Prof Martin Hill
Invasive plant species can be controlled in a safe and sustainable way says Professor Martin Hill, an entomologist at Rhodes University, South Africa. No pesticides, no manual labour, just weevils. Hill says that the massive alien plant problem, which chokes lakes and damages biodiversity in vast areas of Africa, may be controlled quite simply by importing and propagating the plants' natural enemies from the region where the alien plant species originated. Simple? But getting the plan to work requires some goodwill.
The rivers and lakes of Africa have been subjected to invasion by alien aquatic vegetation since the early 1900s. Water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), salvinia (Salivinia molesta), parrot's feather (Myriophyllum aquaticum) and red water fern (Azolla filiculoides), all native to South America, have been recorded in many countries in Africa.
Vast mats of these weeds have affected just about every lake or river in sub-Saharan Africa says Hill. Apart from severely degrading aquatic biodiversity, these weeds have serious economic consequences, hampering the use of water in agriculture and industry.
The control of red water fern near Cradock in the Eastern Cape. Before and after the introduction of the weevil, Stenopelmus rifinasus.
These problem plants pose a huge threat especially in developing regions. They limit biodiversity and endanger food security.
Although these weeds cannot be eradicated, a number of options exist for controlling them. These include the traditional methods of manual removal and mechanical control, and the use of herbicides. While these methods do work, they are expensive and often inappropriate, especially in high biodiversity areas, where indiscriminate 'no-target' effects cannot be tolerated. The search was on for alternative methods.
Enter biological control
A breakthrough in research came from South America, the natural habitat of the water hyacinth. Scientists discovered two weevils and a moth that were able to destroy this weed. These biological control agents have been released into water hyacinth infested areas in several countries and found to be successful.
The most dramatic example of this has been the reduction of water hyacinth on Lake Victoria from more than 20 000ha to less than 2000ha within five years of the release if two weevil species.
The control of water hyacinth on Lake Victoria at Kisumu in Kenya. The left photographs was taken 2 years after the introduction of the two weevils, Neochetina eichhorniae and Neochetina bruchi and the bottom photogtaph some 12 months later (photos Mic Julien, CSIRO, Australia).
Biological programmes to control aquatic weeds, initiated in many areas of Africa from the early 1980s onwards, have successfully brought all of these weeds under control. In some areas however, water hyacinth remains problematic and research continues.
Hill has been researching a grasshopper, Cornops aquaticum, as a possible biological control agent for use in South Africa. This grasshopper from South America may not work in the USA, as it also feeds on certain indigenous plants, but may work in South Africa - it appears that Cornops prefers water hyacinth to South Africa's indigenous plant species.
But how easy is it to turn the science into a successful reality? Hill says that the single most important factor in the success of the biological control programmes has been and will continue to be people power. Turning science into success has involved dedicated individuals who understood the potential of biological control and who drove the projects. According to Hill, these people engaged with communities and involved them in mass rearing and distribution of insects into affected bodies of water.
Biological weed control, says Hill, became relatively simple once the political will was activated.