Grasshoppers Used to Fight "Worst Water Weed"

Bijal P. Trivedi
National Geographic Today

April 1, 2003

Researchers are counting on a small South American
grasshopper—Cornops aquaticum—to help them combat the
"world's worst water weed": the water hyacinth, which chokes waterways
in Asia, Africa, Australia, and the Americas.

Away from its native habitat on the upper Amazon basin, the water hyacinth grows fast and spreads furiously, causing woes for man and beast. Cornops is the latest weapon in what researchers call "biocontrol"—pitting one kind of creature against another. But such methods sometimes cause environmental problems.

"Ideally I would like a magic bullet—one insect that will do 100 percent of the job," said Ted Center, an entomologist and research leader at the Invasive Plant Research Laboratory in Fort Lauderdale, Florida, who is known as "Mr. Waterhyacinth" because he has studied the plant for more than three decades.

But "each introduction has an element of risk," he said, "and the more insects we release the higher the chance of unanticipated results. We need to fit biocontrol to local needs."

Dark green with dazzling purple-blue blooms, the water hyacinth floats on lakes, streams, ponds and rivers. Doubling in size every 7 to 12 days, it can grow up to six feet (1.8 meters) tall and trail three-foot-long (0.9-meter-long) roots.

"The water hyacinth has affected just about every lake, river, and dam in sub-Saharan Africa," said Martin Hill, an entomologist at Rhodes University in Grahamstown, South Africa. "It hinders the extraction of water for industrial, agricultural and domestic purposes. In rural areas it can completely block access to water."

Worldwide Menace

Floating islands of the plants also become breeding grounds for mosquitoes and snails that respectively cause malaria and schistosomiasis.

The water hyacinth invaded the United States during the 19th century.

"The first mention of the plant was in 1884 at an international cotton exposition in New Orleans, where it was given away as a souvenir," said Center.

By the 1890s water hyacinth already occupied the southeast coast of the U.S., infesting Florida, Texas, South Carolina, and Louisiana, which still has the biggest problem.

For decades man has fought the pest with an arsenal of more-or-less costly methods. Mechanical harvesters—a tugboat-mower—are slow. Herbicides are a quick fix, though with dangerous side effects.

In the late 1960s, in South America, researchers discovered two weevils and a moth that destroy the water hyacinth. These creatures have been successfully released in more than 30 countries worldwide. In East Africa's Lake Victoria, for example, two weevil species reduced the coverage of waterhyacinth by 80 percent. (See related article.)

This conquest through biocontrol has spurred the search for other insects to match the range of climates where the water hyacinth thrives.

During the past six years, Center has led three expeditions to South America to find other natural enemies of the water hyacinth—insects and microbes that evolved over millions of years along with the plant and keep it in check.

Finding the Right Bug

Center and his colleagues have conducted surveys from Buenos Aires to northern Argentina, from Iquitos, Peru, to the tributaries of the Amazon, and in Ecuador.

"We tear the plants apart, and pull out all the larvae, and the adults, and see what damage they do," said Center.

Using sweep nets and aspirators the researchers collect insects and take them to a lab in Buenos Aires. There they test an insect to see whether it only has a taste for water hyacinth. Otherwise an introduction could threaten other indigenous species and some cash crops.

The researchers recently took another look at Cornops, rejected during the 1960s because grasshoppers were considered indiscriminate eaters.

For the past five years, Hill has been testing South African relatives of the water hyacinth—about 150 species in all—to see whether the grasshopper will deviate from its favorite food.

"South Africa has 22,000 indigenous plant species—we can't release insects that will eat other species," Hill said.

After five years in a secure lab, Cornops is now deemed safe for biocontrol use in South Africa. This year Hill expects to release several thousand of the insects into a water hyacinth-plagued body of water.

Researchers are looking for an effective, environmentally safe weapon against the water hyacinth in southeast Asia, higher-altitude regions of Africa and elsewhere.

In the U.S., though, Cornops doesn't work. The grasshopper has a taste for pickerel weed—an indigenous plant. The search for a magic bullet goes on.

National Geographic Today, 7 p.m. ET/PT in the United States, is a daily news journal available only on the National Geographic Channel. Click here to learn more about it.

Got a high-speed connection? Watch National Geographic Today in streaming video.