Exotic
or nonnative aquatic plants can have serious impacts on wetland
ecosystems. For decades, Louisiana has been battling the water
hyacinths and other non-native plant species that choke its
waterways. Now a new invader, giant salvinia (Salvinia molesta),
has made it appearance in Louisiana waters. Native to Brazil,
this plant has been spread to Australia, New Zealand, New
Guinea, Africa, and India, each time with disastrous results.
Worldwide, giant salvinia ranks only behind water hyacinth
as the most damaging aquatic weed. In the U.S., giant salvinia
has been found and eradicated in plant nurseries and ponds
in South Carolina, Florida, Missouri, Texas, and Virginia.
In late 1998, a dense concentration of the plants was discovered
in Toledo Bend Reservoir on the Texas-Louisiana border. It’s
feared that the plant will spread to other waterbodies in
the state and, indeed, it has already been found in Bayou
Teche.
Giant
salvinia does its damage by growing into dense mats that can
eventually cover an entire waterbody with vegetation as thick
as three feet. These mats smother native plants and phytoplankton
by blocking the penetration of sunlight into the water. With
no phytoplankton present and no water surface open to the
air, available dissolved oxygen rapidly becomes depleted,
killing the fish in the waterbody. The loss of native aquatic
plants and open water also destroys the value of an area as
waterfowl habitat.
Solid
mats of giant salvinia may also reduce or eliminate boating
and fishing opportunities simply because boats are not able
to penetrate the mats. Salvinia infestations can also devastate
rice, crawfish, and catfish farming operations.
Another
threat, especially in a subtropical climate such as Louisiana's,
is that the plant mats can slow currents, causing increased
siltation that fills in waterbodies until they disappear.
Dead plants that sink from the bottom of the mat may accumulate
on the waterbottom, where the lack of oxygen prevents decay.
Thick mats also serve as floating islands that allow other
plants species to sprout and grow on top of them.
Very few
animals are likely to use giant salvinia as food, because
its high levels of crude ash, lignin, and tannins make the
plant low in nutrients and hard to digest.
Giant
salvinia is fairly easy to identify. The oblong floating leaves
are one-half to one and one-half inches long, larger than
those of its smaller, more frequently found cousin, common
salvinia. Young plants have leaves that lie flat on the water's
surface. As the plants mature and grow into mats, the leaves
fold, become compressed upright, and develop a chain-like
appearance. Both species of salvinia also have many small
spike-like hairs on the upper surface of the leaves. A foolproof
method of separating giant salvinia from common salvinia is
to examine the hairs with magnifying glass. Giant salvinia'
s hairs split into four prongs that rejoin at the tips to
form a structure that resembles an egg beater or cage. The
four prongs on common salvinia do not rejoin at the tips.
Once giant
salvinia becomes established, eradication is very difficult.
The most effective herbicide sprays are not permitted for
use in the United States. The effectiveness of sprays is also
hampered by the leaves' surface hairs, which shed spray particles.
On thick mats, only the plants on the surface of the mat are
exposed to the sprayed herbicide.
Raking
the weed from the water is only moderately effective for a
short time. Removing all of the plants is nearly impossible.
A single plant can, under ideal conditions, multiply to cover
40 square miles in only three months.
Freezing
weather will reduce the number of plants but not eliminate
them, and the survivors rapidly multiply when warm weather
returns. Researchers have studied several insects, including
a moth, an aquatic grasshopper, and a weevil, for their potential
to biologically control giant salvinia, but only the weevil
has any proven effectiveness.
Because
of giant salvinia's negative ecological effects, the difficulty
of its eradication, and its explosive growth rate, the best
control is to prevent its introduction, or at least attack
the problem early before it gets out of control.
Some spread
of giant salvinia into new areas is caused by water currents
and the movements of wildlife and waterfowl. Most spreading
seems to occur when aquarium or water garden plants are discarded
and when boaters do not remove plant fragments from their
boat trailers. Both of these methods of spread can easily
be prevented.
The Louisiana
Department of Wildlife and Fisheries requests that the public
report any sightings of giant salvinia by ca1Iing (337) 948-0255.
* Louisiana
State University Agricultural Center
** Louisiana Department of Wildlife & Fisheries
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