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Saving A Lake From An Invasive Plant

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

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

“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,” the father of eight lamented.

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 called salvinia molesta.

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

Epoch Times Photo
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 country has initiated a nature-based solution to control the obnoxious weed. 

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

On July 27, 2021, a small quantity of the weevils were 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,” says Dr. Aristide Takoukam Kamla, founder of the African Marine Mammal Conservation Organization (AMMCO) in Cameroon.

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

The plant salvinia molesta 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 constitute main vectors spreading it.

Epoch Times Photo
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 per cent of the country’s industries’ discharges often released in the open spaces,” Kenfack Voukeng Sonia Nadège said to The Epoch Times. Kenfack is a Cameroonian weed scientist working with Green Connection, a local environmental conservation non-governmental organization.

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

Lake Ossa and the Sanaga River—the largest River in Cameroon—are connected by a less than 2 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 per cent of the locals used to depend on fishing for livelihood, according to the  environmental group Global Water Partnership. But today, it 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.

It was first collected by Australian researchers from the native range of salvinia molesta in southern Brazil in 1980.

Epoch Times Photo
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,” says Purcell.

“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.”

Dr. Arnold Pieterse, 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 [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 reproduces asexually, a single tiny plant can “eventually multiply and recover a whole water system,” according to Purcell, the Australian biologist.

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 species—have not yet been tried in Lake Ossa as they could cause adverse effects on living organisms and the environment.

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

Epoch Times Photo
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,”  he added.

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 explained.

On the other hand, the biologist sees brighter days for troubled Lake Ossa, according to Julie Coetzee, deputy director and manager of the Aquatic Weed Biocontrol Programme 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.”

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

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

Nalova Akua

Nalova Akua is a Cameroonian multimedia freelance journalist.

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