No. 3, April 2010
CGIAR Systemwide Program
on Integrated Pest
Management (SP-IPM) is a
global partnership that draws
together the diverse IPM
research, knowledge and
expertise of the international
agricultural research centers
and their partners to build
synergies in research outcomes
and impacts, and to respond
more effectively to the needs
of farmers in developing
SP-IPM Technical Innovation
Briefs present, in short, IPM
research findings and
innovations for the
management of pests, diseases
and weeds in agricultural
This and other IPM Briefs are
Invasive floating water weeds – killing life and commerce
Ajuonu O, Tamo M, Neuenschwander P, Toko M, Beed F, Hounkpe C
Weeds by definition are plants that grow in the wrong place. When their seeds or other plant parts are transported to other regions
where their natural enemies are absent, they can multiply unhindered. Indigenous plants, especially those that are adapted for
invading disturbed areas, can also become weeds. The first category is a particularly good target for classical biological control.
Insects, mites and micro-organisms that feed on them are imported from their original area and released against the new invader.
Against indigenous plants however, biological control is far less promising.
By the end of 1980s, many of the water bodies in West Africa were invaded by alien plant species considered to be among the
world’s worst aquatic weeds: water hyacinth Eichhornia crassipes, water lettuce Pistia stratiotes, and water fern Salvinia molesta.
They were accidentally or deliberately introduced as ornamentals or for use in aquariums from their native range South America to
many parts of the world where they have become invasive.
In their new geographic range, aquatic weeds multiply rapidly through both vegetative and sexual reproduction; favoured by the
absence of their natural enemies and eutrophic nutrient-rich conditions. Consequently, large biomass is produced, most times
covering the entire surface of water bodies. The plants threaten the survival of lakeside and riparian communities, kill other aquatic
life by blocking out light, harbour the carriers of such diseases as malaria and bilharzia, impede transport, threaten biodiversity,
and stifle commerce, fishing and irrigation. In the Congo Basin and in Nigeria,
floating water weed have even led to the abandonment of settlements along
rivers and lagoons, which became inaccessible.
According to a 2004 report by the African Development Bank (AfDB), annual
losses in West Africa vary from US$28-56 million for fisheries, US$4-6 million
for health, US$7-14 million for hydro-energy, and US$36-76 million for
agriculture. In 1999, the invasion of water fern threatened the rich bird life of
the Senegal River Delta, a World Heritage Site that has been designated a
Wetland of International Importance under the Ramsar Convention (Diop and
Water way blocked by water hyacinth. – F. Beed
Floating water weeds can be controlled by several methods, which all have their drawbacks. They include physical means, i.e.,
hand weeding - the oldest method - or diverse mechanized equipment, chemical herbicides, and biological control. The latter
consists of the release of host specific natural enemies, usually insects, mites, also pathogens, introduced from the native range
of the weed into the invaded area to reduce the abundance of the weed to a level at which it no longer causes a problem.
Utilisation of the water weeds is sometimes considered as a means of control, because large amounts are removed. Some species
can be used for the production of biogas, as feed for livestock, for the production of paper, as organic manure, for the treatment of
polluted water, and for making ropes, mats, crafts, furniture, etc.
Past control initiatives
Physically removed water hyacinth (foreground)
is quickly replaced(background). – F. Beed
Physical removal and herbicides were first seen as the means of choice,
but both methods were not sustainable as they only provided temporary
control. During a conference in 1988 in Nigeria, participants opted for
biological control as the most appropriate method. In Australia, South
Africa, Sudan and USA, successful biological control of the three weeds
had already been documented. Therefore, in collaboration with other
institutions, the International Institute of Tropical Agriculture (IITA)
implemented the first classical biological control of floating weeds in West
Africa. In 1991, the weevil Neochetina eichhorniae was released against
water hyacinth in the Ouémé and Niger watersheds in Bénin (Van Thielen
et al., 1994). Releases of the weevils Neohydronomus affinis against
water lettuce in Bénin, and Cyrtobagous salviniae in the Congo against
water fern, followed thereafter.