| Two
species of the invasive, aquatic salvinia cause problems
in Louisiana. Common salvinia, Salvinia minima, which is
native to Central and South America, was first identified
in Florida in the 1930s. From there it has spread to every
southern state in the United States. Giant salvinia, Salvinia
molesta, also native to South America, has spread to almost
every temperate and tropical country worldwide. It has become
known as the most noxious of all aquatic weeds.
While
giant salvinia at maturity is quite a bit larger than its
cousin the common salvinia, the two can be distinguished
from one another through close observation of the upper
leaves. Both species have many small spike-like hairs on
the upper leaf surface. The giant salvinia’s hairs
split into four prongs that rejoin at the tips to form a
structure that resembles an eggbeater or cage. The four
prongs on common salvinia do not rejoin at the tip. The
giant’s young plant leaves lie flat on the water similar
to the common salvinia in size and appearance, however as
the giant salvinia grows and matures the leaves begin to
fold upward and inward.
Giant
salvinia is much more damaging than its smaller cousin because
of its ability to grow into dense mats that can cover entire
water bodies with a thick layer of vegetation. These mats
smother native plants by blocking the penetration of sunlight
into the water. Other consequences of giant salvinia are
reduced dissolved oxygen in waters shaded out, causing fish
kills and reduced production of submerged aquatic plants.
This severely reduces 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 push through the thick mats.
Controlling
salvinia is difficult and can be quite costly. Common salvinia
does not tolerate salinities and dies in a few days at levels
at or above 3-4 parts per thousand. Giant salvinia is more
tolerant requiring salinities of 7-10 parts per thousand
for a week to begin killing it. In areas where salt water
is not an option, such as in fish ponds and freshwater marshes,
herbicides are an option. Diquat dibromide, commonly sold
under the brand name of REWARD, is a contact herbicide which
is effective at controlling salvinia. Another option is
to use an aquatic labeled formulation of glyphosate, commonly
known as the roundup family of herbicides. It is widely
sold under many brand names. When applying these two herbicides
it is important to add a surfactant which breaks the surface
tension of the herbicide mixture allowing it to coat the
plant and stick to it long enough to work.
When
controlling salvinia in waterfowl hunting areas, time of
application should be considered. It is most desirable to
control unwanted vegetation without killing desirable waterfowl
foods if possible. The ideal time is early in spring before
the peak of growing season when infestations are light,
water levels allow easy access and it is less costly to
spray small areas of growth. This should be followed by
spot spraying throughout the summer to maintain control.
In
ponds with no flow, another herbicide option is to use Sonar
or Avast, which requires a long residence time to be effective.
It is quite costly, but one application gets control for
up to a year.
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