Posted on: Sunday, November 28, 2004
Arizona to pit weevil vs. waterway weed
By Arthur H. Rostein
Associated Pres
TUCSON, Ariz. — Federal and state naturalists and researchers hope a tiny bug will perform like David against Goliath on a noxious weed potentially threatening western Arizona waterways.
Actually, they'll settle for the pinhead-sized salvinia weevil being enough of a presence to simply help keep the rapidly growing giant salvinia weed in check along the Colorado River.
"It's considered the world's worst aquatic invasive plant," said Cheryl Riley, a natural resource specialist with the U.S. Bureau of Reclamation's field office in Yuma.
Salvinia molesta — a free-floating water fern — is described as a hitchhiker that can be transferred between bodies of fresh water when small pieces of the plant get stuck on boat propellers. It reproduces quickly and can clog rivers, streams, canals, ponds and lake surfaces.
"It can double its mass under ideal conditions every 2.2 days, meaning that it could grow to form a mat 38 square miles in size in three months," said John Caravetta, associate director of the Arizona Department of Agriculture. "The 'wow' factor is definitely there."
The species is native to Brazil but has found its way around the world, becoming a freshwater threat in Australia, Malaysia, Hawai'i and parts of the U.S. Mainland, especially Western states such as Arizona.
Hawai'i officials avoided an environmental disaster in 2003 at Lake Wilson on O'ahu, where 29 federal, state and city agencies worked for four months to kill and remove Salvinia molesta that covered 95 percent of the lake's surface. Officials feared it would spread to other waterways.
When cultivated, it is lush and beautiful, said Earl Andress, head of the biocontrol portion of the salvinia control program for the U.S. Animal and Plant Health Inspection Service in Brawley, Calif. "It looks great in ponds, and that's how it got here, through the aquatic plant industry."
In 1999, salvinia was found in the lower Colorado River in Imperial County, Calif., then outside Blythe, Calif., in a drainage ditch feeding runoff into the Colorado.
Today on the Colorado River, the weed is known to have clogged irrigation canals. It also has to be cleared frequently from screens that keep debris out of water intakes at irrigation canals, at the Central Arizona Project — which supplies drinking water to Tucson and Phoenix — and at the Imperial Dam, about 20 miles north of Yuma.
The weed can also hamper boating and other water recreation and it can choke off fish and aquatic life.
"Most of the fish and the plants will die, because it will pull nutrients out, it will starve the water for oxygen, will choke off that environment both physically and through light not getting through," Caravetta said.
There have been no known fish kills in the Colorado, however.
To control the salvinia, officials see great potential in Cyrtobagous salviniae, an insect that is also a native of Brazil and is the weed's principal predator. It has been used effectively for years in Australia and Papua New Guinea to keep salvinia in check, Andress said.
"It is the only effective measure that's working there," Andress said. "They've tried mechanical removal, sprays with herbicide, booms. The weevil is the only thing that works."
Riley said the multiagency Lower Colorado Giant Salvinia Task Force is pursuing an integrated pest management program that also includes using machines and herbicides — Hawai'i's primary tools in its fight against the weed on Lake Wilson.
Floating booms are used to help steer the salvinia through an irrigation district drain to a mechanical harvester that pulls the plants out of the water to be dried and burned.
Caravetta said that, generally, "we have not been able to get the weevil established in numbers that would have a significant impact."
The weevil works on the weed like this:
The female lays its eggs in a salvinia leaf. Each larva eats its way to the stem and burrows inside, killing a growing terminal.
The insect, which has roughly a 45-day egg-to-adult life cycle, will always leave some of the weed so future generations have food, Andress said. The weed is the weevil's only food source.
He called the phenomenon "one of the beauties of biocontrol."
"As a pest population increases, so does the natural enemy," he said.
In Brazil, "the weevil maintains its own level of food source," Riley said. "We're working to achieve that as well."
The weevil in turn serves as food for birds, several predatory insects and possibly fish.
Andress said the biocontrol program has an excellent chance of success and should have no unintended consequences.
"We're hoping that we're going to get this weed in check before it develops into a serious problem," he said.
