Watch Out Water-hyacinth! - Brief Article

Agricultural Research, March, 2000 by Jim De Quattro

New Jungle Enemies Are Coming

You can't get to Iquitos, Peru, without a boat or a plane. But this jungle-locked city of 350,000 in the rainforests of the upper Amazon River is the business and tourism hub of Peru's eastern lowlands.

In the late 19th century, rubber made Iquitos a major trade center. Today, tourists can visit old rubber-baron mansions like Casa de Hierro (Iron House), designed by Gustave Eiffel of Paris.

Iquitos has a different appeal for Agricultural Research Service entomologist Hugo Cordo. "This region may be the world's richest source of natural enemies of water-hyacinth," he says. Cordo leads the ARS South American Biological Control Laboratory in Buenos Aires, Argentina.

Water-hyacinth, Eichhornia crassipes, is a free-floating perennial herb. The plants grow about 3 feet tall as they float on the water's surface, with stems intertwining to form dense mats.

In the Amazon the plant is held in check by natural enemies such as insects and microbes. These organisms stress the plants, controlling the mat's expansion. But water-hyacinth has escaped to friendlier waters, especially since the 1800s. Often, visitors, drawn by its lush leaves and blue-to-lavender flowers, have taken it home as an ornamental.

A Floating Nightmare

Out of its enemies' reach, waterhyacinth has become the worst floating aquatic weed in many tropical and subtropical parts of the Americas, Asia, Australia, and Africa. In Africa it infests every major river and nearly every major freshwater lake. In the United States, it flourishes in hundreds of bodies of water in Hawaii and California and throughout the South from Texas to the Carolinas.

Today, increased cooperation by governments and scientists around the world is turning up the heat on water-hyacinth. The more unique natural enemies that scientists can find and evaluate, the more likely they can deploy new biological control cadres suited to the weed's various growth stages and to different climates and other conditions.

At worst, this plant may be a killer. In the Sepik area of Papua New Guinea, it has been blamed for making people starve. According to Australian scientists K.L.S. Harley, M.H. Julien, and A.D. Wright, people "could not access subsistence gardens, hunting areas, catch fish, or travel to market to sell and buy produce" because of dense water-hyacinth mats.

More typically, water-hyacinth damages water quality by blocking sunlight and oxygen and slowing the water's flow. Capable of doubling within a couple of weeks, it can grow faster than any other plant. By choking out other vegetation, it makes an area unusable by plants and animals that live in or depend on the water. Fish spawning areas may vanish.

In the Florida Everglades of the United States, the snail kite (Rostrhamus sociabilis) is endangered partly because this bird can't find apple snails--its favorite food--where the weed has smothered the snail's favored food plants. In some parts of the world, the mats form habitat for disease-carrying mosquitoes as well as snail species that are intermediate hosts for schistosomiasis, among the world's worst parasitic diseases.

Uncontrolled, water-hyacinth robs water from potential drinking and irrigation supplies. The mats can block boat travel. Chunks of mat can break free to clog downstream pump stations supplying water for drinking, irrigation, and hydropower.

Chemicals and mechanical removal, the primary weapons against the weed, are costly and often ineffective.

Searching for Its Nemesis

Scientists believe that the best bet for a long-term solution is to introduce one or more natural enemies as biological controls.

Two decades ago, Cordo and ARS entomologist Jack DeLoach in Temple, Texas, led an effective biological control program at Argentina's Dique los Sauces reservoir. In the 1970s, ARS researchers Ted Center and Neal Spencer were the first to release in the United States two South American weevils (Neochetina bruchi and N. eichhorniae) and the water-hyacinth borer (Sameodes albiguttalis).

These and other organisms are being deployed in more than 20 other countries, including Australia, Cuba, Egypt, Honduras, Indonesia, Malaysia, Mexico, Panama, South Africa, Thailand, Vietnam, and Zimbabwe. There have been many successes, but results have been variable and the weed continues to cause problems.

"For years," says Cordo, "we thought most of the best potential biological control agents were already found."

"But until now," Center notes, "no one had really looked for them in the upper Amazon. That is probably the area where water-hyacinth originated--where you might expect to find the greatest diversity of natural enemies." Center leads ARS' Aquatic Plant Control Research Unit in Fort Lauderdale, Florida.

This scientific optimism brought Cordo, Center, and three other scientists to Iquitos in late April 1999. The others were entomologist Martin Hill with South Africa's Plant Protection Research Institute and plant pathologists Harry Evans and Djami Djeddour of CABI Bioscience in England.