If looks were everything, no one would have a problem with the water hyacinths that adorn Lake Victoria. Their lush leaves sprout a gorgeous violet blossom. Their beauty must have enchanted whoever first plunked these plants native to South America into this giant body of water bordered by Tanzania, Uganda and Kenya in the highlands of Central Africa.

But when it arrived in Lake Victoria, the world's second-largest freshwater lake and the economic wellspring for 30 million people in the surrounding countries and beyond, the hyacinth did not merely thrive. Feeding on the lake's surplus of nutrients and basking in steady equatorial temperatures, it multiplied so relentlessly that it very nearly became The Thing That Ate Lake Victoria.

"The water hyacinth," said Festus Osewe, one of the thousands of Lake Victoria fishermen idled by the weed, "brings so many problems."

As the hyacinth multiples--doubling the area it covers as quickly as every five days--it forms vast mats miles and miles long that block the harbors of fishermen whose annual catch typically totals $400 million. Its tendrils still waves, creating breeding grounds for malaria, sleeping sickness and bilharzia, a common ailment in Africa spread by water-borne parasites. Broad, floating leaves provide perfect cover for poisonous snakes and man-eating crocodiles.

The weed has trapped a navy (Ugandan patrol boats were caught in a bay filled with towering three-foot "bull" hyacinth) and darkened cities (floaters more than once clogged the intake pipes at the power station that supplies Kampala, the Ugandan capital).

"It has a whole range of impacts that collapse into an ugly something," said Timothy Twongo, of Uganda's Fisheries Research Institute, which has battled the explosive species since the mid-1990s.

That battle has taken many forms. At one point, a Tanzanian witch doctor was paid $340 to banish the weed; he ended up returning the money. But in the latest incarnation, the campaign to control Lake Victoria's water hyacinth represents an especially vivid chapter in one of the longest running debates in environmentalism.

That would be the contest between technology and "sustainable solutions." Or, in this case, bug vs. machine.

"I am betting on the insect," Twongo said.

The machine is a Swamp Devil, a $300,000 contraption that is part barge, part grinder. Uncrated last week in nearby Kisumu, Kenya, it stands as the first conspicuous manifestation of an $70 million World Bank program designed to save Victoria with the help of the three countries that share its shores.

"We are the world's leading manufacturer of surface water management equipment," said David A. Dauffenbach, engineering vice president of Aquarius Systems, the North Prairie, Wis., firm that won the $1.3 million World Bank contract for mechanical harvesting.

As he spoke, a pair of hippos rustled in shoulder-high hippo grass, one of the troublesome secondary growths that have followed the pioneering hyacinth. The whirring teeth of the Swamp Devil would slice through that as well, Dauffenbach said.

Although proven elsewhere, including Florida, the Swamp Devil has yet to attack Lake Victoria. The first stop will be here in Homa Bay, a scenic port town 200 miles northeast of Nairobi surrounded by lopsided mountains, vast sky and . . . less hyacinth every day.

"You can see the leaves which are turning brown," said Zachary Onyando Auko, another idled fisherman, pointing to the hyacinth mat that has surrounded the city's pier intermittently for three years, but less impressively lately. The withering foliage is freckled with tiny nut-brown bites. The blossoms are not there at all.

"It's the beetles," Auko said.

The beetles are Neochetina eichhorniae and Neochetina bruchi. Tiny, nocturnal and voracious, they were introduced into the lake by Kenyan scientists in 1997, following the lead of Uganda. Both countries conducted tests demonstrating that the insects, also known as weevils, ate nothing but hyacinth.

No one, however, expected them to eat so much so fast. Today, the weed has all but disappeared from Ugandan shores. It shows signs of real stress on the Kenyan side as well, more than twice as quickly as officials expected.

"We thought that five years would be the minimum," said Gerald Ochiel of the Kenya Agricultural Research Institute. Instead, the same conditions that fed the hyacinth appear to have nurtured its nemesis. "The weevils had a field day."

There is widespread acknowledgment that a role remains for "mechanical harvesting," especially at high-traffic sites. The weevils have yet to clear Homa Bay of the weed that stopped regular ferry service and forced more than one pregnant woman, en route to the Homa Bay District Hospital from nearby islands, to give birth in a boat.

But advocates of sustainable technology nonetheless enjoy casting the insect's astounding progress as a John Henry fable, with the Swamp Devil in the role of the steam-powered drill.

"The idea of mechanical harvesters cleaning up the lake is absurd, because with a little bit of applied ecology you don't need to spend these ridiculous amounts of money," said Leslie Kaufman, a marine biologist at Boston University who studies Lake Victoria.

Total cost to date of the weevils, also picked up by the World Bank: perhaps $50,000.

Some question whether the weevil did it alone. In Uganda, when 80 percent of the shoreline was choked by the green necklace, officials are rumored to have resorted to wholesale use of herbicides. Uganda denies the rumor, which carries an extra stigma because of the recent history of "chemical fishing" on the lake. The European Union stopped importing Lake Victoria fish earlier this year after fillets showed traces of poisons fishermen had dropped overboard to bring them to the surface.

And, in truth, the weevil was not welcomed warmly. "The problem is with the perception of introducing things in Lake Victoria," said Geoffrey Howard, of the International Union for Conservation of Nature and Natural Resources. "And that begins with the Nile perch."

The Nile perch is a huge fish often seen on postcards dwarfing the fisherman beside it. After being introduced into the lake in 1950s, it proceeded to eat almost every other kind of fish there, including scores species of the brightly colored haplochromines for which Victoria was famous among aquarium owners.

Today, even though the Nile perch has been firmly established as an export, the only good thing fishermen here say about the hyacinth is that its roots provide cover in which smaller species can elude the mammoth fish.

CAPTION: The relentless spread of water hyacinths, a plant native to South America, traps a taxi boat in Homa Bay, Kenya, on Lake Victoria.

CAPTION: The competing methods of fighting the water hyacinth in Lake Victoria are a voracious beetle, in man's hand, and the Swamp Devil barge-grinder, in background.