has been very effective at the time against salvinia molesta in the Senegal River. It is host-specific for salvinia and does not form any risk to the environment,” Pieterse said.

Because salvinia plants reproduce asexually, a single tiny plant can “eventually multiply and recover a whole water system,” according to Purcell.

In summer and at high temperatures, the plant will often grow faster than mechanical harvesters can remove the plant.

“The weevil population follows the growth of the weed closely. In spring and summer, salvinia increases and, in turn, the weevil then begins to breed and build up populations that reduces [kills] the salvinia,” Purcell said.

Herbicides—another option of weeding out the invasive alien weed—haven’t yet been tried in Lake Ossa, as they could have adverse effects on living organisms and the environment.

Herbicides and mechanical controls “must be reapplied indefinitely as the plant regrows each season,” Purcell said.

The mass-rearing of salvinia weevil—the natural enemy of the invasive aquatic fern salvinia molesta. The insects are said to have the ability to self-perpetuate and feed specifically on the weed. (AMMCO)

Though considered the most effective method compared to manual removal or chemical control (use of herbicides), scientists insist biological control will likely work in Cameroon, even as drawbacks have been registered in some countries.

“The effectiveness varies from site to site depending on environmental parameters, temperature, nutrient availability and water flow, shade, and so on,” Purcell said.

“Fifty-thousand tons of salvinia on Lake Moondarra were killed by weevils over a 400-hectare infestation. Excellent control was achieved on Lake Moondarra, where within 14 months 200 hectares of salvinia were replaced by open water. This is a best-case scenario. Control in other areas may not be as effective if shaded.”

One drawback with biological control is that the invasive plant never disappears.

“Some salvinia must be tolerated, as this sustains a population of the weevil and complete eradication never occurs,” Purcell said.

On the other hand, Purcell sees brighter days ahead for Lake Ossa, as does Julie Coetzee, deputy director and manager of the Aquatic Weed Biocontrol Program at Rhodes University, South Africa.

“Because Cameroon is tropical, the prospects for successful biological control are high,” she said.

“I would predict that there will be a significant reduction in cover within 18 months, if not less. While the process is not perceived as quick, in comparison to herbicide, it is sustainable in the long term. Patience is key.”

Purcell is also hopeful, but cautions that one of the enemies of control of salvinia and other aquatic weeds is “eutrophication, inflows of chemical fertilizers into aquatic system from agriculture and sewerage, which can stimulate the growth of aquatic weeds, sometimes faster than the weevils can control the plants.”

Those inflows into aquatic systems should also be regulated to improve the chances of control, he said.

This report was supported by the GLF-Climate Tracker Drylands Fellowship.