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Mar 2000 - Contents PDF Previous Story Next Story





WATCH OUT WATER-HYACINTH! NEW JUNGLE ENEMIES ARE COMING

 



Above, grey-white nymphs of Taosa
plant hoppers feed on heavily damaged
water-hyacinth. The sap-sucking insects not
only damage and weaken the weed,
they introduce plant pathogens.
(K8800-2)

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.


Petioles of water-hyacinth densely
punctured by oviposition marks
of Taosa plant hoppers.
(K8800-1)

A Floating Nightmare

Out of its enemies' reach, water-hyacinth 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.



The Megamelus plant hopper (about 3mm
long) may provide badly needed help in controlling water-hyacinth.
(K8843-1)

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.

 


The natural beauty of water-hyacinth's
flower and foliage has helped it
spread to become a floating
nightmare in many tropical areas.
(K8801-1)

"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.

They searched for natural enemies along 180 kilometers of the upper Amazon and
the two rivers that converge to form it—the Ucayali and Marañon.

Into the Thick of It

On April 27, guide Andrés Guerra motored the group down the Amazon in a small
aluminum boat. "We were looking for water-hyacinth growing in cochas," Cordo
says. Cochas—bayous and depressions away from the river channel—are typically
concealed behind the wall of rainforest lining a river channel.

 


Thrypticus fly (about 2 mm long).
(K8843-2)

Eventually the group stopped at an area Guerra said was near cochas. With a
machete, he hacked low branches and vines to clear the boat's path through the
flooded forest. "For a half hour," says Cordo, "we thought we were heading to
nothing. But suddenly, the cochas were there. We steered into the water-hyacinth
mat and began using our hands and a sweep net to collect insect and plant
samples."

In all, the scientists collected hundreds of natural enemies and plant samples
at 30 sites in 7 days. From the first day, the excitement was about a tiny
insect, a water-hyacinth fly that none of the scientists had seen before.

In Buenos Aires, Cordo's team had already been testing several promising
insects, including three species of Thrypticus flies collected since 1996 in
Argentina. Now, it appeared they had found a new Thrypticus.


Water-hyacinth in bloom.
(K8801-2)

The female Thrypticus deposits an egg in a water-hyacinth petiole—the stalk that
attaches the leaf to the stem. The young larva feeds on the inner tissue. Within
the petiole it digs ultranarrow tubes called mines. The mines have one or more
tiny spurs that exit the petiole's outer skin. "Water doesn't enter these tiny
orifices," Cordo says, "and we don't know why the fly makes them. But they may
serve as doorways for pathogens to enter and further weaken the plant."

Each Thrypticus species has a unique mining pattern. One is shaped like the
letter "U"; another is C-shaped. But at site 1—and later, other sites—the
scientists found ringlike mines with several orifices. "We believe the
Thrypticus that made these is a new species," Cordo says. "And it appears to be
a specialist in attacking very young petioles."

A Bountiful Harvest

During the trip, the scientists also found as many as three new species of Taosa
plant hoppers. Only one had been reported in the scientific literature. Taosa
and Megamelus plant hoppers are sap-sucking insects that, like their whitefly
cousins, can transmit plant pathogens. Cordo says the Taosa especially "are
impressive because of their impact in combination with a pathogen we have not
yet identified. Infested plants were short, weak, and full of spots made by the
pathogen."

The insect identifications are preliminary, but 11 species new to science have
been collected so far by the ARS research team on water-hyacinth and its
relatives in Iquitos and northern Argentina since 1996: six Thrypticus, three
Taosa, and two Megamelus species.

The ARS scientists at Buenos Aires have been conducting numerous studies of the
insects' biology and behavior. They will screen colonies of insects they collect
to find out which might do the most damage to water-hyacinth.

They are also making sure water-hyacinth is the only important plant attacked.
"Along with crops, this means testing ornamentals and plants in natural
settings," Cordo says. "We've already determined that the new Thrypticus and one
of the Megamelus do not attack plants in other families. And they will not
attack the one U.S. plant in the water-hyacinth family that we don't want to
hurt." That plant, pickerel weed, is used by many small aquatic animals.

Cordo, Center, and Hill, the South African scientist, are collaborating to
determine how best to rear Megamelus and Thrypticus for lab and outdoor tests.
"With luck," Cordo says, "Thrypticus might be ready to import in 2 or 3 years,
for testing first in the Fort Lauderdale lab. With Megamelus, that time may come
sooner, since we already know how to rear small lab colonies of it."—By Jim De
Quattro, ARS.

This research is part of Crop Protection and Quarantine, an ARS National Program
(#304) described on the World Wide Web at
http://www.nps.ars.usda.gov/programs/cppvs.htm.

Hugo Cordo is at the USDA-ARS South American Biological Control Laboratory,
Hurlingham, Argentina; telephone and fax 54-11-4662-0999 . His mailing address
is Agricultural Counselor, ARS Lab, U.S. Embassy Buenos Aires, Unit 4325, APO AA
34034-0001.

Ted D. Center is at the USDA-ARS Aquatic Plant Control Research Unit, 3205
College Ave., Fort Lauderdale, FL 33314; phone (954) 475-0541, ext. 103, fax
954-476-9169.

 

"Watch Out Water-hyacinth! New Jungle Enemies Are Coming" was published in the
March 2000 issue of Agricultural Research magazine.

 

   

 

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