Big gains from small creatures
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.
People power
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.
More information:
Professor Martin Hill (m.p.hill@ru.ac.za)
is
based at Rhodes University in South Africa. He will be delivering a lecture at the Sasol Scifest 2004: "Putting
insects to work".
More information on the Sasol Science festival and Professor Hill's talk at: www.scifest.org.za
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