Aquatic weeds throughout the world pose significant threats to water security. These threats are not only to biodiversity and ecological processes but also land and river communities that rely heavily on water resources – especially in developing countries. Wetlands, rivers and lakes throughout Africa have been invaded by several aquatic weed species.
These include some of the most damaging worldwide, such as water hyacinth, Kariba weed and water lettuce. These aquatic weeds not only threaten one of Africa’s most valuable resources – water – but also the well-being of these communities that rely heavily on the abundant resources that Africa’s waters offer.
Invasive aquatic weeds are an ongoing problem. For example, in South Africa alone, more than 400 sites are infested with alien aquatic plants. Every year new sites are being recorded, some of them having major socioeconomic implications. The impact that aquatic weeds have include:
reducing water storage capacity, specially potable water;
impeding flow and interfering with navigation;
promoting habitat for mosquitoes; and
spreading water borne disease.
Biological control is the way forward
Control technologies exist for these weeds. These are chemical, mechanical and biological control methods. Chemical control involves the use of registered herbicides, which are usually sprayed directly onto the plants. Mechanical control is the use of mechanical harvesters or manual labour to remove plants from the system where they are problematic.
Biological control is the use of host specific organisms, usually insects, in weed biological control. Many herbivorous insects have evolved and developed an intricate relationship with plants. Often, the herbivore is bound to a small group of closely related plant species and even a single species. Biological control takes advantage of this close relationship.
Scientists can spend around five years on host specificity testing, impact assessments and a cost-benefit analysis. Before release, the biological control agent must be host-specific, damaging to the plant population and the benefits must outweigh the potential risks.
These have been widely implemented throughout Africa. Even though these control efforts have been successful, there is often no continuity in these programmes. Many of them are externally funded and when the funding dries up, programmes are discontinued. When more funding becomes available the wheel is reinvented despite extensive research being available.
This is one of the main reasons biological control is the most cost-effective and sustainable method of control. Irrespective of funding cycles, the host specific biological control agents work tirelessly, feeding on their target weed.
One example of biological control is with the invasive plant water hyacinth which has infested Lake Victoria. Aquatic weeds were a massive problem in this region, affecting the local fishing industry. It was brought down from 20,000 hectares to just 2000 over a couple of years using just two biological control agents. These were two weevil species, Neochetina eichhoriae and N. bruchi.
Massive weed infestations in Lake Kariba, where the Kariba weed (Salvinia molesta) got its common name, were reduced through biological control alone. It reached such low levels that any impact to the community and ecosystem are considered negligible.
Time for Africa to come together
With the growing level of research and understanding of the mechanisms behind the invasion of many these aquatic weeds, there is no reason for them to be a problem.
By understanding the processes behind both successful and unsuccessful biological control programmes, current management plans can be adapted for better results. With the ongoing research in biological control in South Africa, new agents are being cleared for release on a regular basis. The most recent being the delphacid leaf hopper (Megamelus scutellaris) against water hyacinth.
One unit in particular is committed to solving aquatic weed problems in Africa using knowledge and capacity grown in Africa.
This research group has already grown capacity in Africa by training master’s and PhD students in aquatic weed biological control from several countries including Mozambique, Ghana, Cameroon, South Africa and Zimbabwe. The group is also in the process of creating a clearing house for the knowledge on the identification, impacts and control of these weeds.
This would involve setting up networks bringing the right people together, facilitating the control efforts and, probably most importantly, sourcing funding. There is a belief that within Africa there is a growing capacity, knowledge and enthusiasm to control these weeds. In line with the Millennium Assessment Plan, knowledge created in Africa to solve Africa’s aquatic weed problems is the way forward.
Initially, the clearing house will be set up in South Africa for the spreading of appropriate technology and knowledge. That is with the hope that Africa will take responsibility for its own aquatic weed problems.
The vision and ultimate goal is for there to be aquatic weed combatants at institutions around Africa. The knowledge grown in Africa should be freely available for people though out the continent willing to take responsibility of a problem in their country, region or maybe just the pond in their back garden.
As long as Africa comes together, there is no reason for aquatic weeds to still be a problem.