Where does giant salvinia come from?
Giant salvinia (Salvinia
molesta) comes from South America, mainly Brazil and Argentina. It likely came to the United States via the
aquatic plant trade.
Why is giant salvinia a problem?
Giant salvinia forms dense mats that can clog waterways and
prevent the passage of sunlight into the water.
This affects any kind of boating activities such as duck hunting and
fishing, and also reduces the amount of dissolved oxygen in the water, which
can have negative consequences for fish, native plants, and waterfowl.
Does giant salvinia flower or produce seeds?
No. Giant salvinia is
a type of fern, so it does not flower or produce seeds. The plants are sterile and only reproduce by
budding or fragmentation.
How can giant salvinia be controlled?
Herbicides and biological control using the salvinia weevil
are two of the most efficient control methods for giant salvinia. Other methods are mechanical removal, either
by hand or machine, and water level drawdowns.
Beware of mechanical mowers that shred plants, as these can promote new
bud growth from fragmentation. Drawdowns
can be effective if done correctly, but they require enough time for all of the
plants to dry out completely, and often leave wet areas that cannot be drained
entirely.
Can giant salvinia be eradicated?
It is highly unlikely that giant salvinia will be
eradicated. Because it is so widespread,
it would be nearly impossible to kill or remove every single plant. Only a small amount of plant material is
needed to start a new infestation.
How do I know if I have giant salvinia?
Giant salvinia can be easily recognized by its floating,
light green fronds and egg beater-shaped trichomes (leaf hairs). It may be confused with common salvinia (Salvinia minima), a related species
which has smaller leaves and forked trichomes that are not fused at the
tip. Giant salvinia is sometimes
confused with species of duckweed, but it has much larger leaves and many
hydrophobic trichomes. Correct
identification is extremely important to successful control.
What is the LSU AgCenter doing to control giant salvinia?
The LSU AgCenter conducts research on giant salvinia control
methods including herbicides and biological control using the salvinia
weevil. Currently the LSU AgCenter
manages several salvinia weevil rearing ponds that are used to supply weevils
to the Louisiana Department of Wildlife and Fisheries (LDWF) for statewide
distribution.
Where does the salvinia weevil come from?
The salvinia weevil (Cyrtobagous
salviniae) comes from giant salvinia’s native range in South America. The weevils we have here are originally from
Brazil and came by way of Australia in the early 2000s.
What is the salvinia weevil life cycle?
Adult salvinia weevils lay eggs during warm parts of the
year (typically March-October) in giant salvinia buds or rhizomes. Larvae hatch in about 10 days and begin to
feed on the plant, eventually burrowing inside it. The larvae will begin to pupate in the
plant’s underwater “roots” (actually modified leaves) about 23 days after
hatching and emerge as adults 10-15 days later.
Adults may begin to mate and lay eggs 5 days after emergence, and can
continue to reproduce for several months.
Reproduction stops during the winter months, when adults overwinter and
commence egg laying in the spring.
Development from egg to adult takes about 45 days, and several
generations can occur in one season.
How do salvinia weevils control giant salvinia?
Adult salvinia weevils feed on young leaves and buds of
giant salvinia, which inhibits new plant growth. As the larvae burrow and feed inside the
rhizome, they disrupt the flow of nutrients from the roots to the leaves,
causing the plant to turn brown and sink.
A sufficient density of salvinia weevils can effectively clear a water
body of giant salvinia in a period of months to years.
Will the salvinia weevil eat anything other than giant salvinia?
Even if they run out of food, salvinia weevils will not move
onto crops or garden plants. Extensive research has shown that salvinia weevils
can only survive and complete their life cycle on members of the genus Salvinia. In the U.S., this includes giant salvinia (Salvinia molesta) and common salvinia (Salvinia minima). There are no native North American plants in
the Salvinia family, making the
salvinia weevil an extremely safe agent for biological control.
Will salvinia weevils control common salvinia as well as giant?
In Louisiana we have two biotypes of the salvinia weevil,
one that specializes on giant salvinia and one that specializes on common
salvinia. Even though they are the same
species, each biotype differs slightly in size and host plant preference. Weevils of the giant salvinia biotype will
feed on common salvinia, but may not proliferate on it due to its comparatively
small size. For this reason we do not
recommend releasing weevil-infested giant salvinia into common salvinia
infestations.
What happens to the salvinia weevils after all the giant salvinia has died?
If no other giant salvinia plants are nearby, the weevils
will probably die along with their host plant.
However, small amounts of giant salvinia are typically still present
even after the majority has died, and these can serve as refuges for the
weevils until they encounter more salvinia plants. The salvinia weevil population will tend to
track giant salvinia’s yearly growth cycle, increasing from spring to summer when
plants are actively growing and decreasing over winter.
What happens to salvinia weevils in the winter?
Salvinia weevil eggs and larvae cannot survive below 62°F,
so adults must overwinter until spring temperatures allow them to commence egg
laying. Adults overwinter on surviving
salvinia plants and can feed when temperatures are above 55°F. When temperatures drop, adults typically
cling to the plant’s underwater fronds where they are less exposed to the
elements. Salvinia weevil adults can
survive short periods of below zero temperatures but are at high risk of
mortality if ice forms.
Can salvinia weevils fly?
Yes, salvinia weevils are capable of flying short distances
during certain times of the year.
However, they seem to be weak fliers.
Their primary means of dispersal is by walking across plants or floating
on plants as they are carried by water currents.
How do I know if I have salvinia weevils?
The only way to be sure is by monitoring the salvinia
infestation. An easy method is to check
new buds for signs of weevil feeding damage.
Damaged growing tips will have a chewed appearance with small holes or
leaf scars. Large holes accompanied by
frass (insect droppings) are likely caused by the Samea moth and not weevils.
Photos of weevil damage are available in the LSU
Bug Biz Series. Weevil presence may
also be determined by Berlese extraction of plant material. For more information on weevil population
monitoring see our guide (soon to come), How to Release and Monitor Salvinia
Weevils for Biological Control of Giant Salvinia in Louisiana.
How are salvinia weevils reared?
The LSU AgCenter manages several outdoor rearing ponds. These ponds are constructed for ease in
monitoring the weevil population over time.
The ponds are fertilized to promote healthy giant salvinia growth and
managed for pests including fire ants and Samea
moths. Weevils are stocked year-round
and harvested a few times each year. For
more information see our guide, How
to Rear Giant Salvinia Weevils in Outdoor Ponds.
Does giant salvinia biocontrol work?
Weevil-infested giant salvinia plants are released into the
middle of giant salvinia infestations and allowed to spread as the weevil
population grows. Over time, the weevil
population will catch up with the giant salvinia and prevent further spread,
then eventually kill the plants. In
southern LA, giant salvinia biocontrol has been very effective, and results can
be seen as early as three months after the weevil population reaches an
adequate density. However, it can take
several years and multiple release attempts before the weevil population
reaches this level. For this reason,
biocontrol has had less success in north LA, where weevil establishment has
been hindered by cold winter temperatures.
How can I get salvinia weevils?
Currently the LSU AgCenter’s weevils are provided to the Louisiana Department of Wildlife and Fisheries (LDWF), and distributed statewide to various organizations. Weevils may be available to the public on a limited basis - for more information, contact Dr. Rodrigo Diaz (RDiaz@agcenter.lsu.edu) or Maggie Keller (Mkeller@agcenter.lsu.edu).
How many weevils do I need to release?
This depends on the size of the infestation, time of year,
and desired timeframe of control.
Typically, one 20-gallon tote containing adults, pupae, eggs, and larvae
is released per site, with sites being spaced at least 100-200 feet apart. Material used to fill the totes is harvested
when the weevil population density is around 30 adults per pound. It is important to continue to monitor
release sites to determine if the release was effective, or if more releases
are needed. Knowing the size of the
weevil population is important for guiding future management decisions.
When and where should weevils be released?
The best time to release weevils is in the spring, before
the giant salvinia has had a chance to reach its rapid growth stage. This way, the weevils have the best chance of
keeping up with the rate of infestation.
Releases should be made in locations with steady water levels and plenty
of direct sunlight, as these promote the best plant quality. Weevils should be released into green,
healthy plants with plenty of new buds.
It is important to confirm that giant salvinia is present prior to
making releases! Otherwise, releasing
weevil-infested material could potentially inoculate giant salvinia into a
water source that previously did not have any (for example, if only common
salvinia is present).
How soon will I see results?
Biocontrol can take months to years before results are
visible. The weevil population needs
time to grow and expand to cover the giant salvinia infestation. This is dependent on the size of the infestation,
number and size of the releases, and location.
As a rule results will be faster further south, where temperatures are
warmer and the weevils can develop faster.
It is important to continue to monitor release sites regularly so that
necessary actions including supplemental releases may be made.
Can I use other control methods in addition to weevil releases?
Effective biological control requires patience. However, an integrated approach can be
beneficial to controlling giant salvinia while the weevil population
establishes. Herbicides and mechanical
removal can be used at the same time as biological control, as long as the
weevils are undisturbed. The best
approach is to target removal zones away from weevil release sites. Keep in mind that large patches of sinking
giant salvinia can cause a temporary reduction in dissolved oxygen, which may
be harmful to fish and other freshwater animals. For this reason applying herbicide in strips
or small patches is preferable to applying over large areas. Small areas with weevils can be protected
during cold fronts with row cover, which can protect weevils from cold exposure
and reduce mortality.
