T. D. Center, D. L. Sutton, V. A. Ramey and K. A. Langeland
Two types of biological control are used in aquatic plant management, the introduction
approach and the augmentation-manipulation approach.
Introduction Approach
Native plant communities evolve with a complex relationship of natural controls that keep
them in balance. These natural controls may include environmental restraints, competing species,
herbivores (plant eating organisms) and pathogens. When plants are introduced into new areas,
they
often have a competitive advantage over native plants because natural controls of the introduced
plants are not present. This competitive advantage may allow introduced plants to cause
problems
such as displacing native plant communities. The introduction approach to biological control is
the
purposeful introduction of natural controls such as insects and pathogens, to help provide balance
among native and introduced plants. The natural control is usually referred to as the biological
control
agent or biocontrol agent. Small numbers of biocontrol agents are released, where they
previously
did not exist, so that they can increase to a point where they are in balance with the target pest
(weed)
and establish a self-perpetuating population that reduces weed density.
The most attractive aspect of the introduction approach is that it can be permanent and
self-perpetuating. Once established, additional releases are usually unnecessary so additional
expenses
are avoided. However, exceptions occur when it becomes necessary to move field-collected
bioagents to new locations. While the initial expense is high, over the long run the introduction
approach is among the least expensive control options. Benefit to cost ratios of this approach
have
been estimated at 50 - 100:1 or even higher.
Augmentation-manipulation approach
The augmentation-manipulation approach involves releases of the biological control agent at
levels capable of providing control and at strategic times. For example, herbivorous (plant
eating)
fish are used in this manner.
Stocks of bioagents must be maintained for the augmentation-manipulation approach and
systems must be developed to efficiently distribute the biocontrol agent. The desirability of this
approach is its greater predictability. The level of control attained is dependent upon the quantity
of
bioagents used. The augmentation-manipulation approach lends itself to commercial enterprise
because business may profitably supply biological control organisms to management
interests.
Advantages of using bioagents to manage aquatic weeds include: (1) longevity of control once
the organism has become established, (2) constant pressure against the growing weeds, (3) low
long-term cost, (4) high effectiveness against specific weeds, (5) minimum impact on nontarget
species
(introduction approach) (6) in the case of some fish, the production of an edible protein
product.
Herbivorous Fish
Triploid grass carp. Triploid grass carp (Figure 13) are
specially produced in hatcheries and possess three sets of chromosomes instead of the normal
two.
This abnormal condition causes sterility, so these are the only exotic fish that can be legally used
for
aquatic weed control in most states. A permit is usually required for possession and use of
triploid
grass carp. Because they cannot reproduce, the number of fish present in an area can be
regulated.
Triploid grass carp prefer to consume submersed plants, so they are effective controls of this
type of vegetation. However, as the name, grass carp implies, they consume many terrestrial and
aquatic grasses. Grass clippings provide a ready food supply that can be used to rear fish to an
appropriate size for stocking.
Grass carp also browse tips of young, tender emergent plants which often provides control
of emergent species. Although young grass carp feed on filamentous algae such as
Cladophora and
Spirogyra, they are not effective for control of most filamentous algal species and
they do not control
phytoplankton.
The ability of grass carp to consume aquatic plants depends on the size of both plants and
fish.
Other factors such as age, gender, and population density of the fish and species, abundance, and
location of the vegetation influence the feeding behavior of the grass carp.
Because predators like birds, snakes and other fish are normally present, grass carp that are
1 pound (10-12 inches) or larger should be stocked to maximize survival.
Stocking rates of 20 - 25 grass carp per acre effectively control hydrilla. A dense stand of
weeds requires more fish than a sparse stand, but grass carp should not be stocked in extremely
dense
infestations where low oxygen levels may occur. In these instances, herbicides or mechanical
methods should be used to reduce plant density followed by stocking of low numbers of grass
carp
to prevent regrowth.
Triploid grass carp should not be released in large open systems until
additional research data is available because of their nonselective feeding behavior and the
lack of predictability of their movements in open systems.
Tilapia. Tilapia are tropical species that can suppress growth
of softer aquatic vegetation such as filamentous algae and bladderwort when stocked at high
density
(300 per acre). Two species of tilapia have been considered for aquatic weed control. The Blue
tilapia (Tilapia aurea) feeds entirely on algae (planktonic and filamentous) but does
not readily
consume larger, coarser vegetation. The redbelly tilapia (T. zilli) feeds on larger
submersed
vegetation rather than algae. However both species reproduce rapidly and consume both the
vegetation and small animals that are important food sources for desirable fish populations.
Therefore
use of tilapia can have unwanted environmental consequences.
Tilapia will not overwinter in water below 65 F. This is a benefit from an environmental
standpoint, but annual restocking is necessary in temperate climates unless a warm water supply
(such
as a thermal spring or power plant cooling effluent) is available as a refuge during winter. In
tropical
climates, where they do overwinter, they are prolific and can be detrimental to sportfish
populations.
Use of tilapia for aquatic weed control is legal in some states but not in others.
Before stocking any type of fish for biological control of aquatic weeds,
check with appropriate state agencies to determine state regulations.
Insects
A foreign insect species must be extensively tested and proven to be host-specific (can not
reproduce in the absence of the host) before it can be released in the U.S. These tests are
designed
to demonstrate that the bioagent will not feed appreciably or reproduce on any plant other than
the
target weed. This ensures that it will not harm crop plants or other desirable species.
Alligator weed. Alligator weed, a native of South America,
was
the first aquatic weed target for biocontrol because it was difficult to control with herbicides.
Three
host-specific South American insects were found and eventually released. These include the
alligator weed flea beetle (Agasicles hygrophila) which was released in
1964; the
alligator weed thrips (Amynothrips andersoni) which was released in 1967; and the
alligator weed stem
borer (Vogtia malloi), a moth which was released in 1971.
Agasicles hygrophila
Both larvae and adults of alligator weed flea beetle feed on leaves and severely defoliate
extensive stands of alligator weed in most southeastern states. They are less effective in more
temperate areas and have little effect north of South Carolina. Their effectiveness in more
temperate
areas can be enhanced by collecting and releasing large numbers of adults early in the year,
especially
after cold winters that diminish populations. Alligator weed flea beetles can be obtained for
release
by contacting the United States Corps of Engineers, Jacksonville District.
Vogtia malloi
The alligator weed stem borer is a small, brown moth. Because of its high dispersive ability,
this insect readily moves to new areas where it can become a very important biocontrol agent.
Damage by the alligator weed stem borer causes the stems to collapse and the mat acquires a
"flattened" appearance. This, often in conjunction with flea beetle defoliation, provides very
effective
alligator weed control.
The alligator weed thrips is the least well known of the three alligator weed bioagents.
Although they can locally devastate alligator weed populations they are not an effective
biocontrol
agent because they do not readily disperse.
Water hyacinth. Three species of insects have been released
for control of water hyacinth. The first was the mottled water hyacinth weevil (Neochetina
eichhorniae) (Figure 15) that was released in Florida in 1972. The second was the
chevroned
water hyacinth weevil (Neochetina bruchi), which is quite similar to the first. It was
released in
Florida in 1974. The third insect was a moth, the water hyacinth borer (Sameodes
albiguttalis) which
was released in 1977.
Neochetina species
A good indication of the presence of water hyacinth weevils is the occurrence of distinctive
adult feeding scars on the leaves. Mature larvae can often be found in the petiole bases or in the
stem.
Adults of both species only fly intermittently and are usually unable to disperse from site to site.
Occasionally, however, they develop flight muscles and dispersal follows. The weevils
(especially
the chevroned) have been the most effective of the water hyacinth insects.
Outbreaks of water hyacinth borers are devastating to young water hyacinth stands and
sometimes prevent regrowth after herbicidal control. Because plants are attacked only briefly
during
early mat development, however, water hyacinth borer population seems sporadic. During this
limited
period they effectively slow the rate of mat development.
Sameodes albiguttalis
Water hyacinth weevils and borers have suppressed water hyacinth growth in many areas.
This
control is sometimes manifested as sudden declines but these are rare. Instead, long term decline
of
the plant population typifies the normal biocontrol pattern. When stressed by insects, the mats
may
persist for extended periods but fail to expand during favorable growing periods. The associated
stress also increases vulnerability to diseases, frosts, and other factors. This longterm process is
not
helpful if the immediate removal of a water hyacinth infestation is required but probably reduces
the
amount of other control measures that must be used.
Water lettuce. A small South America weevil,
Neohydronomus
affinis (Figure 16), was released on a 75-acre water lettuce infestation on Kraemer Island
in
Lake
Okeechobee, Florida during April 1987. By April 1989, the weevil population increased to
extremely
high densities and thoroughly suppressed the water lettuce population.
Neohydronomous affinis
These results have now
been
repeated at other sites. This insect previously controlled water lettuce in Australia and South
Africa.
A moth, Namangana pectinicornis, which is native to Southeast Asia, has been used
successfully in
Thailand. It has recently been released in Florida and should become a very effective control
agent.
Hydrilla. Worldwide surveys were begun in 1981 to find
bioagents of hydrilla. Several species of insects were found and two species from India and one
from
Australia have been released. Four species of Australian insects are now under evaluation. One,
a
small, stem-boring weevil, Bagous new species, is presently being considered for
release. Other
promising insects are known from different parts of the world that have not yet been
evaluated.
Bagous affinis
A weevil that feeds on hydrilla tubers, Bagous affinis (Figure 17), occurs
naturally
in India
and Pakistan. It is specific to hydrilla and was first released at Lake Tohopekaliga on 30 April
1987
and subsequently in several other sites in Florida. Additional studies are needed to determine if
this
weevil can control hydrilla tubers.
Hydrellia pakistanae
Two species of leaf-mining flies, Hydrellia pakistanae (Figure 18) from India and
an
undescribed Hydrellia species from Australia, have been released in Florida. The
larva burrows within
and destroys up to 12 leaves during the course of its development. These flies show a great deal
of
potential for hydrilla control if they can be successfully colonized in the field.
Parapoynx diminutalis
A small aquatic moth, Parapoynx diminutalis (Figure 19), that is native to
Southeast Asia was
accidentally introduced into Florida, probably with imported aquarium plants. It is now very
widespread within the state and seems to be quite specific to hydrilla in the field. They
sometimes
severely damage hydrilla, leaving the stems completely defoliated. Although little is known
about
it,
the occurrence of this moth seems sporadic and unpredictable and occurrences do not result in
acceptable levels of control.
Pathogens
The use of plant pathogens for biological control of aquatic weeds has a great deal of intrinsic
appeal. Suspensions of spores can be readily formulated and easily applied to the weed, much
like
a herbicide. In theory, these could then be self-sustaining. If so, this approach would encompass
the
best advantages of both biological and herbicidal control. Unfortunately pathogens tend to be
environmentally sensitive and can be rendered ineffective by extremes of temperature or
humidity.
The introduction approach would seem ideal for the use of pathogens. Restrictions regarding
the importation of plant pathogens from abroad tend to prohibit this approach and limit the scope
to
native pathogens. One fungal pathogen (Cercospora rodmanni), has been formulated
as a
mycoherbicide for water hyacinth. However, it has not been very effective and research in this
area
in continuing.
Snails, manatees, etc.
Two snails (Marisa cornuarietis and Pomacea australis) have been
studied as potential
biocontrol agents for aquatic weeds. Large numbers will control several species of submersed
aquatic
plants under confined conditions. However, snails are not currently under consideration as
biocontrol
agents for aquatic weeds because of environmental risk associated with the purposeful
propagation
of prolific, generalized herbivores, they are intermediate hosts for certain fish and human
parasites
and they are not effective under natural, unconfined conditions.
Manatees or sea cows (Trichechus manatus) have been experimentally used,
mainly in canals,
for aquatic weed control in Florida. Manatees effectively removed submersed and floating plant
species. However, during winter heaters were required to keep manatees warm. In a study of
King's
Bay (Crystal River, Florida), conducted by the U.S. Fish and Wildlife Service, biologists found
that
10 times as many manatees as normally wintered there could not consume the existing hydrilla
biomass, much less keep up with the growth of plants.
Other biological controls for aquatic weeds that have been suggested and/or tested include
ducks, geese, crayfish, nematodes, viruses, and water buffalo. Any of these may be useful under
highly specialized conditions, but none have proven practical.
Mechanical control involves the use of machines to cut or remove aquatic plants.
Mechanical
control provides immediate, tangible results especially when the weeds are removed from the
water
body. Because of its high cost, mechanical control is generally practical only for small areas like
marinas, swimming areas, boating trails, or where other methods are unfeasible or undesirable.
Rapid
regrowth by some aquatic weeds often necessitates repeated mechanical control operations
during
a single growing season.
The first large machine used for aquatic plant management was designed and built for the
Army Corps of Engineers in 1900. It crushed water hyacinths and discharged them back into the
water. In 1937, the "Kenny" was invented which could crush 200 acres of water hyacinths per
month.
In the 1940s, "saw boats" were used to cut submersed plants which were also left to decompose
in
the water.
Because of the water quality problems associated with the decay of the plants, modern
harvesters are designed to remove the harvested material. Working widths on these machines
range
from 4-18 feet with working depths of 1.5-9 feet. They can haul up 10-13 tons. Some systems
include shuttle barges that transport plants to shore while the cutter/harvester continues to
work.
The choice of a machine to manage a particular weed problem depends on the plant species
in question, the disposal system to be used, and the management objectives for the water body.
No
one system is universally effective. A number of factors, including costs, should be considered
when
selecting a mechanical harvesting system.
One purpose that has been suggested for mechanically harvesting aquatic plants is to remove
nutrients from over enriched waters. All plants require nutrients for growth and these nutrients
are
stored in plant tissue. Therefore harvesting aquatic plants simultaneously removes nutrients from
aquatic ecosystems. However, because the water content of most problem aquatic plants is 90%
or
greater, the quantity of nutrients removed is low in relation to the mass of plant material
harvested.
Mechanical harvesting as a means of reducing nutrients in natural waters has not been shown
cost
effective.
During the past 100 years, many researchers throughout the world have attempted to find
practical uses for harvested aquatic plants. Because utilization is most efficient when the plants
are
deliberately grown in easily harvested aquacultural systems, it is unlikely to be applicable as an
aquatic
weed management practice. Except for some small local operations, no market currently exists
for
harvested aquatic weeds.
Mechanical removal is an important method of aquatic plant management in certain
circumstances because of several advantages it has over other methods. Immediate control can
be
achieved in small areas and the water can be used immediately, as compared to water use
restrictions
as associated with some herbicides. Rotting plants do not remain in the water to deplete oxygen.
Algal blooms following weed control are not as likely as compared to other methods because
nutrients bound in plants are removed.
Use of mechanical harvesting for aquatic weed control is limited because of several
disadvantages. It is usually higher in cost and much slower than other methods and there are
high
maintenance and repair costs. Some water bodies are not suitable for because of water depth and
presence of obstructions. A suitable area for disposal of harvested plants must be available.
Wildlife
and desirable vegetation is removed with harvested weeds. Plant fragments drift to infest new
areas.
Increased turbidity may result from disturbance of sediments.
Water level manipulation refers to the raising of water levels to control aquatic vegetation by
drowning or lowering to control aquatic vegetation by exposing them to freezing, drying or heat.
Use
of water level manipulation for aquatic plant management is limited to reservoirs with adequate
water
control structures.
Drawdown, which refers to the lowering of water level is more commonly used than raising
water levels. Drawdown has been used in lake management for many years to oxidize and
consolidate flocculent sediments, to alter fish populations, and for aquatic weed control. In
addition
to the need for an adequate water control structure, use of drawdown for aquatic plant
management
may also be restricted by considerations such as water use patterns (e.g. disruption of recreational
or agricultural use) or a predictable source of water for refilling.
Drawdown is usually conducted during winter months so that plants are exposed to both
drying and freezing. Summer drawdown can also be effective but usually results in greater
impact
to recreational water use, stresses fish populations and has a greater potential to enhance the
spread
of emergent plants such as cattails, rushes and willows.
Drawdown alters the composition of aquatic vegetation, but doesn't always
produce desirable
changes. The responses of various aquatic plant species to drawdown vary widely (Table 5) and
sometimes unpredictably. Brazilian elodea is sensitive to drawdown and is often
controlled for up
to three years by this method. In contrast, drawdowns only partially control hydrilla, a near
relative
of Brazilian elodea, when it is growing in sandy lake bottoms and have little effect when hydrilla
is
growing in organic sediments. The hydrilla tubers that are produced deep within the hydrosoil
are
protected from desiccation and can survive several consecutive drawdowns. In
general submersed
aquatic plants have variable responses to drawdown while emergent plants readily tolerate
them.
The advantages of drawdown as a method of aquatic plant management include
the low cost
(unless recreational or power generation is lost) and secondary benefits of sediment oxidation
and
consolidation and fisheries enhancement. Potential undesirable effects of drawdown include
reductions of desirable species, increases of undesirable tolerant species like hydrilla, expansion
of
undesirable species to deeper areas, the creation of floating islands, and the loss of storage water
and
recreational benefits if insufficient water is available to refill the basin.
All plants require a certain amount of light to grow. Submersed aquatic plants can sometimes
be controlled or suppressed by reducing light penetration into the water. Light penetration can be
reduced by the use of special pond dyes, special fabric bottom covers or fertilization.
Even though dyes are not pesticides, only those that are approved for use in ponds should be
used. These specially produced dyes block the kind of light that plants need for photosynthesis
and
are not toxic to aquatic organisms, humans or animals that might drink the treated water. Dyes
are
only effective in ponds that have little or no flow through them and they are generally effective
only
in water of 3 feet or greater depth. Therefore, the water body can remain productive for fish.
Various materials, including black plastic and specially manufactured bottom covers, have
been used prevent rooted aquatic plants from growing. Gases that are produced on pond bottoms
accumulate under nonpermeable bottom covers, such as plastic, and cause them to float to the
surface. However, specially made bottom covers can be effective for preventing submersed
aquatic
plant growth. In addition to preventing light from reaching the pond bottom these materials also
physically prevent rooted aquatic plants from becoming established. These special materials are
very
expensive and must be replaced routinely. Therefore, their use is restricted to ornamental ponds,
swimming areas or around boat docks (care must be taken to prevent the bottom cover from
becoming tangled in boat propellers).
Another method of reducing light penetration into water is to increase the amount of
phytoplankton in water by adding fertilizer to ponds. Fertilization can be an effective method of
controlling aquatic weeds in ponds but it is not recommended in large water bodies. Fertilization
must be done correctly and must be continued once started. Even if it is done correctly
fertilization
can have adverse effects such as increasing the potential for fish kills and deleterious blue green
algal
blooms. Additional information of pond fertilization can be found in pond management and
aquaculture publications that are available from Cooperative Extension Offices.
