A TINY, newly discovered Amazonian fly that attacks water-hyacinth weeds has for the first time been reared in large numbers. "This is our researchers' latest step to building a new team of natural enemies to stop water-hyacinth, a nasty aquatic weed," said Floyd P. Horn, administrator of USDA's Agricultural Research Service.

Dense water-hyacinth mats infest hundreds of ponds, lakes and streams in California, Hawaii and throughout the South from Texas to the Carolinas. The plant is native to South America. The mats rob water from drinking and irrigation supplies, block boat travel and clog water-pumping stations. They also damage water quality, blocking light and oxygen and slowing the water flow. And they choke out other plants - and the fish and other animals that rely on these plants or on access to open water.

The 1.5-millimeter fly, discovered last year by ARS researchers, is a new species of Thrypticus. Immature flies feed inside the weed's petioles, the tiny stalks that attach leaves to stems. In addition, the flies' tunneling may enable microbes to enter and further weaken or kill the plants.

"These flies - and other new species the scientists discovered in the upper Amazon basin - could become the first new insects imported to attack water-hyacinth in about 25 years," Horn said. "Biological control with insects and other natural enemies, such as fungi, is essential to a long-term solution. Today's primary weapons - herbicides and mechanical removal - can be costly and are often ineffective."

ARS entomologist Hugo Cordo and colleagues found the new species last April near Iquitos, Peru. Cordo leads ARS' South American Biological Control Laboratory in Buenos Aires, Argentina. ARS is USDA's chief scientific wing.

In early to mid-December, Cordo's research team released several hundred adult Thrypticus water-hyacinth petiole mining flies on water-hyacinth plants in a 6-foot-square cage outside the Buenos Aires lab. The flies mated, and large numbers of their adult offspring began appearing in January. "With luck," Cordo said, "Thrypticus might be ready to import in two or three years" for testing in an ARS quarantine laboratory in Florida.

In the 1970s, three South American insects were found and tested in Argentina and imported and released in the U.S. by ARS entomologists Neal Spencer and Ted Center, who was with Cordo on the 1999 expedition. "Today these insects help check water-hyacinth in the U.S. and more than a dozen other countries," Horn said. "But having a more diverse crew of natural enemies should increase our success against this invasive weed species."

In the Amazon, insects and microbes put water-hyacinth under stress. This controls the mat's expansion. But the weed has escaped to many countries where these natural enemies aren't present. Visitors attracted by the plant's lush leaves and blue-to-lavender flowers have often taken it home as an ornamental. That's apparently how it reached the U.S. in the late 19th century.

The Iquitos region "may be the world's richest source of natural enemies of water-hyacinth," Cordo said.

Cordo's expedition colleagues were Center, who leads the ARS Aquatic Plant Control Research Unit in Fort Lauderdale, Fla., Martin Hill of South Africa's Plant Protection Research Institute, and Harry Evans and Djami Djeddour of CABI Bioscience in England. They searched 180 kilometers of the upper Amazon River and the Ucayali and Marazon Rivers that converge to form it. They collected hundreds of natural enemies and plant samples at 30 sites over seven days.

Cordo's research team has discovered 11 new insect species on water-hyacinth and its relatives in Peru and northern Argentina since 1996. They include six Thrypticus species, three Taosa plant hoppers and two Megamelus plant hoppers. Researchers are studying the insects' biology and behavior and screening them to identify the best candidates for biological control. They are also making sure water-hyacinth is the only plant attacked. "Along with crops, this means testing ornamentals and plants in natural settings," Cordo said.