By Conrad Mwanawashe
ZIMBABWE is winning the war against aquatic Alien Invasive Species (AIS), one of the drivers of biodiversity loss, that have threatened food security and human livelihoods due to their impact on agriculture, forestry, fisheries and human health.
However, Zimbabwe, known to have diverse habitats and vegetation ecosystems, sees biological control of AIS as one of the potential management strategies that is sustainable.
Aquatic invasive alien plants that have been threatening Zimbabwean water bodies include Kariba weed (Salvinia molesta), Water hyacinth (Eicchinia crassipes), Water lettuce (Pistia stratiotes), Parrot’s feather (Myriophyllum aquaticuum) and Red water fern (Azolla filliculloide).
Among these aquatic weeds, Kariba weed and Water hyacinth have emerged as problematic aquatic weeds in most of the country’s water bodies. Kariba weed was first observed in Zimbabwe in 1955 in Kariba dam. Efforts were made to control the weed and the exercise was very successful using the bio control agent, Salvinia weevil (Cyrtobagous salviniae).
Water hyacinth has been reported to be present in Zimbabwe as early as 1940 and has subsequently spread to other parts of the country. These aquatic weeds are known to invade water bodies reducing aquatic available oxygen, restricts recreation, fishing, clog irrigation infrastructure, alters water quality and reduces dam capacities.
“Of late we have been receiving some reports of resurfacing of the weed. Then it prompted us to start mass breeding of the biological control agent which is Kariba weed weevil. We are multiplying this agent so that we harvest and introduce it in affected water bodies so that we manage them sustainably.
“We are not recommending use of chemicals in management of water weeds because chemicals are costly and they can affect or they threaten our non-invasive species, in other words, our biodiversity and aquatic ecosystems will be affected. They affect aquatic ecosystems integrity and the beneficiaries of those water bodies will fail to access the services offered by those ecosystems,” said Nathan Sangombe, Research Officer, Plant Protection Research Institute (PPRI).
PPRI principal fields of research are:
- Plant Pathology
- Plant Entomology
- Plant Nematology
How biological control works – Setting nature upon nature
Biological control is a natural method of control, using natural means to address problems that occur in nature.
The balance that we see in nature is as a result of the interactions, between the plants, animals, and other physical factors. They work together to regulate each other.
Therefore, biological control, is just trying to mimic nature.
There are organisms that feed on plants, thereby reducing or regulating the population of those plants and that is the basis of biological control.
“We have weeds that originated from somewhere without their enemies, their natural enemies, which are also called antagonists. So, when we introduce biological control, we are trying to reestablish that relationship between the invasive alien weeds and their co-evolved natural enemies, co-evolved in the sense that they developed together such that their relationship is so intricate that those organisms only feed on that particular invasive alien plant. So, this is what we will be trying to achieve,” according to Dr Gerald Chikowore a Researcher from Rhodes University.
Dr Chikowore said advantages of biological control include that it’s a self-perpetuating technique. Once you introduce a biological control agent and it establishes, what happens is that it’s now there for good. It’s now working in sync with its natural host.
“So, you don’t have to go in there again to reintroduce over and over again once you’ve reached the level of control that is necessary. Of course, at first, you might want to build up higher populations so that you impact on the weeds quite quickly. But once the system is now set up, it becomes self-perpetuating,” he added.
Biological control is cost effective
“When you look at the cost of biological control versus chemical control, you would find that biological control is much cheaper. You may be alarmed by the higher initial capital outlay that you invest into research to develop a biological control agent.
“But as you go further, the cost-benefit analysis actually improves, because once you’ve introduced it, it’s established, you’re no longer incurring costs on the same particular weed again. When you use chemicals, chemicals by nature are toxic not only to the plants, but to other life forms.
“As opposed to biological control, which is only targeted towards the target weed, chemical control can have some other blanket effects, some unintended effects, some non-target effects. You need to avoid that, particularly when you’re dealing with aquatic environments where you’ve got fish living in it, you’ve got other microorganisms living in there. and you also have other plants that are native to this region, that exist in those particular ecosystems.
“Therefore, you don’t want a solution that when you apply it, it just kills everything. We want something that is selective, that is specific. That’s where biological control comes in.
“When you compare biological control with mechanical management, you’ll find that mechanical control is labour intensive and the equipment that is required to achieve the level of control that you desire might be quite expensive.”
“Also, just like chemical, it’s also recurrent. You have to go back over and over again. And it would take you ages, or it would be like a lifelong commitment to keep on managing a particular water body. Whereas when you’re dealing with biological control, once established, once the level of sufficient level of control has been achieved, it becomes sort of permanent.
“From the human aspect, when you’re dealing with water, you wouldn’t want water that is always mixed with pesticides, with chemicals, yet insects dwell on their plant, they feed on their plant, kill their plant, and if there’s no more food, starvation takes over, their populations diminish,” said Dr Chikowore.
Plant Protection Research Institute has successfully managed water bodies in Borrowdale Brooke and Chinyika Dam in Goromonzi and is currently managing the Shangani Dam where the biological control agent was introduced and it established well.
The Kariba weed has a potential to double its biomass within four to ten days and can also form some thick mats and can completely cover a water body.
Role of research in weed management
“In finding solutions to problems research is very important, especially because you really need to start doing your work in the laboratory, and then you do it in the glasshouse to check whether your bioagent is working. Then you can spread it in the field when you are quite sure that it won’t have any effect to non-targets,” said Plant Protection Research Institute Acting Head Mrs Grace Hama.
For the studies host-specific insects are used so that when introduced that biological control agent, will only target the target species.
Studies are also carried out before the biological control agent is released into water bodies, host-specific studies, where different species that are evolutionary related from closely related to distantly related are taken. This is referred to as the phylogenetic relationship which describes the evolutionary connections between organisms, typically visualized as a “tree of life” or phylogenetic tree. This is important so as to eliminate any chances of multiplying a biological control agent or a potential biocontrol agent that can be a pest.
At Chinyika Dam, about 1,400 biological control agents were introduced while at Borrowdale Brooke, 600 weevils were introduced. Then they multiplied.
Biological control requires “a lot of patience” because you need to build higher population densities.
“It’s a game of numbers. When we build population densities, then the impact will be imminent,” said Sangombe, .
When the water hyacinth weevil was released at Lake Chivero, the number of daughter plants, which can act as some seeds were reduced, plant height and flowering of the plant was also reduced.
“Then to augment the efforts of that weevil, we imported a new biological control agent in June 2023, which is called water hyacinth plant whopper. We carried out some specific tests, and we started our releases in September 2023,” said Sangombe.
