16 Apr 2014

The Salvinia Molesta weed, which has been ravaging the Chobe, Linyanti and Okavango water systems for decades is now under control, according to the department of water affairs.

In an interview with the head of the Aquatic vegetation control unit in the department of water affairs in Maun, Dr Naidu Kurugundla said the invasive weed was first discovered in Botswana at Kazungula 66 years ago.

Dr Kurugundla said the introduction of a scientific monitoring programme had made a significant improvement to keep the weed under control in many parts of the country. The weed is controlled by introducing small weevils that only feed on it. 

Salvinia molesta, locally known as Mochimbamo weed is a free floating plant that does not attach to the soil, but instead remains buoyant on the surface of the water.

The weed prefers to grow in slow moving waters such as those found in lakes, ponds, streams, ditches, marshes and rivers.

The weed prefers nutrient rich waters such as those found in eutrophic water or those polluted by waste water.

Salvinia Molesta or Kariba weed as is often called after it threatened to clog Lake Kariba in 1970, if left uncontrolled, it can form dense vegetation mats that reduce water-flow and lowers light and oxygen levels in the water.

The dark environment in the water then affects living organisms and other water species, including fish and submerged aquatic plants. The weed is said to be a serious threat to the tourism industry.

It has the ability to alter wetland ecosystems and cause wetland habitat loss, posing a severe threat to socio-economic activities that are dependent on open flowing quality water bodies. The invasive weed originates in the South eastern Brazil.

Giving a historical background of the weed, Dr Kurungndla said the Department of Water Affairs had every reason to be concerned when the weed was first discovered in 1948.

Dr Kurugundla said during those times the biggest challenge was finding the effective control agent for the weed, as those available proved to be ineffective. Botswana was to realise later that the weed had reached the Zambezi basin, threatening the Chobe, Linyanti and Kwando.

Dr Kurugundla said by the early 1970’s the three water systems were completely covered by Salvinia and an improvement was realised after a spraying exercise in 1976, but the weed was still not controlled.

A major breakthrough in the salvinia control mystery was made in 1981 when a certain Australian, Dr Wendy Forno from Council of Scientific Industrial Research Organization was sent to Brazil to find the specific biological control for salvinia molesta.  Dr

Forno found a small weevil that feeds only on the salvinia molesta , which she took back to Australia.

After breeding it, the weevil was released into a completely covered lake and in twelve months the weed had been completely controlled.

Botswana was only to learn about this new development in 1986, and immediately contacted the research centre in Australia to send a scientist who would help in controlling the weed.

Australia sent Dr Forno to Botswana, and she trained people on how to use Salvinia weevils to control the weed.

Unknown to Botswana, the Namibia had in 1983 gone to Australia and imported 450 adult weevils into the Katima Mulilo area and bred them before they were released in the Caprivi at Sitwa. This move was to help Botswana in its endeavor as these weevils then followed the system into Botswana.

In the same year that Dr Forno brought the weevils and trained people on how to breed and use them to control the weed, it was discovered that both Xini and Bodumatau lagoons were completely covered by the weed.

Dr Kurugundla explained that a truck load of salvinia which had weevils was released into the two lagoons and in nine months the two lagoons were cleared of the weed.

He further noted that at the same time, the weed was spreading in the Moremi Game Reserve, through various means such as floods, wild animals, fishermen, boats and canoe movements.

In 1996 he found that total control of the weed had still not been accomplished as there was no proper monitoring programme in place.

Dr Kurugundla then started a scientific monitoring programme that involved surveying all the infested areas, and releasing the weevils there.

The programme also involved setting up monitoring sites in both the Okavango Delta and Kwando/Linyanti systems.

By 2003, all the infestations in all the affected areas had been significantly been minimised, and were being monitored.

He said currently there are 60 salvinia monitoring sites in the Okavango Delta, while the Kwando Linyanti has 49.

To date, Dr Kurugundla observed that the Salvinia weevils and the salvinia weed have reached an equilibrium, which is needed to achieve a complete control of the weed.

He said it meant new infestations would almost immediately be controlled by the weevils that are readily available in the system, adding that monitoring and assessments continue throughout all the sites. ENDS