Introduction
Water hyacinth, Eichhornia crassipes, is a free-floating (but
sometimes rooted) freshwater plant of the family Pontederiaceae (Figure 1).
It has attractive flowers and these have been the reason for its spread
around the world. Water hyacinth does not cause problems in its native
range, the Amazon basin in South America, but outside that area it is a
menace. It impacts on human health, food security, water and power supplies,
navigation, biodiversity and natural ecosystem functions in many tropical
and subtropical regions because of its ability to quickly invade and choke
freshwater bodies. It can grow almost anywhere in the tropics, affecting
stationary or slow-moving water bodies such as small roadside drains and
ponds to large lakes like Lake
Victoria in Central Africa or slow moving rivers like the Sepik River system
in Papua New Guinea (PNG).
Water hyacinth is regarded as the world’s worst water weed (Holm et al.,
1979) and is among the 100 most troublesome invasive alien species in the
world (www.issg.org/database) .
Distribution
Water hyacinth is native to Brazil. However, it is now a cosmopolitan weed
having being spread from it native range and reported from most countries
between 400 N and 450S (Holm et al., 1977). In the Pacific it is reported
from American Samoa, Cook Islands, CNMI, Fiji, French Polynesia, FSM, Guam,
Marshall Islands, New Caledonia, PNG, Samoa, Solomon Islands, and Vanuatu
(Waterhouse, 1985; PIER Database, 2003).
Water hyacinth goes by different names in the Pacific: wota haisin (PNG),
riri vai (Cook Islands), bekabe kairanga, jal khumbe (Fiji); bung el ralm
(Palau), (PIER Database, 2003).
Description
Water hyacinth is a perennial herb that grows on the surface of freshwater.
It has very short stems that are not visible because they stay underwater.
Leaves are rosetted, with glossy leaf blades ovate to round on spongy
air-filled petioles varying from 10 cm to one metre tall. Plants growing at
the water’s edge or in less crowded conditions, such as over open water, are
much shorter and may have swollen air-filled petioles that help the plant to
stay afloat on the surface (Figure 2). The floral parts are characterised by
a tall inflorescence stalk with about 8-15 irregular individual mauve or
purplish flowers. The middle upper petal of the flower has a distinctive
yellowish blotch.
Problems
The formation of stolons results in massive and continous mats of
interlocking plants that can clog up rivers and choke the life out of ponds
and lakes (Figure 2). Some of the most common problems caused by water
hyacinth infestations include:
- Disruptions to water transportation (by canoes, dinghies, and larger
vessels). This was a serious issue in the Sepik River during the early
1990s (Julien and Orapa, 2001). The weed formed thick mats that covered
the surface of river canals and oxbow lakes, obstructing people’s access
to communication, schools, health centres, government services, food
gardens, fishing grounds and local markets for sale of crops;
-
 Effects on fish and wildlife can be immeasurable. In severely affected
water bodies, fish may die as a result of increasingly anaerobic (lacking
oxygen) conditions. Populations of wildlife like water birds, amphibians
and reptiles may also be affected;
- Changes in water chemistry. Thick mats of water hyacinth affect the
chemistry of the water by absorbing large amounts of nitrogen, phosphorus
and other nutrients and locking these nutrients up. It may be useful for
cleaning polluted water bodies of heavy metals but its presence is not
good for normal areas. In addition, the weed physically prevents sunlight
(required for photosynthesis) from entering the water and directly affects
the growth and survival of microscopic, oxygen-producing phytoplanktons
and zooplanktons vital for the food chain and survival of larger organisms
like fish;
- Loss of water from the surface of ponds and lakes is known to be
higher through evapotranspiration than through normal evaporation from the
surface of water. This can be a serious issue for irrigation or livestock
watering ponds in dry areas.
- Damage to infrastructure. The build-up of water hyacinth against fence
lines and low bridges, and its growth in dams, irrigation canals, and
floodwater drainage systems in urban areas can result in serious and
costly damage to these structures;
- Obstruction of water flow. The thick mats prevent free flow of water.
This can lead to higher incidences of water-borne diseases or promote
conditions favourable to disease vectors like mosquitoes and snails and
harbour harmful animals like snakes.
Management of water hyacinth
Prevention
The ideal method of preventing the introduction of this alien invader is
through public awareness, particularly on the part of people living near
rivers and lakes. The aim should be to prevent the weed being introduced
from another area, even for personal use in small ornamental ponds or tanks.
In some countries, water hyacinth is a prohibited plant by law. When new
outbreaks occur, the plants should be destroyed as soon as possible because
timing is critical. The longer they are left, the more seeds will be
produced and deposited. Long distance spread of water hyacinth is always
facilitated by people who find it as an attractive ornamental plant.
Physical or mechanical control
Water hyacinth is easy to pull out of the water or mud and can be killed by
spreading it out on dry ground to dry for a few days in the sun. It can then
be burnt – with diesel if partially dried. Slashing water hyacinth plants
while they are still growing in the water does not kill them but instead
promotes further growth and must therefore be avoided.
Chemical control
Herbicides mainly 2,4-D and Glyphosate can kill water hyacinth, but the
Secretariat of the Pacific Community (SPC) does not recommend the use of
herbicides in waterways because of danger to people using the water
resources and to aquatic life such as fish.
Biological control
Using the natural enemies of a weed to suppress its growth is called
biological control. A number of effective natural enemies (biocontrol
agents) of water hyacinth have been
released worldwide for its control. Some of these are available in a number
of countries in the Pacific Islands region. The most widely used agents are
the weevils Neochetina eichhorniae and N. bruchi, which
together have given good control of water hyacinth and are available in
Papua New Guinea and some areas of Australia. N. eichhorniae (Figure 3) was
released and is available in Fiji (since 1977), Solomon Islands (since 1982)
and Vanuatu (in 2004). Additional host-specific biological control agents
available in the Pacific region include the pyralid moths Niphograpta
albiguttalis and Xubida infusellus. Both moths are present in Australia but
only X. infusellus is established at one locality in PNG (Julien and
Orapa, 2001).
Biocontrol agents can take anything between 2 – 6 years to become
effective. Once established, biological control is permanent and
selfsustaining, requiring very little or no
additional intervention in the long term as the weed and biocontrol agent
reach a point where they live in a state of equilibrium with each other.
Their populations then fluctuate seasonally with little impact on people.
Plant vigour and sizes should be reduced (Figure 4) and the weed should be
too weak to form large floating mats.
Integrated control
 A weed management programme that integrates physical and biological control
methods has been used successfully in a number of places including the Sepik
River in PNG, where it is common practice to slash the sides of canals,
enabling the current to carry mats of water hyacinth to the sea.
Erecting floating booms of bamboo or dry lightweight logs at the entrances
to lakes or tributaries of larger rivers could help prevent the spread of
water hyacinth into unaffected waterways.
References cited
- Holm, L.G., D.L. Plucknett, J.V. Pancho, and J.P. Herberger. 1977. The
World’s Worst Weeds. Distribution and Biology. University Press of Hawaii,
Honolulu, Hawaii. 609 pp.
- ISSG. 100 of the World’s Worst Invasive Alien Species. A selection
from the Global Invasive Species Database.
http://www.issg.org/ database .
- Julien, M.H. and W. Orapa. 2001. Insects used for biological control of
the aquatic weed water hyacinth in Papua New Guinea. PNG Journal of Figure
3: Adult Neochetina eichhorniae weevil (left), cause scarring of leaves
(middle), and larvae (right) damage the crowns of water hyacinth plants,
reducing their vigour.
- Agriculture, Forestry and Fisheries 44(1 & 2): 49-60.
- PIER Database 2003. http://www.hear.org/pier
- Waterhouse, D.F. 1985. The occurrence of major invertebrates and weed
pests in the South West Pacific. In: P. Ferrar and D. Stechmann (eds).
Proceedings of a workshop on the biological of pests in the south west
Pacific. ACIAR/GTZ and Government of Tonga, 17-25 October 1995, Vaini,
Tonga.
For more information, contact:
Warea Orapa, Coordinator,
Weed Management
SPC Land Resources Division
Private Mail Bag, Suva, Fiji Islands
Phone: (679) 3379284.
Email: WareaO@spc.int
Photos supplied by Warea Orapa.
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