Agronomy Department, Center for Aquatic Plants
University of Florida,
Institute of Food and Agricultural Sciences
Gainesville, FL 32653
Cite as follows: Langeland, K.A. 1996. Hydrilla
verticillata (L.F.) Royle (Hydrocharitaceae), "The Perfect Aquatic
Weed". Castanea 61:293-304.
ABSTRACT
The submersed macrophyte hydrilla (Hydrilla verticillata (L.F.)
Royle), which is native to the warmer areas of Asia, was first discovered in
the United States in 1960. A highly specialized growth habit, physiological
characteristics, and reproduction make this plant well adapted to life in
submersed freshwater environments. Consequently, hydrilla has spread rapidly
through portions of the United States and become a serious weed. Where the
plant occurs, it causes substantial economic hardships, interferes with
various water uses, displaces native aquatic plant communities, and
adversely impacts freshwater habitats. Management techniques have been
developed, but sufficient funding is not available to stop the spread of the
plant or implement optimum management programs. Educational efforts to
increase public and political awareness of problems associated with this
weed and the need for adequate funding to manage it are necessary.
INTRODUCTION
Colonization of the land by ancestral marine autotrophs, which began long
before the mid-Paleozoic, gave rise to evolution of vascular plants
(Sculthorpe 1967). Ecological adaptability has allowed this group to evolve
species that have colonized diverse terrestrial habitats from desert to
tundra. A small group of plants, 1 per cent by most liberal estimates,
returned to life in aquatic and marine environments (Sculthorpe 1967). These
fresh water and marine vascular plants, as a group, are particularly
fascinating because of numerous adaptations they have evolved as they
returned to the submersed environment. One of the most studied aquatic
vascular plants is hydrilla (Hydrilla verticillata (L.F.) Royle
(Hydrocharitaceae)). Hydrilla could easily be called the perfect aquatic
plant because of the extensive adaptive attributes it possesses to survive
in the aquatic habitat. These characteristics allow Hydrilla to be an
aggressive and competitive colonizer of aquatic habitats. Hydrilla has
become a serious pest in North American waters. This paper will discuss
hydrilla as "the perfect aquatic weed" in North America.
IDENTIFICATION
Here are pictures and
drawings of hydrilla and infestations.
Hydrilla is highly polymorphic, its appearance can vary considerably
depending upon the conditions under which it is growing (Verkleij et
al. 1983; Pieterse et al. 1985). It grows submersed in water
and generally is rooted to the bottom, although sometimes fragments will
break loose and survive in a free-floating state. Erect stems can be quite
long when the plant grows in deep water. Branching is usually sparse until
the plant grows to near the water surface, then branching becomes profuse.
Many horizontal above-ground stems (stolons) and underground stems
(rhizomes) are also produced. Leaves are 2-4 mm wide, 6-20 mm long, and
occur in whorls of 3-8. The leaves have 11-39 sharp teeth per cm along the
margins and often have either spines or glands on the underside of the
midrib. The midrib is also often red. Adventitious roots are usually glossy
white unless growing in highly organic sediments in which case they take on
the reddish brown color of the sediment, or they can have a green cast
caused by the presence of chlorophyll when exposed to light.
Hydrilla can be either monoecious or dioecious with both male and female
flowers singly froma spathe (Cook and L#nd 1982, Pieterse 1981). Female
flowers consist of three whitish sepals and three translucent petals, are
10-50 mm long, 4-8 mm wide, attached at leaf axils, are clustered toward the
tips of the stems, and float on the water surface. The stem tips from which
female flowers arise are often very compact and have very short leaves.
Female flowers are resistant to wetting and when returned to the water
surface after submergence will immediately re-float. A submerged female
flower has been described as an inverted bell filled with a large bubble.
Male flowers have three whitish red or brown sepals that are up to 3 mm long
and 2mm wide. They have three whitish or reddish linear petals that are
about 2mm long and they have three stamens which are formed in leaf axils.
Male flowers are released and float to the surface as they approach
maturity. Thousands of these free floating male flowers are sometimes
observed in windrows on ponds (Langeland and Schiller, 1983). Both male and
female flowers are produced singly from the spathe.
Hydrilla produces hybernacula, turions in leaf axils and tubers
(subterranean turions) terminally on rhizomes. Turions are very compact
dormant buds that are produced in leaf axils and fall from the plant when
they mature. These structures are 5-8 mm long, dark green, and appear to be
spiny. Tubers are formed terminally on rhizomes or stolons and can be found
30 cm deep in the sediment. They are 5-10 mm long and are off-white to
yellow unless they take on darker colors from organic sediments.
DISTRIBUTION
Hydrilla is probably native to the warmer regions of Asia (Cook and L#nd
1982). It is a cosmopolitan species that occurs in Europe, Asia, Australia,
New Zealand, the Pacific Islands, Africa, Europe, South America, and North
America. Although hydrilla occurs in temperate areas, it tends to be more
widespread in tropical areas of the world.
Hydrilla was discovered in the United States in 1960 at two Florida
locations, a canal near Miami and in Crystal River (Blackburn et
al. 1969). It spread throughout the state very rapidly. By the early
1970s it was established in major water bodies of all drainage basins in the
state. In 1988, the Florida Department of Natural Resources estimated over
20,000 ha of water in Florida contained hydrilla (Schardt and Nall 1988).
Hydrilla continues to spread in Florida and in 1995 covers 40,000 ha of
water in 43% of public lakes. Hydrilla is now found in all Gulf Coast
states, Atlantic Coast States as far north as Maryland and Delaware, and in
the western states, California, Washington, and Arizona.
It is evident that there have been at least two hydrilla introductions
into the United States because at least two different forms occur. Florida
populations are dioecious female, as are all wild populations thus far
observed as far north as Lake Marion in South Carolina. Most populations
north of Lake Marion are monoecious. The exceptions are a dioecious
population in Wilmington, North Carolina and both dioecious and monoecious
plants in Lake Gaston, which borders North Carolina and Virginia (Ryan
et al. 1995).
A major question that remains is how far north in the United States
hydrilla will spread and whether it will be a problem in the northern states
as it is in the southern states. The northernmost monoecious hydrilla
population occurs at approximately 40o north latitude in the United States.
In Poland and the Soviet Union, hydrilla occurs near 50o north latitude.
These latitudes are similar to US and Canadian border and suggest the
northern limit for hydrilla colonization in the northern hemisphere.
However, hydrilla does not seem to spread readily from existing populations
in Northeastern Europe (Cook and L#nd 1982). Still, how far north in the
United States hydrilla will thrive and be a weed problem remains a
question.
Research suggests that the monoecious strain is better adapted to the
temperate climate because it can form tubers more quickly during short
photoperiods (Spencer and Anderson 1986, Van 1989) and also during long
photoperiod (Van 1989). This may explain the distribution of the monoecious
and dioecious populations along the Atlantic coast, or the distribution
could be coincidental.
BIOLOGY AND PHYSIOLOGY
Hydrilla can establish and then displace native aquatic plants such as
pondweeds (Potamogeton sp.) and eelgrass (Vallisneria
americana Michaux). While all aquatic plants have developed adaptations
for life in the aquatic environment, hydrilla seems to be a couple of steps
ahead of other submersed plants. Research has identified many of the
characteristics that enable hydrilla to exist and compete so effectively.
Some of these characteristics are very simple and effective while others are
complex and of scientific interest.
The growth habit of hydrilla enables it to compete effectively for
sunlight. It can elongate very rapidly, up to one inch per day, until it
nears the water surface. Near the water surface it branches profusely and
produces greater stem density than other submersed aquatic plants. One half
of hydrilla standing crop occurs in the upper 0.5 m of water column (Haller
and Sutton 1975). By producing this mat of vegetation on the water surface
hydrilla is able to intercept sunlight to the exclusion of other submersed
plants. Hydrilla makes efficient use of available nutrients. Hydrilla tissue
is composed of approximately 90% water (Van et al. 1976).
Therefore, the plants can produce a great deal of fresh plant material from
a limited supply of the essential plant nutrients carbon, nitrogen and
phosphorus.
Hydrilla is able to grow under a wide range of water chemistry
conditions. It is commonly found in oligotrophic (low nutrients) to
eutrophic (high nutrients) lakes (Cook and L#nd 1982). It can grow in water
up to about 7% the salinity of seawater (Haller et al. 1974) or
higher (Steward and Van 1987); and it tolerates a wide range of pH, but
tends to grow better at pH 7 (Steward 1991).
Hydrilla is adapted to use low light levels for photosynthesis (Van
et al. 1976, Bowes et al. 1977). This means that hydrilla
can begin to photosynthesize earlier in the morning and thus successfully
compete with other aquatic plants for limited dissolved carbon in the water
. The low light requirement (1% of full sunlight or less) also allows
hydrilla to colonize in deeper water than other aquatic plants. Hydrilla has
been found growing at a depth of 15 m in Crystal River and commonly occurs
in water 3 m deep in Florida lakes.
Submersed plants are subjected to constraints on photosynthesis in
comparison to terrestrial plants. Owing to the 104x slower diffusion rate of
carbon dioxide in water than air, efficient use of bicarbonate ion as a
dissolved inorganic carbon source is an important competative characteristic
for existence in the aquatic environment. Hydrilla can use free carbon
dioxide from surrounding water when it is available and can switch to
bicarbonate utilization when conditions favors its use i.e., high pH and
high carbonate concentration (Salvucci and Bowes 1983). These conditions
occur in highly productive waters during warm water and high photosynthesis
conditions. Under these conditions, hydrilla can also switch to C4-like
carbon metabolism, characterized by low photorespiration, and inorganic
carbon fixed into malate and aspartate (Holaday and Bowes 1980).
Hydrilla is very efficient at reproducing itself and maintaining itself
during adverse conditions. It can reproduce itself in four different ways.
These are: fragmentation, tubers, turions, and seed.
Almost 50% of hydrilla fragments that have a single whorl of leaves can
sprout a new plant that a new population can grow from, and greater than 50%
of fragments with only three whorls of leaves can sprout (Langeland and
Sutton, 1980). This means that small amounts of hydrilla on boat trailers,
bait buckets, draglines, and from aquariums can spread the plant from place
to place.
Turions are formed terminally on rhizomes (commonly called tubers or
subterranean turions) and in leaf axils (commonly called turions or axillary
turions). One single subterranean turion has been shown to produce over 6000
new turions per m2 (Sutton et al. 1992), and 2,803 axillary turions
can potentially be produced per m2 (Thullen 1990). Subterranean turions can
remain viable for several days out of water (Basiouny et al.1978),
and for over four years in undisturbed sediment (Van and Steward, 1990).
They also survive ingestion and regurgitation by waterfowl (Joyce et
al. 1980), and herbicide applications (Haller et al.
1990).
Seed production is probably of minor importance to hydrilla reproduction
compared to its successful vegetative reproduction. Although seed production
and viability is low compared to many other weeds (Langeland and Smith
1984), the importance of seed production has not been well researched and is
not adequately understood. Seeds of many plants can be ingested by birds,
carried for long distances, and passed through the gut in a viable
condition. If this proves to be true for hydrilla seed, it may prove to be
an important means of natural, long distance dispersal.
IMPORTANCE
Hydrilla causes major detrimental impacts on water use. In drainage
canals it greatly reduces flow, which can result in flooding and damage to
canal banks and structures. In irrigation canals it impedes flow and clogs
intakes of pumps used for conveying irrigation water. In utility cooling
reservoirs it disrupts flow patterns that are necessary for adequate cooling
of water. Hydrilla can severely interfere with navigation of both
recreational and commercial craft. In addition to interfering with boating
by fisherman and waterskiers in recreational waters, hydrilla interferes
with swimming, displaces native vegetation communities, and can adversely
impact sportfish populations. The economic impacts of these water uses to
real estate values, tourism, and user groups can be staggering. For example,
an economic study on Orange Lake in North Central Florida indicated that the
economic activity attributed to the lake was almost $11.0 million and during
years that hydrilla completely covers the lake these benefits can be
virtually lost (Milon et al. 1986). Cost of hydrilla management is
also extremely high, especially when funding is insufficient for adequate
management. An estimated $10.0 million is necessary to manage hydrilla in
Florida public waters in 1994-95 and $14.5 million will be necessary in
1995-96, as hydrilla continues to expand (Jeff Schardt, Florida Department
of Environmental Protection, personal communication). Highly transparent water is often considered desirable by the public and
large populations of submersed aquatic macrophytes, such as hydrilla, will
tend to increase water clarity (Canfield et al. 1984). The exact
reasons for this increase in water clarity are not completely understood but
it probably results from a combination of factors which include lowering
sediment re-suspension and reduction of phytoplankton populations by
compartmentalizing nutrients. Regardless, large amounts of aquatic
macrophytes are necessary to cause substantial increases in water clarity
(Canfield et al. 1984; Canfield and Hoyer 1992). The endeavor to benefit sportfish or waterfowl habitat or produce clear
water has resulted in deliberate dispersal of hydrilla by individuals unwary
of the severe detrimental impacts that can be caused by the plant.
Detrimental impacts caused by hydrilla far outweigh beneficial impacts and
it is usually more difficult to manage than native plant populations, which
it displaces. MANAGEMENT Hydrilla is managed differently in different types of waters, which
depends on water uses. Therefore different methods or combination of methods
are used depending on the desired end result. In water conveyance systems,
the end result may be no vegetation, whereas in recreational waters the goal
is usually to improve the environment by selectively controlling hydrilla
amongst native vegetation. Management methods include herbicides, grass carp
(Ctenopharyngodon idella Val.), and mechanical removal. Insects
have been released for classical biological control agents and others are
under study. The herbicide active ingredients, copper, diquat, endothall, and
fluridone can be used to selectively control hydrilla to some extent,
depending on the associated plant community. Copper, diquat and endothall
are fast acting contact herbicides that have relatively broad spectrums on
submersed aquatic plants. They are used to selectively control hydrilla by
injection of liquid herbicides, from trailing hoses, under floating leafed
vegetation such as spadderdock (Nuphar sp.) or around emergent
vegetation such as bulrush (Scirpus sp.) (Langeland et al.
1991). Granular endothall can be used in the same manner. Fluridone is only
effective for whole-pond applications or large scale (>2 ha.)
applications in large water bodies and its selectivity is dependent on
application rates, contact times, and timing of applications. For example,
fluridone has been used to manage hydrilla in Lake Okeechobee with minimum
to no long term impact on a native vegetation community consisting of
southern naiad (Najas guadalupensis (Sprang.) Magnus), eelgrass,
pondweed (Potamogeton illinoensis Morong), and American lotus
(Nelumbo lutea Willd.) (Langeland et al. 1991). Grass carp is a herbivorous fish that is effective for controlling
hydrilla (Van Dyke et al. 1984). Possession of this fish is illegal
in most states because of the potential environmental damage that could
result if escaped fish establish a breeding population. Sterile, triploid
grass carp (Malone 1984) are also effective (Cassani and Caton 1986) and are
now available and legal by permit in some states in the U.S. In small ponds
or lakes and canal systems, with adequate control structures, and where
total removal of vegetation is acceptable, triploid grass carp stocking is
highly recommended. They have been used to selectively manage hydrilla in
water detention ponds where emergent vegetation was desirable but this use
is unpredictable (personal communications with contractors). Because they
are non-specific herbivores, an adequate method of recapturing the fish has
not been developed, and because stocking rates for partial control have not
been established, grass carp are rarely used in large multi-purpose lakes
where aquatic vegetation is desirable for sportfish and waterfowl
habitat. Specialized machines are used for mechanically removing hydrilla.
However, this is not a widespread practice because of the high cost
involved, which is often over $1000 per acre and because of logistical
constraints in large water bodies. Because of hydrilla's rapid growth rate,
up to six harvests are required annually (McGehee 1979). Mechanical removal
is mainly used for hydrilla management in proximity to domestic water supply
intakes, in rapidly flowing water, and when immediate removal is
necessary. A commonly asked question is if there is a use for harvested plant
material that would help defray the high cost of harvesting. Research has
been conducted to determine the feasibility of using harvested hydrilla for
practical purposes, such as cattle feed (Easley and Shirley 1974, Bagnall
et al. 1978). Considering the high cost of harvesting hydrilla and
its low nutritive value and fiber content compared to its wet bulk very
little return can be derived from the product. Some of the earliest research for classical biological control of
hydrilla was with snails (Blackburn and Taylor 1968). Snails are very
effective at consuming large amounts of hydrilla when they are present in
high density in enclosed experimental areas. However under natural
environmental conditions they are not effective. Likewise, plant pathogens
have been isolated that are effective against hydrilla under experimental
conditions (Charudattan and Lin, 1974; Charudattan and McKinney 1978), but
not under natural conditions. Insects offer promise as biological suppressants for hydrilla, but as yet
none has been shown to effectively fit into management programs. Here are pictures and more
information about biocontrol insects for hydrilla. Extensive, worldwide surveys for natural hydrilla enemies were begun in
1981 in a cooperative study between the University of Florida-IFAS, United
States Department of Agriculture, and U.S. Army Corps of Engineers. Over 40
species of insects have been found that feed on hydrilla. Several of these
are presently being evaluated as potential hydrilla biosuppressants in the
United States and other insects from Australia are under consideration
(Center 1992). Bagous affinis Hustache is a weevil that was
discovered in Pakistan and India. This is not a truly aquatic insect, but
the adult lays its eggs on rotting wood and other organic matter and after
hatching the larvae burrows through the sediment until it encounters a
hydrilla tuber (Bennett and Buckingham 1991). The tuber is then destroyed as
the insect feeds on it while it completes its life cycle. This insect will
only be potentially useful in combination with lake drawdown or
intermittently wet and dry shorelines. Another un-named Bagous sp.
has been released in U. S. but has not become established. Hydrellia
pakistanae Deonier is a leaf mining fly that is very promising as a
hydrilla biosuppressant (Buckingham et al. 1989). H.
pakistanae is established in Florida but it's impact on hydrilla is
undetermined. An aquatic moth, Parapoynx diminutalis Snellen, was accidentally
introduced into the United States (Del Fosse et al. 1976). The
larvae of this moth can frequently be found feeding in large numbers on
hydrilla, however extensive damage does not occur until late in the growing
season after hydrilla is already at problem levels. Although the moth larvae
sometimes defoliates large areas of hydrilla, the viable stems remain and
the plant remains a problem. Predators, such as fish, also limit the density
of P. diminutalis populations (Perkins 1978) and it does not appear
to be an effective biosuppressant for hydrilla. Even manatees or sea cows (Trichechus manatus) have been
considered for biological control of hydrilla. A study conducted by the U.S.
Fish and Wildlife Service reported that over 1000 manatees, 10 times the
actual number of 116, would be needed to consume just the standing biomass
of hydrilla in Kings Bay (Crystal River, Florida) (Etheridge et al.
1985). The manatee is not presently considered for use as a potential
biological control for hydrilla because it's numbers are too few, it is not
well suited for moving from place to place, and it is an endangered
species. The use of drawdown for aquatic plant management is limited to those
lakes or ponds that have sufficient water control structures and hydrologic
characteristics to adequately control water level, and where drawdown will
not interfere with other primary water uses such as domestic or irrigation
supplies, navigation, or hydrologic power. Following hydrilla life cycle
research it was suggested that drawdown may be used successfully for
hydrilla management by timing drawdowns to prevent tuber formation in the
fall and vegetative regrowth and sprouting of tubers in the spring (Haller
et al., 1976). Large scale tests of this drawdown schedule for
hydrilla control in Florida have demonstrated that hydrilla can be
temporarily controlled, but tubers remained dormant and viable in organic
hydrosoils. Also, other areas were quickly colonized by fragments from
unaffected areas (Haller and Shireman 1983). Similarly, drawdown for
hydrilla control in North Carolina and Virginia, where lake bottoms had a
high clay content, was unsuccessful (Hodson et. al 1984, Langeland
and Pesacreta 1986). The old saying "an ounce of prevention is worth a pound of cure" is very
applicable to hydrilla control. In states such as Florida where hydrilla is
widespread, it is difficult to totally prevent movement of the plant between
public lakes. However, an aggressive educational program can prevent many
heartaches for private pond owners and may prevent the spread of hydrilla
into new areas of the country. State and federal agencies can help by
developing and implementing programs to educate the public about the
problems that can arise from the introduction of non-native aquatic plants,
such as hydrilla, to lakes rivers and ponds. These programs should be
directed toward water resource user groups, such as fishing clubs and
aquaculturists and also to aquarium hobbyists. Programs should include
information on ways to identify these plants and to prevent their
introduction, by careful checking and then removal of plant fragments from
boats and trailers. SUMMARY Hydrilla was introduced into the United States about 35 years ago (ca.
1960). Because of unique biological and physiological characteristics and an
aggressive growth habit, hydrilla has established itself in a wide range of
aquatic habitats. Once established in a system it can alter the environment
detrimentally by replacing native aquatic vegetation and affecting fish
populations. Monetary losses occur when waterfront property values are
reduced as a result of these environmental impacts or when interference with
boating access reduces recreational use of the water body. In urban and
agricultural situations hydrilla interferes with the movement of water for
drainage or irrigation purposes and again, monetary or property losses can
result. Through scientific research, innovative aquatic plant management
programs, and educational programs we have dealt with many of the challenges
presented by this weed. However, hydrilla management costs millions of
dollars annually and many water resources are diminished because of hydrilla
infestations that cannot be remedied. Many challenges remain and it is hoped
that further advances in hydrilla management will be made in the years to
come. LITERATURE CITED Bagnall, L. O., K. E. Dixon and J. F. Hentges, Jr. 1978. Hydrilla silage
production, composition and acceptability. J. Aquat. Plant Manage.
16:27-31. Basiouny, F. M., W. T. Haller and L. A. Garrard. 1978. Survival of
hydrilla (Hydrilla verticillata) plants and propagules after
removal from the aquatic habitat. Weed Sci. 26:502-504. Bennett, C. A. and G. R. Buckingham. 1991. Laboratory biologies of
Bagous affinis and B. laevigatus (Coleoptera:
Curculionidae) attacking tubers of Hydrilla verticillata
Hydocharitaceae). Ann. Entomol. Soc. Amer. 84(4):421-428. Blackburn, R. D. and T. M. Taylor. 1968. Snails for aquatic weed control.
In Proc. Weed Sci. Soc. Am. p. 51. Blackburn, R. D., L. W. Weldon, R. R. Yeo and T. M. Taylor. 1969.
Identification and distribution of certain similar-appearing submersed
aquatic weeds in Florida. Hyacinth Contr. J. 8:17-23. Bowes, G. A. S. Holaday, T. K. Van and W. T. Haller. 1977. Photosynthetic
and photorespiratory carbon metabolism in aquatic plants. In
Proceedings 4th Int. Congress of Photosynthesis, Reading (UK) pp.
289-298. Buckingham, G. R., E. A. Okrah and M. C. Thomas. 1989. Laboratory host
range tests with Hydrellia pakistanae (Diptera: Ephydridae), an
agent for biological control of Hydrilla verticillata
(Hydrocharitaceae). Environ. Entomol. 18(1):164-171. Canfield, D. E., Jr., and M. V. Hoyer. 1992. Aquatic macrophytes and
their relationships to Florida lakes. Final Report submitted to Bureau of
Aquatic Plants, Florida Department of Natural Resources, Tallahassee, FL
32303. 599 pp. Cassani, J. R. and W. E. Caton. 1986. Growth comparisons of diploid and
triploid grass carp under varying conditions. Progr. Fish-Cult.
48:184-187. Center, T. D. 1992. Biological control of weeds in waterways and public
lands in the Southeastern United States of America. In Proceedings,
Vol. 1, First International Weed Control Congress, Melbourne Australia. Ed.
J. H. Combellack, K. J. Levick, J. Parsons and R. G. Richardson. Charudattan, R. and C. Y. Lin. 1974. Isolates of Penicillium,
Aspergillus, and Trichoderma toxic to aquatic plants. Hyacinth
contr. J. 12:70-73. Charudattan, R. and D. E. McKinney. 1978. A Dutch isolate of Fusarium
roseum "Culmorum" may control Hydrilla verticillata in
Florida. In Proceedings 5th EWRS International Symposium on Aquatic
Weeds, Amsterdam (Netherlands) p. 219-224. Colle, D. E. and J. V. Shireman. 1980. Coefficients of condition for
largemouth bass, bluegill, and redear sunfish in hydrilla-infested lakes.
Trans. Amer. Fish. Soc. 109:521-531. Cook, C.D.K. and R. L#nd. 1982. A revision of the genus
Hydrilla (Hydrocharitaceae). Aquat. Bot. 13:485-504. Del Fosse, E. S., B. D. Perkins and K. K. Steward. 1976. A new US record
for Parapoynx diminutalis (Lepidoptera: Pyralidae), a possible
biological control agent for Hydrilla verticillata. Fla. Entomol.
59: 19-20. Easley, J. F. and R. L. Shirley. 1974. Nutrient elements for livestock in
aquatic plants. Fla. Sci. 39:240-245. Esler, D. 1989. An assessment of American Coot herbivory of hydrilla. J.
Wildl. Manage. 53:1147-1149. Estes, J. R., W. A. Sheaffer and E. P. Hall. 1990. Study I. Fisheries
studies of the Orange Lake chain of Lakes. Florida Game and Fresh Water Fish
Commission, Completion Report as Required by Federal Aid in Sport Fish
Restoration Wallop-Breaux Project F-55-R Lower Ocklawaha Basin Fisheries
Investigations, Tallahassee, Florida. 86 pp. Etheridge, K., G. B. Rathburn, J. A. Powell and H. I. Kochman. 1985.
Consumption of aquatic plants by the West Indian Manatee. J. Aquat. Plant
Manage. 23:21-25. Haller, W. T., A. M. Fox and D. G. Shilling. 1990. Hydrilla control
program in the Upper St. Johns River, Florida, USA. In Proceedings
of the EWRS 8th Symposium on Aquatic Weeds 8:111-116. Haller, W. T., J. L. Miller and L. A. Garrard. 1976. Seasonal production
and germination of hydrilla vegetative propagules. J. Aquat. Plant Manage.
14:26-29. Haller, W. T., D. L. Sutton and W. C. Barlowe. 1974. Effects of salinity
on growth of several aquatic macrophytes. Ecology 55(4):891-894. Haller, W. T. and J. V. Shireman. 1983. Monitoring study for Lake
Ocklawaha lake management plan, Final Project Report 1979-1983. U.S. Army
Corps of Engineers, Jacksonville District, Jacksonville, FL 32232, Contract
Nos. DACW 17-79-C-0084, DACW 17-81-C-0010 and Center for Aquatic Weeds,
University of Florida, 7922 N.W. 71st St., Gainesville, FL 32606. 329
pp. Haller, W. T. and D. L. Sutton. 1975. Community structure and competition
between hydrilla and vallisneria. Hyacinth Contr. J. 13:48-50. Hodson, R. G., G. J. Davis and K. A. Langeland. 1984. Hydrilla management
in North Carolina. Water Resources Research Institute of the University of
North Carolina, Raleigh Report No. 217. 46 pp. Holaday, A. S. and G. Bowes. 1980. C4 acid metabolism and dark CO2
fixation in a submersed aquatic macrophyte (Hydrilla verticillata).
Plant Physiol. 65:331-35. Johnson, F. A. and F. Montalbano III. 1984. Selection of plant
communities by wintering waterfowl on Lake Okeechobee, Florida J. Wildl.
Manage. 48:174-178. Joyce, J. C., W. T. Haller and D. E. Colle. 1980 Investigation of the
presence and survivability of hydrilla propagules in waterfowl. Aquatics
2:10-14. Langeland, K. A., F. B. Laroche, D. G. Shilling, W. M. Andrew, and J. A.
Rodgers. 1991. Selective hydrilla management in Upper Mayakka Lake.
In Proceedings Florida Lake Management Society Second Annual
Meeting, Orlando Florida. pp. 1-8. Langeland, K. A. and G. J. Pesacreta. 1986. Management program for
hydrilla (a monoecious strain) in North Carolina. Water Resources Research
Institute of the University of North Carolina, Raleigh, Report No. 225. 26
pp. Langeland, K. A. and C. B. Smith. 1984. Hydrilla produces viable seed in
North Carolina lakes. Aquatics 6:20-22. Langeland, K. A., and D. L. Schiller. 1983. Hydrilla in North Carolina.
Aquatics 5:8-14. Langeland, K. A. and D. L. Sutton. 1980. Regrowth of hydrilla from
axillary buds. J. Aquat. Plant Manage. 18:27-29. Malone, J. M. 1984. Triploid white amur. Fisheries (Bathesda)
9(2):36. McGehee, J. T. 1979. Mechanical hydrilla control in Orange Lake, Florida.
J. Aquat. Plant Manage. 17:58-61. Milon, J. W., J. Yingling and J. E. Reynolds. 1986. An economic analysis
of the benefits of aquatic weed control in North-Central Florida, Economics
Report No. 113, Food and Resource Economics, Agricultural Experiment
Station, Institute of Food and Agricultural Sciences, University of Florida,
Gainesville 32611. 52 pp. Perkins, 1978. Approaches in biological control of aquatic weeds. In
Proceedings EWRS Symposium on Aquatic Weeds, Amsterdam (Netherlands).
pp.9-15. Pieterse, A.H. 1981. Hydrilla verticillata-a review. Abstr. of
Trop. Agric. 7:9-34. Pieterse, A.H., J.A.C. Verkleij, and P.M. Staphorst. 1985. A comparative
study of isoenzyme patterns, morphology, and chromosome number of
Hydrilla verticillata (L.f.) Royle in Africa. J. Aquat. Plant
Manage. 23:72-76. Porak, W. F., S. Crawford, D. Renfro, R. L. Cailteux and J. Chadwick.
1990. Study XII. Largemouth bass population responses to aquatic plant
management strategies. Florida Game and Fresh Water Fish Commission,
Completion Report as Required by Federal Aid in Sport Fish Restoration
Wallop-Breaux Project F-24-R, Tallahassee, Florida. 86 pp. Ryan, F. J., C. R. Coley, and S. H. Kay. 1995. Coexistence of monoecious
and dioecious hydrilla in Lake Gaston, North Carolina and Virginia. J.
Aquat. Plant Manage. 33:8-12. Salvucci, M. E. and G. Bowes. 1983. Two photosynthetic mechanisms
mediating the low photorespiratory state in submersed aquatic angiosperms.
Plant Physiol. 73:488-96. Schardt, J. D. and L. E. Nall. 1988. 1988 Florida Aquatic Plant Survey,
Florida Department of Natural Resources Technical Report No.89-CGA.
Tallahassee. 118 pp. Sculthorpe, C. D. 1967. The Biology of Aquatic Vascular Plants Edward
Arnold (Publishers) Ltd., London. 610 pp. Spencer, D. F. and L. W. J. Anderson. 1986. Photoperiod responses in
monoecious and dioecious Hydrilla verticillata. Weed Sci.
34:551-557. Steward, K. K. 1991. Growth of various hydrilla races in waters of
differing pH. Florida Scientist 54:117-125. Steward, K. K. and T. K. Van. 1987. Comparative studies of monoecious and
dioecious hydrilla (Hydrilla verticillata) biotypes. Weed Sci.
35:204-210. Sutton, D. L., T. K. Van and K. M. Portier. 1992. Growth of dioecious and
monoecious hydrilla from single tubers. J. Aquat. Plant Manage.
30:15-20. Thullen, J. S. 1990. Production of axillary turions by the dioecious
Hydrilla verticillata. J. Aquat. Plant Manage. 28:11-15. Tucker, T. 1987. How to fish hydrilla. Bassmaster 20(9);30-34. Van, T. K. 1989. Differential responses in monoecious and dioecious
Hydrilla verticillata. Weed Sci. 37:552-556. Van, T. K., W. T. Haller and G. Bowes. 1976. Comparison of the
photosynthetic characteristics of three submersed aquatic plants. Plant
Physiol. 58:761-768. Van, T. K. and K. K. Steward. 1990. Longevity of monoecious hydrilla
propagules. J. Aquat. Plant Manage. 28:74-76. Van Dyke, J. M., A. J. Leslie, Jr. and L. E. Nall. 1984. The effects of
grass carp on the aquatic macrophytes of four Florida lakes. J. Aquat. Plant
Manage. 22:87-95. Verkleij, J.A.C., A.H. Pieterse, G. J.T. Horneman and M. Torenbeek. 1983.
A comparative study of the morphology and isoenzyme patterns of Hydrilla
verticillata (L.f.) Royle. Aquat. Bot. 17: 43-59.
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