杏吧原创

BIOLOGICAL control, the natural alternative to pesticides, may have met its
match on the tranquil waters of Lake Victoria. The lake is being choked by water
hyacinth, which has clogged lakeshore ports and destroyed a vital fishing
industry. Biologists had pinned their hopes on introducing weevils to attack the
plants, but over the past two years the insects have made little or no impact on
the beautiful weed. Now Uganda鈥檚 fisheries department wants to blitz the plant
with chemicals. A recent pilot study carried out for the government concluded
that two herbicides are safe and effective at killing water hyacinth. One also
has the advantage of being cheap. It is 2,4-dichlorophenoxyacetic acid, best
known for its use a generation ago in the jungles of Vietnam as a compound of
the notorious Agent Orange.

These sound like desperate measures but even ecologists are becoming
convinced they are necessary. Water hyacinth first invaded the world鈥檚 largest
tropical lake almost a decade ago, and now covers more than 10 000 hectares. The
weed has defeated all efforts to expel it, including the introduction of the
weed-eating weevils into the lake a couple of years ago. Such biological pest
control has in recent years been heralded as an ecologically friendlier,
cheaper, more effective and longer-lasting replacement for toxic pesticides. But
the experience in Lake Victoria highlights a major weakness of
biocontrol鈥攐nce an invading species has become really well established, a
predator has little chance of overwhelming it.

European biologists first found water hyacinth in the S茫o Francisco
River in eastern Brazil. They admired its big flowers and dark, leathery leaves
and planted it in botanical gardens round the world. Since then, as its admirers
have gone home, the weed has stayed behind and flourished with a vengeance. It
is, according to Paul Woomer, an agricultural scientist at the University of
Nairobi, 鈥渙ne of the fastest growing and most aggressive plants on Earth鈥�. An
infestation typically doubles in size within a fortnight. The plant usually
spreads by sending out short runner stems that bear daughter plants. But it also
produces large numbers of seeds that can survive in soils and lake sediments for
up to 15 years.

World domination

This fecund colonist has spread to 53 countries, from the backwaters of
Bengal to the mighty rivers of China, from Fiji to the Australian outback, from
Louisiana bayous to the wetlands of Cambodia, blocking waterways, destroying
fisheries and harbouring diseases. Only Europe has escaped. Its spread in Africa
in the past decade has been especially aggressive. Almost no large water body
remains untouched. The weed entered Lake Victoria in 1989 after migrating from
the headwaters of the Zaire River. It came via the River Kagera in Rwanda, the
same river that five years later bore into the lake the bodies of 40 000 victims
of that country鈥檚 genocide. For nine years now, the Kagera has carried into the
lake a constant stream of hyacinth clumps鈥攅nough to cover about three
hectares a day.

From the lake鈥檚 western shores, the weed has spread across open waters and
round shallow, muddy shorelines. It now covers four-fifths of the Ugandan shore
and an estimated 2000 hectares around the Kenyan port of Kisumu. Here, ships now
take five hours to push through the weed to dock. Local fishing boats stand no
chance. At Kusa Bay, south of Kisumu, water hyacinth has filled the bay and
destroyed the livelihood of a 2000-strong fishing community, leaving 52 fishing
boats marooned, the fish warehouses permanently locked and the old shorefront
hotel derelict. From the beach, there is hyacinth as far as the eye can see. 鈥淲e
now have to fish the rivers, but the small fish there go bad before we can get
them to market, and there are no fish at all in the dry season,鈥� says Charles
Vorster, a village fisherman.

One estimate puts the economic cost of this exotic invader at $150
million a year. As well as destroying fisheries it has damaged bridges and
clogged hydroelectric turbines, power station cooling systems and water supply
inlets. But efforts to physically remove the mats of vegetation, some up to two
metres thick, have proved an arduous and fruitless task. Fishermen in Kusa Bay
last year spent three days a week clearing the weed by hand in an attempt to
make a path for their boats. But the weed kept returning, and they gave up. Such
work is, in any case, dangerous. The weed harbours snakes, crocodiles, hippos,
the snails that carry bilharzia, the mosquitoes that carry malaria and sewage
that may contain cholera bacteria. One village on the lakeshore lost five people
to crocodiles and three more to hippos in a single year.

At the hydroelectric plant on Owen Falls in Uganda, four boats fitted with
rakes and conveyor belts struggle to keep the weed clear of the turbines. At
Port Bell outside the capital, Kampala, they spend $12 000 a week to
remove the weed mechanically, without ever getting the better of it. A
purpose-built floating harvester made in Britain sank on its first day in the
lake. Even when they work, mechanical harvesters 鈥渏ust create space for the
plants to grow into鈥�, says Geoffrey Howard, director of the World Conservation
Union in Kenya.

Biocontrols have scarcely fared better. Uganda first introduced two species
of South American weevils to the lake more than two years ago. The Kenyan
Agricultural Research Institute (KARI) added more last year. The weevils spend
their nights munching the leaves. If they eat enough, the leaves dry out and
weevil larvae can tunnel into the stem, eventually killing the weed. That is the
theory. In places, monitors have spotted leaves nibbled by the weevils. But so
far the bugs have made no visible impression on the size of the infestation.
Outside the Kibos fisheries research station near Kisumu Airport, where large
numbers of weevils have been introduced, water hyacinth grows with alacrity.

Weevils have successfully limited the spread of smaller infestations in
India, Thailand, Florida, Java and Zambia. But critics say the releases in Lake
Victoria have been too little and too late. 鈥淜ARI has been struggling to rear
the insects,鈥� says Hans Herren, director-general of the International Centre of
Insect Physiology and Ecology (ICIPE), which runs a lakeside research station at
Mbita Point, on the eastern shore of the lake. He believes that biocontrol has
failed because the government did not consult outside scientists. 鈥淲e could rear
lots of weevils here,鈥� says Kwesi Ampong-Nyarko, an agroecologist with the
ICIPE. 鈥淲e have the capacity, but KARI didn鈥檛 want us involved at the start. It
was politics.鈥�

Weevils, he says, 鈥渨ould have been effective in the beginning in Lake
Victoria, but not now when the mats are two or three kilometres wide. The bugs
can only eat so much.鈥� Brian O鈥橰iordan of the London-based Intermediate
Technology Development Group agrees: 鈥淎t KARI, they are now talking about having
an effect in ten years鈥� time. This is unlikely to be effective. And in any case,
ten years is too long. It needs something immediate.鈥� Even if the animals now
being distributed into the lake thrive, they will not eliminate their prey. They
will merely establish a balance between the two species. The vital question is
where that balance will lie. Woomer points out that in tests at old clay pits
beside Lake Victoria, weevils have prospered while water hyacinth continued to
occupy the entire water surface. This is a bad sign. He believes biocontrol
agents alone are unlikely ever to reduce the weed significantly in the open
waters of the lake.

The killing fields

The biocontrollers have not lost all hope. They are looking for new, more
virulent, predators. Their menagerie currently includes a South American
grasshopper, moths from Australia whose larvae feed on hyacinth leaves and a
rust fungus. Bioprospectors are busy in the Amazon basin looking for other
natural predators.

Ecologists say that Lake Victoria will never be freed of alien weeds until
its wider ecological problems are solved. 鈥淓ven if water hyacinth is controlled,
it will be replaced by other weeds,鈥� says Howard. Les Kaufman of Boston
University, who has studied the ecosystem for a decade, blames a huge influx of
nutrients. The hyacinth thrives on sewage from cities such as Kampala and
Kisumu, effluent from the sugar factories, paper mills, tanneries and breweries
springing up across the lake basin, and silt washing into the lake as the
catchment鈥檚 forests are chopped down and converted to fields. In the past two
decades, adds Kaufman, these nutrients have turned a once clear, well-oxygenated
lake into a muddy, stratified water body, with no oxygen in its bottom layers.
The fish may die, but water hyacinth loves it.

Last year the World Bank launched a five-year Lake Victoria Environmental
Management Project, aimed at cleaning up the lake鈥檚 catchment by reducing
industrial pollution and soil erosion. It allocated $8.3 million
specifically for controlling water hyacinth. The aim is to persuade Uganda,
Kenya and Tanzania to cooperate on a strategy to reduce infestations by a
mixture of mechanical, biological and perhaps chemical means. Howard is cynical
about such plans. 鈥淔oreign experts gather like crows at a carcass to sell their
methods,鈥� he says. 鈥淏ut in five years we will still have the hyacinth.鈥�

So what should be done in the meantime? Local groups, such as the
Kisumu-based Friends of Lake Victoria, advocate cottage industries based on
harvesting the hyacinth. In Bangladesh, small projects turn hyacinth into
baskets and furniture, or dry, boil and bleach it to make paper and fibreboard.
Hyacinth can also make fertiliser, feed animals, generate 鈥渂iogas鈥� in anaerobic
fermenters or be pyrolysed to make charcoal briquettes. The stumbling block for
all these schemes is that 95 per cent of the weed is water, and it can take
months to rot down. The return on the laborious and potentially dangerous task
of harvesting is meagre. In most studies, says Woomer, 鈥渢he value of water
hyacinth is consistently less than the cost of recovery and processing鈥�.

Chemical weapons

And that leaves herbicides. 鈥淟ake Victoria is now in such a state that only
wholesale use of chemicals can do the job within a respectable time frame,鈥� says
Garry Hill, a hyacinth expert at the Centre for Agriculture and Biosciences
International (CABI) in Ascot, Berkshire. After experimenting with harvesters
and weevils, the Ugandan government fisheries department has called in the
American herbicide specialist Aquatics Unlimited. Last year it ran trials in the
lake with glyphosate (produced by Monsanto under the brand names Roundup and
Rodeo) and 2,4-dichlorophenoxyacetic acid.

Both have been used before in Africa on smaller infestations of the weed. The
side effects of glyphosate are reportedly small. But 2,4-D, which acts fastest
and can obliterate at least half of the hyacinth within a week, has major side
effects. In Zimbabwe, it was sprayed onto Lake Chivero outside Harare from the
1950s to 1971, and again in 1990 when the weed suddenly took over a third of the
lake. Spraying was stopped following a public outcry over fish deaths. Sudan
used it to fight water hyacinth in the 1960s and 1970s, but banned the chemical
in 1983.

Jim Findlay, a South African consultant who works for Monsanto, points out
that 2,4-D is banned in many countries from water that is intended for human,
animal or agricultural use within seven days of spraying. His own report finds
that 80 per cent of the chemical is still present in the water after two weeks.
It is, he says, toxic to animal life and can evaporate from water, causing
鈥渄rift鈥� that threatens crops along the shore, such as bananas, beans and
cassava. For these reasons it is 鈥渢otally impractical in the vast majority of
African areas鈥�.

Nonetheless, the Ugandan government and Aquatics Unlimited concluded that
both glyphosate and 2,4-D 鈥減ose very low risk to humans and the environment鈥�.
Crucially, they also found that 2,4-D costs less than half as much to apply as
glyphosate. The fisheries department wanted immediate action but last October
Uganda鈥檚 National Environmental Management Authority rejected the application to
spray chemicals. Woomer points out that fast-acting 2,4-D could kill many fish,
as rotting hyacinth consumes the oxygen in enclosed bays. Despite this the
fisheries department has appealed against the ruling and is confident that it
will get the go-ahead later this year.

In future there could be a new fusion between the chemical and biological
approaches. CABI is seeking aid funds to develop a mycoherbicide, which would
use extracts from native African fungi to kill the water hyacinths. The idea is
based on a similar product launched in March this year and targeted at locusts.
Researchers at CABI aim to synthesise a mycoherbicide to fight water hyacinth
within three years. 鈥淚t looks like herbicide use will be a continuing feature of
water hyacinth control in Africa for the foreseeable future,鈥� says Hill. But he
hopes a mycoherbicide will be 鈥渕uch easier on the environment鈥� than other
chemicals.

Meanwhile the case for 2,4-D seems to be gaining ground, perhaps
unconsciously promoted by the widespread use of military metaphors to describe
the problem and diagnose a solution. Woomer calls for 鈥渇ull naval, biological
and chemical warfare鈥� against water hyacinth. And in warfare, at least, 2,4-D
has a track record.