Water lettuce - Pistia stratiotes
Water lettuce - Pistia stratiotes
Water lettuce - Pistia stratiotes
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Department of Agriculture, Fisheries and Forestry<br />
Biosecurity Queensland<br />
<strong>Water</strong> <strong>lettuce</strong><br />
<strong>Pistia</strong> <strong>stratiotes</strong><br />
Fact sheet<br />
DECLARED CLASS 2 PEST PLANT<br />
PP19 December 2013<br />
<strong>Water</strong> <strong>lettuce</strong><br />
<strong>Pistia</strong> <strong>stratiotes</strong><br />
Fact sheet<br />
DECLARED CLASS 2 PEST PLANT<br />
<strong>Water</strong> <strong>lettuce</strong> is a free-floating aquatic weed that rapidly<br />
forms dense mats covering rivers, dams and irrigation<br />
canals. It can restrict water flow, increase water loss by<br />
evapotranspiration and can serve as a breeding ground<br />
for mosquitoes. I creates a desne cover on the water<br />
surface that prevents light penetration and exchange of<br />
gasses. This leads to a drop of oxygen in the water whish<br />
is detrimental to the aquatic ecosystem. <strong>Water</strong> <strong>lettuce</strong><br />
spreads both by vegetative reproduction and by seeds.<br />
There is a big infestation in the Warrego River at<br />
Cunnamulla that is of concern, as much of the Murray–<br />
Darling catchment is currently free of this weed and<br />
all efforts should be made to control this infestation<br />
and prevent further introduction. In all other areas of<br />
Queensland, an integrated control strategy should be<br />
adopted to reduce infestations.<br />
Declaration details<br />
<strong>Water</strong> <strong>lettuce</strong> is a declared Class 2 plant under Land<br />
Protection (Pest and Stock Route Management) Act 2002.<br />
A Class 2 pest is one that has already spread over<br />
substantial areas of Queensland, but its impact is so<br />
serious that we need to try and control it and avoid<br />
further spread onto properties that are still free of the<br />
pest. By law, all landholders must try to keep their land<br />
free of Class 2 pests and it is an offence to keep or sell<br />
these pests without a permit. A local government may<br />
serve a notice upon a landholder requiring control of<br />
declared pests.<br />
Great state. Great opportunity.
Description and general information<br />
As the name suggests, the entire plant resembles a small,<br />
floating, open head of <strong>lettuce</strong>. The fan-shaped leaves are<br />
pale green, have marked parallel veins, are spongy and<br />
covered with hairs and form a floating rosette. The spongy<br />
tissue and the air that gets trapped by the hairs give the<br />
plant its buoyancy. It has a tuft of unbranched, fibrous<br />
roots up to 80 cm long.<br />
Mature plants produce a large number of small, pale white<br />
to green, inconspicuous flowers hidden at the base of the<br />
leaves of the rosette. A membrane separates the male and<br />
female flowers. The fruit is a berry 5–8 cm across with<br />
oblong seeds about 2 mm long, tapered at each end.<br />
The problem<br />
Under tropical conditions, growth is rapid; dense mats<br />
form, often covering rivers, dams and irrigation canals.<br />
These mats, with their long roots, can restrict the flow of<br />
water, increase water loss through evapotranspiration<br />
and reduce light penetration that is necessary for<br />
photosynthesis in submerged aquatic plant life. The lack<br />
of oxygen and the shading significantly impact on native<br />
aquatic animals and plants.<br />
The presence of water <strong>lettuce</strong> is especially undesirable in<br />
the Murray–Darling system as it could possibly spread into<br />
inland New South Wales, Victoria and South Australia were<br />
it is currently not established.<br />
Heavy infestations can affect waterbodies in a number<br />
of ways:<br />
Safety and health risk<br />
Children and livestock may be in danger of drowning if<br />
they become entangled in the roots of a heavy infestation.<br />
<strong>Water</strong> <strong>lettuce</strong> mats may also create a haven for mosquitoes<br />
that are vectors of Ross River fever and encephalitis.<br />
Interference with irrigation and stock watering<br />
Stock may have difficulty gaining access to drinking water<br />
if the surface is completely covered by weeds.<br />
<strong>Water</strong> flow to irrigation equipment is reduced due to the<br />
restrictive action of the roots, which in turn increases<br />
pumping times and costs.<br />
Damage to structures<br />
Under flood conditions, rafts of plant material build up<br />
at fences and bridges that in turn collect other floating<br />
debris. The combined weight may cause these structures<br />
to collapse.<br />
Loss of water<br />
High rates of evapotranspiration through the leaves can<br />
increase water loss during summer.<br />
Degradation of water quality<br />
Heavy water <strong>lettuce</strong> infestations reduce the penetration<br />
of sunlight necessary for native plant growth in water<br />
bodies preventing aquatic photosynthesis and thereby<br />
reduce the availability of oxygen to other organisms.<br />
Heavy weed cover also prevents the exchange of air, which<br />
normally occurs on an open water surface, further reducing<br />
oxygenation of the water.<br />
As the plant dies and decomposes, oxygen is removed<br />
from the water; this causes water pollution and<br />
stagnation. This stagnation affects water quality and may<br />
result in the death of aquatic animals.<br />
Destruction of wildlife habitats<br />
A large infestation of water <strong>lettuce</strong> is a physical barrier<br />
for aquatic and semi-aquatic animals, restricting their<br />
territorial movements and breeding activities.<br />
Recreation and aesthetics<br />
Large infestations of water <strong>lettuce</strong> stop the passage of<br />
boats by clogging the inlets of boat engine<br />
water-cooling systems. The mats of weed also interfere<br />
with swimming and make fishing impossible. The natural<br />
beauty of an open waterbody can be spoilt and further<br />
degraded as native aquatic plants, birds and animals<br />
are displaced.<br />
Life cycle<br />
Active growth of plants begins with increasing<br />
temperatures in spring. Flowering commences early in the<br />
plant’s life–about the fourth or fifth leaf stage; high plant<br />
densitities trigger flowering. After flowering, seeds drop to<br />
the muddy bottom and form a persisting seed bank. The<br />
plants can produce large number of seeds over a season<br />
(several thousands per meter square). Seeds germinate in<br />
late November to early December the next season and float<br />
to the surface as seedlings.<br />
Plants reproduce vegetatively throughout the growth<br />
season. Each mother plants generates several daughter<br />
plants that row on stolons (up to 20 cm long). <strong>Water</strong><br />
<strong>lettuce</strong> is sensitive to frost.<br />
Habitat and distribution<br />
The origin of water <strong>lettuce</strong> remains unclea and was<br />
introduced to Australia as an aquarium and water garden<br />
specimen. As a result of flooding and abandonment,<br />
the plant is now found in a number of areas throughout<br />
eastern Queensland. The plant prefers stationary or<br />
slow-moving streams and could become established in<br />
most areas of the state. <strong>Water</strong> <strong>lettuce</strong> thrives in water with<br />
high nutrient concentrations.<br />
Like other floating plants, water <strong>lettuce</strong> can survive for<br />
long periods when stranded on mud banks or in other<br />
damp situations (such as a roadside culvert where it may<br />
even take root).<br />
2 <strong>Water</strong> <strong>lettuce</strong> <strong>Pistia</strong> <strong>stratiotes</strong>
Map 1 Distribution of water <strong>lettuce</strong> in Queensland<br />
Mechanical control of water <strong>lettuce</strong> can help take<br />
advantage of flooding or water flushes that deposit water<br />
<strong>lettuce</strong> in dams, lagoons and calm water areas of rivers<br />
and creeks. When using this approach it is essential water<br />
<strong>lettuce</strong> is removed before its rapid growth commences.<br />
<strong>Water</strong> <strong>lettuce</strong> can survive if it is deposited or left on moist<br />
banks it. To help prevent the reintroduction of water<br />
<strong>lettuce</strong> into the watercourse, it is essential it is moved<br />
from the waters edge.<br />
Biological control<br />
Two weevils (Neohydromonus pulchellus and Orchetina<br />
bruchi) have been introduced and exert good control in<br />
some areas.<br />
The weevils have proved effective on dams from<br />
Bundaberg to Brisbane. The life cycle of the weevil takes<br />
about three months. Eggs are laid in the fleshy leaves and<br />
the larvae tunnel through the plant tissue. The openings<br />
assist the entry of fungi and bacteria, causing the tissue<br />
to rot.<br />
Biological control is most effective on large infestations,<br />
but it may take several years to achieve satisfactory<br />
control if used in isolation from other methods. To<br />
establish an effective breeding population of biological<br />
control agents, infested plants should be placed in an area<br />
where the water <strong>lettuce</strong> is concentrated.<br />
Your local government office or local Biosecurity<br />
Queensland officer can assist you with protocols and<br />
information on the collection site nearest to you.<br />
Prevention<br />
New infestations may become established when plants<br />
are brought into new areas by flooding or accidential<br />
transport, i.e. through boating. Surveillance and physical<br />
removal at such times may prevent infestation. Public<br />
education to prevent disposal of plants from domestic<br />
situations is necessary.<br />
Control<br />
In most cases the best management approach combines<br />
herbicide, mechanical, fire and biological control methods<br />
with land management changes. It is essential to choose<br />
control methods that suit the specific weed and the<br />
particular situation.<br />
Mechanical control<br />
<strong>Water</strong> <strong>lettuce</strong> removal by hand or racking is a practical<br />
control method often used for small areas or when<br />
numbers are low. The use of water weed harvesters may<br />
also be used in larger areas.<br />
Physical removal is most effective for small infestations<br />
and should be made before flowering and seeding.<br />
<strong>Water</strong> <strong>lettuce</strong> <strong>Pistia</strong> <strong>stratiotes</strong> 3
Herbicide control<br />
Before using any herbicide always read the label carefully.<br />
Apply all herbicides strictly in accordance with the<br />
directions on the label. When treating water which is used<br />
for irrigation purposes, the withholding period should be<br />
followed in accordance with the label recommendations.<br />
Spraying with herbicides is often the only practical method<br />
for large infestations. Autumn applications tend to be more<br />
effective than summer ones.<br />
Table 1 lists the herbicides registered for control of<br />
water <strong>lettuce</strong>.<br />
Integrated control<br />
Integrated control is a sensible strategy that includes<br />
a combination of mechanical, biological and herbicide<br />
methods that complement each other.<br />
First make certain that the weevils are established<br />
on the infestation, and then carry out mechanical<br />
control or a spray program using a selective herbicide.<br />
Selectively controlling strips of the water <strong>lettuce</strong> mats<br />
helps concentrate the biological control insects onto the<br />
remaining weed to increase damage.<br />
Mechanical removal of dead plants will avoid water quality<br />
degradation by masses of rotting weed.<br />
Further information<br />
Further information is available from your local<br />
government office, or by contacting Biosecurity<br />
Queensland (call 13 25 23 or visit our website at<br />
www.biosecurity.qld.gov.au).<br />
Table 1 Herbicides registered for the control of water <strong>lettuce</strong><br />
1, 2, 3<br />
Situation Herbicide Rate Comments<br />
<strong>Water</strong>ways, non potable<br />
water, drains, dams,<br />
margins lakes and<br />
streams<br />
2,4-D acid<br />
(e.g. AFFRAY 300)<br />
2,4-D acid<br />
(e.g. AFFRAY 300)<br />
50 mL/10 L water<br />
or 500 mL/100 L water<br />
Knapsack: coverage 10 L spray solution/100 m²<br />
Avoid causing submersion of sprayed plants<br />
5 L/200 Lwater/ha Boom application by helicopter. Raindrop D8<br />
nozzles angled back at 45 degrees to minimise<br />
spray drift<br />
Aquatic areas<br />
Diquat (20 g/L)<br />
e.g. Watrol non-residual<br />
herbicide®<br />
50 L to 100 L/ha<br />
or 4 L/100 L water<br />
Add wetting agent. Higher rate for denser<br />
infestations or dirty water. Overall spray,<br />
thoroughly wetting foliage. Follow label<br />
instructions<br />
Add wetting agent. Follow label instructions<br />
Aquatic areas (drains,<br />
channels, margins of<br />
streams, lakes and<br />
dams)<br />
Aquatic areas<br />
Diquat (20 g/L)<br />
e.g. Vegetrol herbicide ®<br />
Diquat (20 g/L)<br />
e.g. Vegetrol herbicide ®<br />
Calcium dodecyl<br />
benezene sulphonate<br />
(e.g. immerse)<br />
50 L/ha to 100 L/ha<br />
or 4 L/100 L water<br />
5 L/ha to 10 L/ha<br />
or 0.4 L/100 L water<br />
1:19 with kerosene.<br />
Apply 1 L of mixture per<br />
100 m²<br />
Add wetting agent. Follow label instructions<br />
Sprinkle onto free floating plants and adjacent<br />
water surface lightly, just enough to change their<br />
normal colour. Don’t spray dense solid mats with<br />
no visible water surface.<br />
Use in Summer. Follow label instructions<br />
Glyphosate (360 g/L)<br />
(numerous)<br />
1 L to 1.3 L/ 100 L water<br />
or 6 L/ha to 9 L/ha<br />
Notes:<br />
Diquat is the only product registered for use in water storage areas used for human consumption. Note that 14 days must<br />
elapse after treatment before water can be consumed.<br />
Read the label carefully before use. Always use the herbicide in accordance with the directions on the label.<br />
This fact sheet is developed with funding support from the Land Protection Fund.<br />
Fact sheets are available from Department of Agriculture, Fisheries and Forestry (DAFF) service centres and our Customer Service Centre (telephone 13 25 23). Check<br />
our website at www.biosecurity.qld.gov.au to ensure you have the latest version of this fact sheet. The control methods referred to in this fact sheet should be used in<br />
accordance with the restrictions (federal and state legislation, and local government laws) directly or indirectly related to each control method. These restrictions may<br />
prevent the use of one or more of the methods referred to, depending on individual circumstances. While every care is taken to ensure the accuracy of this information,<br />
DAFF does not invite reliance upon it, nor accept responsibility for any loss or damage caused by actions based on it.<br />
© The State of Queensland, Department of Agriculture, Fisheries and Forestry, 2013.