1

Hydrilla Monitoring and Management in North Carolina 1

I. Background 2

Hydrilla growth and reproduction. Hydrilla (Hydrilla verticillata L.f. Royle) is a submersed, 3

rooted aquatic macrophyte native to Asia. Two distinct biotypes have invaded the continental 4

United States, a dioecious biotype first introduced to Florida and the monoecious biotype found 5

in North Carolina. The monoecious biotype primarily ranges between central Georgia and 6

Connecticut on the Atlantic coast, with populations also occurring in California and Washington. 7

Monoecious hydrilla is the dominant biotype found in the mid-Atlantic states, with limited 8

reports of the two biotypes coexisting in lakes (True-Meadows et al. 2016). Although dioecious 9

hydrilla is no longer present, Lake Gaston once contained both biotypes and was the first 10

location where both were confirmed to exist in a water body at the same time (Ryan et al. 11

1995). 12

Monoecious hydrilla has several adaptations that help it to displace and outgrow native 13

species including rapid vegetative growth rates, low light-compensation point, C4 14

photosynthesis, and turion production. In addition to producing many stolons and rhizomes, 15

hydrilla can elongate towards the water surface where it begins to branch profusely, 16

intercepting sunlight and excluding other plants (Langeland 1996). The combination of these 17

characteristics results in a plant where half of the standing crop is in the top half meter of the 18

water column (Haller and Sutton 1975). Tissue matter of hydrilla is approximately 90% water, 19

which lends to its efficient use of the available nutrients and allows it to prolifically produce 20

plant material from a relatively small supply of essential plant nutrients. The low light 21

compensation point of hydrilla allows it to photosynthesize earlier in the day and in deeper 22

2

water than most other aquatic plant species (Langeland 1996). Hydrilla can switch to C4 23

photosynthesis when water is warm and highly productive. This pathway is characterized by 24

low photorespiration and inorganic carbon is fixed to malate and aspartate. Monoecious 25

hydrilla appears to have some tolerance to pH and salinity, however, reports vary on response 26

to salinity (True-Meadows et al. 2016). Growth is greatest in the summer, with no shoot growth 27

occurring in winter and regrowth in the spring dependent on the sprouting of turions and 28

tubers (Harlan et al. 1985, Sutton et al. 1992, Owens et al. 2012) 29

Monoecious hydrilla has four primary methods of reproduction: fragmentation, axillary 30

turion production, subterranean turion (tuber) production, and possibly seed. Hydrilla can 31

rapidly spread within a lake as well as between lakes due to fragmentation. Fragments with a 32

single whorl of leaves may root and form a new population. Small amounts of hydrilla can easily 33

spread to new lakes in this manner from trailers, bait buckets, aquariums, or intentional 34

spreads for the perceived benefits to fish and waterfowl habitat. 35

Axillary turion production occurs on leaf axils and these appear as compact, green, leaf 36

buds. Subterranean turions, or tubers, are produced at the terminal end of rhizomes. Turion 37

and tuber production varies, but in most North Carolina lakes axillary turions are formed from 38

October through December, while tuber production primarily occurs from June through 39

October, with some production occurring in November and December (Harlan et al. 1985, 40

Meadows 2013). Tubers vary in color from white to red, or black, dependent on the sediment 41

composition. Tubers and turions serve as primary overwintering strategies for monoecious 42

hydrilla as fragments and stems do not overwinter. Tubers also serve as an extremely effective 43

method of spread and maintaining populations in water bodies. Monoecious hydrilla has been 44

3

shown to produce over 6,000 new tubers from a single initial tuber (Sutton et al. 1992). Tubers 45

are typically found in sediment depths up to 8 cm and can occur at densities of over 3,000 46

tubers m2 (Nawrocki 2011). Monoecious hydrilla tubers exhibit germination rates of 90% in 47

laboratory trials but appear to require a chilling period prior to sprouting which may prevent 48

sprouting in the same year as formation (True-Meadows et al. 2016, Carter et al. 1987). Tubers 49

have been observed in undisturbed soil 4 years after production and 6-year old tubers can be 50

viable (True-Meadows et al. 2016). These observations may be indicative of an environmentally 51

imposed dormancy that prevents the depletion of tuber populations. Unlike dioecious hydrilla, 52

sprouted monoecious tubers spread shoots laterally, rather than vertically toward the surface 53

(Van 1989). These tubers can also survive ingestion by waterfowl and exposure to herbicide 54

applications (Langeland 1996). Seed production has been reported in monoecious hydrilla but 55

does not appear to be a major method of spread in the United States. Sexual reproduction of 56

hydrilla is of particular concern because it could result in adaptations to a wider range of 57

environments by aiding in dispersal and overwintering. Genetic variability would increase and 58

could result in more difficult management and control. 59

60

NC Water systems at risk. Since introduction in the 1970’s, hydrilla has become widespread 61

across North Carolina. NCDEHNR (1996) projected potential hydrilla maximum infestations of 62

lakes by region and this projection has held largely true (Table 1). Hydrilla has been present in 63

Lake Gaston since the 1980s and approximately 15% is shallow enough for hydrilla infestation. 64

Additionally, 30-40% of Falls and Jordan lakes (reservoirs near Raleigh) are shallow enough for 65

hydrilla infestation. Native aquatic species are not common in reservoirs due to the lack of a 66

4

seed bank (Smart et al. 1998). The lack of interspecific competition and high nutrient levels also 67

favor rapid hydrilla growth in reservoirs (Nawrocki 2016). Nutrient loading and sedimentation 68

as a result of land development have resulted in increased eutrophication over time in North 69

Carolina Piedmont reservoirs (Nawrocki 2016). Hydrilla has been shown to allocate more 70

resources to photosynthetic tissue with increased nutrient levels. This leads to surface mat 71

formation and shading out of plant growth beneath the hydrilla mat. Sedimentation and 72

increased chlorophyll cause turbid water conditions, decreasing light penetration and creating a 73

disturbed habitat that favors hydrilla growth (Nawrocki 2016, Langeland 1996, Steward and Van 74

1987). Increased water levels shade out most submersed plant species and inundates floating 75

or emergent species. Natural adaptations of hydrilla to turbidity and low light provide it with 76

further advantages over other aquatic plant species present in reservoirs when water turbidity 77

is high. 78

North Carolina natural lakes and rivers have the potential for severe hydrilla infestations 79

(Table 1). Many are shallow enough to support extensive hydrilla populations. These lakes can 80

be habitat to many endemic species, including those under federal protection. Hydrilla 81

infestations drastically alter the lake and threaten the habitat these species require. However, 82

preventative action and early treatment can reduce the potential for hydrilla invasion. Hydrilla 83

was discovered in Lake Waccamaw in 2011, and treated in 2013. Subsequent monitoring on 84

Lake Waccamaw found tuber banks in relatively low densities, with no hydrilla tubers found in 85

2018. Continued monitoring and management efforts are pivotal to ensuring hydrilla does not 86

dominate this rich natural resource. Hydrilla was first observed in the Eno River in 2005 and 87

infested over 20 miles of river in Orange and Durham counties (NCWRC 2019). Hydrilla has also 88

5

been discovered in the Deep River, this is particularly concerning due to potential damage to 89

the federally endangered Cape Fear Shiner (Notropis mekistocholas) (NCWRC 2019). Hydrilla 90

infestations have also been noted in the Chowan and Cape Fear rivers. 91

92

Agencies and Regulatory Authorities. As with many states, multiple state agencies have 93

jurisdiction over various aspects of water resource management. North Carolina also a broad 94

definition of “waters of the State”: G.S. 143-212(6). Nuisance species such as hydrilla fall under 95

jurisdiction of various several state and federal agencies with authority on North Carolina water 96

bodies. Hydrilla is a federally listed noxious weed and is classified by North Carolina 97

Department of Agriculture and Consumer Services (NCDA&CS) as a Class A noxious weed. 98

NCDA&CS adopted and maintains the Noxious Weed List that regulates prohibited plant species 99

in North Carolina. This list contains all species on the federal noxious weed list in addition to 100

several species approved by the NC Board of Agriculture that are deemed noxious weeds for 101

the state (NC Aquatic Nuisance Species Management Plan). Transport and sale of species on 102

this list is prohibited without a permit or unless exempt by the provisions of the Noxious Weed 103

Regulations. 104

The North Carolina Wildlife Resources Commission (NCWRC) has hydrilla listed as an 105

aquatic nuisance species and have public outreach programs to educate users of the waterways 106

on these species and methods to mitigate the spread of these species. NCWRC implemented a 107

habitat enhancement program to deter the introduction and spread of invasive weedy species 108

in 2013 and have invested heavily into hydrilla management efforts. NCWRC is also responsible 109

for regulation and permitting of game and nongame fish stocking. Unlicensed fish stocking has 110

6

the potential to degrade habitat and water quality of a water body. These conditions may allow 111

invasion or expansion of existing noxious weed populations such as hydrilla due to adaptations 112

(rapid growth, low light-compensation point, etc.) these species have. Triploid grass carp, a 113

common hydrilla control method, are only allowed for aquatic vegetation management and 114

permits for stocking and purchase are available through NCWRC. Two of their notable projects 115

include Harris Lake and the Eno River. Harris Lake has been stocked with sterile grass carp with 116

herbicide treatments around boat docks to reduce the risk of spread to other water bodies. Low 117

concentration fluridone drip systems have proven effective in managing hydrilla populations in 118

the Eno River. 119

NCWRC often works in conjunction with North Carolina Department of Environmental 120

Quality (NCDEQ) Division of Water Resources in hydrilla management efforts. The Aquatic 121

Weed Control Act of 1991 directed NCDEQ to create the Aquatic Weed Control Program to 122

assist citizens and local governments within North Carolina with aquatic weed infestations. The 123

North Carolina Aquatic Weed Control Council (NCAWCC) was previously an interagency 124

committee that was formalized by the Weed Control Act and distributes state-appropriated 125

funds to treat nuisance species. The Aquatic Weed Control Program focuses on early response 126

to limit spread and mitigate the long-term impacts of aquatic weeds (NCDEQ 2019). This also 127

gave NCDEQ the authority to remove aquatic weeds in state water bodies and established the 128

North Carolina Division of Water Resources within NCDEQ. Aquatic nuisance species are often 129

controlled with pesticides, and therefore are subject to the Clean Water Act. The Environmental 130

Protection Agency has authorized North Carolina to administer National Pollutant Discharge 131

Elimination System (NPDES) permits, which are required for any pesticide application to or near 132

7

waters of the United States. General permits can be obtained to cover small-scale treatments, 133

but permits must be obtained for applications where any threshold is exceeded (NC Aquatic 134

Nuisance Species Management Plan). 135

The US Army Corps of Engineers (USACE) is also involved in hydrilla management and 136

research. USACE maintains four reservoirs in North Carolina and works with state agencies 137

including NCWRC and North Carolina State University to perform vegetation surveys and 138

determine invasive species locations. The USACE in conjunction with these other agencies have 139

performed spot treatments and grass carp stockings to control hydrilla and other nuisance 140

species observed on the USACE reservoirs. The US Army Engineer Research and Development 141

Center (ERDC) is involved in invasive species management. ERDC has provided invaluable 142

research, technical support, and funding to invasive plant management efforts, especially 143

involving hydrilla control. 144

145

Stakeholders. North Carolina lakes and reservoirs are multi-purpose systems with numerous 146

stakeholder groups and often conflicting opinions on management practices. In developing 147

management programs, it is important that stakeholder interests and perceptions be 148

considered. Thus, it is important to include them during the information gathering stage and 149

also in the public notification stage. However, most stakeholders are not topical experts and 150

should not be included in decision making roles. Public education programs should also be 151

implemented when scientific justification for a management program is not well understood by 152

stakeholders. 153

8

Stakeholder perception is often based on how the stakeholders use the water resource. 154

Recreational users include boaters, swimmers, water skiers, etc., and these individuals 155

generally desire little to no vegetation in the areas they frequent. Anglers and waterfowl 156

hunters also use waterbodies for recreational activities, but these groups generally use 157

vegetation as targets for their activities and thus prefer moderate (anglers) to high (waterfowl 158

hunters) amounts of vegetation. Power generation companies utilize the water to generate 159

electricity and need to maintain safe and efficient operation without risk from nuisance 160

vegetation. These entities have a Shoreline Management Plan approved by the Federal Energy 161

Regulatory Commission which may specify vegetation components of the system that must be 162

maintained. Many municipalities also utilize these water systems as sources of potable water 163

and, therefore, need safe and reliable supplies without risk of contamination or supply loss. 164

Public perception of how their drinking water is managed can be an important consideration in 165

management programs as well. Several lake and reservoir systems are also highly developed 166

and these property owners desire unhindered access to the water and conditions that support 167

property value. Finally, as most waters within North Carolina meet the statutory definition of 168

“waters of the State”, North Carolina state agencies should also be considered stakeholders. 169

170

Survey Methods. There are a variety of survey methods for monitoring and mapping aquatic 171

macrophyte infestations. These surveys can better document the abundance, presence and 172

distribution of desirable and target species for management purposes. Point-intercept methods 173

can be used for plot studies with a recommended minimum of 30 points per plot, or whole-lake 174

or basin surveys, with a recommended minimum of 100 points (Madsen and Wersal 2018). 175

9

Point intervals less than 10-20 meters are often difficult to sample and line transects are 176

suitable replacement in these instances (Madsen and Wersal 2018). Presence/absence 177

techniques in point-intercept surveys can be used to rapidly collect large quantities of data in 178

the field and require no sample analysis (Madsen 1999). Semi-quantitative estimates of 179

abundance or distribution used with point-intercept surveys have been used, but can slow 180

survey efficiency and can hinder survey efficiency and survey analysis (Madsen and Wersal 181

2018, Howell and Richardson 2019). Occurrence data collected from point-intercept analysis 182

will follow a binomial or Poisson distribution, and a chi-square test is the most appropriate 183

statistical analysis (Madsen and Wersal 2018). However, if occurrence data has been collected 184

from the same sites a more advanced statistical analysis such as McNemar's test for 185

dichotomous response variables. Point-intercept surveys are the industry standard for 186

assessments of presence and distribution of aquatic macrophytes but are prone to subjectivity 187

and are inefficient compared to some other survey methods. 188

Estimation of plant abundance is valuable to management of aquatic macrophytes 189

because aboveground biomass is often the most problematic and a nuisance for stakeholders. 190

Quadrat surveys are the most commonly used methods for sampling abundance of emergent 191

and floating aquatic plant macrophytes. Quadrats of 0.1 m2 are often sufficient, with 10 to 20 192

samples required for statistical significance (Madsen and Wersal 2018). Sampling devices such 193

as dredges, rakes, and coring samplers are often used for submersed aquatic vegetation, and 194

are effective tools at collecting above and belowground biomass. With a 15-cm sampler 30 to 195

100 samples should be collected for statistical significance (Madsen and Wersal 2018). 196

Assuming normal distribution, this data can then be analyzed using parametric statistical tests 197

10

such as t-tests or ANOVA. Although these methods of biomass sampling are informative, they 198

can become tedious, labor-intensive and costly, prompting a shift to non-destructive methods 199

of biomass sampling. 200

Hydroacoustic technology has become a widespread survey tool for mapping submersed 201

aquatic vegetation in the past several decades (Howell and Richardson 2019, Madsen and 202

Wersal 2018). With advances in commercially-available hydroacoustic systems, less expensive 203

electronics can be utilized by management programs for assessments of submersed 204

macrophyte abundance. Hydroacoustic mapping offers a repeatable, non-destructive 205

monitoring opportunity for management applications and ecological growth patterns (Howell 206

and Richardson 2019). Hydroacoustic mapping also has potential for use in formulating 207

recommendations for herbicide treatments and grass carp stocking for hydrilla management 208

(Howell and Richardson 2019). ArcGIS software provides an effective analysis and post-209

processing tool and can be used in conjunction with statistical software such as R and RStudio 210

packages for statistical analysis. 211

Due to complications with turbidity and water reflectance, aerial mapping has limited 212

applications for submersed aquatic macrophytes such as hydrilla unless the infestations have 213

reached the water surface. However, this technology has proved useful in monitoring, mapping 214

and analysis of emergent and floating plant infestations. Unmanned aerial vehicles (UAV) 215

provide a platform for small imagers and can enhance boat-based survey practices. Innovation 216

and improvement of UAV technology will provide a valuable tool to augment aquatic plant 217

management practices. 218

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With the variety of monitoring technologies available to aquatic plant management, an 219

integrated approach is needed to provide an efficient and objective approach to monitoring 220

aquatic macrophyte abundance and management efforts. Remote sensing provides a labor and 221

time efficient alternative to in situ sampling and can be used for environmental assessment and 222

modeling, which can inform management needs (Silva et al. 2008). Remote sensing technology 223

can be paired with geographic information systems can further enhance the utility of the 224

information provided (Lehmann and Lachavanne 1997, Shaw 2005). The development of boat-225

based multiple sensor arrays (MSA) to passively detect submersed, emergent, and floating 226

vegetation in conjunction with fixed point sampling, and in situ observations provides an 227

innovative approach for mapping and post-processing through statistically analyzable 228

descriptions of aquatic macrophyte abundance (Howell 2017). This method of integrated 229

monitoring was developed using commercially available echosounders, a multispectral imager, 230

agronomic optical sensor, action cameras and software packages. Post-processing and data 231

analysis can be performed using ArcGIS for plot and normalized difference vegetation index 232

(NDVI) analysis, Matlab and Canopeo for image processing, and R and RStudio for statistical 233

analysis (Howell 2017). The concurrent use of multispectral camera and agronomic optical 234

sensor provides finite mapping options for distinguishing SAV from floating and emergent 235

macrophytes, while dual transducer arrangements provide precise SAV detection. Although 236

limitations to integrated systems such as the MSA are still being improved upon, it provides a 237

passive and repeatable option for monitoring and objectively analyzing management efforts. 238

There are a variety of monitoring and survey options available for aquatic plant 239

management. The industry standard of point-intercept methods provides an easily analyzed 240

12

method of obtaining occurrence and distribution data. Hydroacoustic sensors provide a 241

relatively easy to use measurement of submersed macrophyte abundance, with commercially 242

available sensors and post-processing software. Advancements and innovation with sensor 243

technology will allow management professionals to combine a variety of sensor technologies 244

for a more holistic method of data collection and analysis, such as seen with boat-based MSA 245

systems. Further development of MSA systems will work to overcome current limitations and 246

provide aquatic plant management programs with a valuable tool for surveys and monitoring. 247

Educating professionals in aquatic plant management about the strengths and limitations of 248

survey methods will help to create new industry standards for aquatic plant management. 249

Citizen science may also be incorporated into vegetation surveys. Stakeholders have a 250

vested interest in their waterbodies and many would like to participate in improving the overall 251

quality of their system. Volunteer programs also provide an opportunity to educate 252

stakeholders more thoroughly about water systems and aquatic plant management. Lake 253

Gaston has had an active and effective volunteer monitoring program for over 10 years. A 254

summary of the Lake Gaston Volunteer Survey is provided in Appendix 1. 255

Recent advances in unmanned systems, sensor technology, and machine learning offer 256

numerous opportunities for making aquatic plant surveying more efficient as well as more 257

informative. Unmanned aerial systems have been demonstrated to detect hydrilla in shallow 258

water and machine learning has been demonstrated to identify hydrilla from hydroacoustic 259

imagery. These technologies should be further evaluated for incorporation into management 260

programs. 261

262

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Management Options. There are a variety of non-chemical management options for hydrilla 263

control. These options should be included, as appropriate, within a complete integrated pest 264

management (IPM) plan for the specific pest (hydrilla) in the specific situation. In general, IPM 265

plans include knowledge gathering (literature and surveys) and using a combination of 266

management techniques as part of an ecosystem based strategy for the best long term results 267

both from a pest management standpoint but also in reducing adverse effects. Previous 268

research has indicated that at least seven years of management are needed in North Carolina 269

to reduce the hydrilla tuber bank to near zero levels (Figure 1; Nawrocki 2016). Stopping 270

management activities for a growing season can also result in replenishment of the tuber bank 271

and loss of management investment (Figure 2). 272

Prevention is a common control strategy for any pest and may include boat inspections 273

and sanitation. North Carolina Sea Grant has taken steps to educate boaters on proper methods 274

for inspecting and cleaning boats to prevent further spread of hydrilla. Hydrilla can easily be 275

caught in between the boat and trailer bunks, and is also commonly entangled in axles, lights, 276

transducers, and motor propellers. Self-inspections and signage reminding boaters of potential 277

hydrilla spread are useful for reducing the transport of hydrilla and other aquatic invasive 278

species between lakes. However, several states, including New York and Idaho have 279

implemented boat inspection stations to further prevent spread of these invasive species. 280

Inspection stations are especially beneficial at high use boat ramps because they are an 281

effective way to inform boaters about invasive species spread, reduce the spread of hydrilla, 282

and empower boaters to protect the lakes. 283

14

The water garden and aquarium trade are likely responsible for some of the early 284

outbreaks of hydrilla in the United States. While hydrilla is listed as a federal and state noxious 285

weed, making it illegal to import, transport, and sell the plant, noxious species such as hydrilla 286

can easily be exchanged between individuals and transported inconspicuously. Additionally, it is 287

still possible to purchase invasive species under different terminology than that monitored. 288

Monitoring and education of people in the aquarium and water garden trade is a relatively 289

simple preventative measure that can help further prevent the spread of hydrilla in North 290

Carolina. 291

Waterfowl present a unique risk to the spread of hydrilla due to the possibility to 292

transport turions, fragments, and seeds. Hydrilla tubers and turions may survive ingestion and 293

regurgitation by waterfowl (Joyce et al. 1980). Hydrilla may also be intentionally spread due to 294

the perception that it improves fish and waterfowl habitat and feeding (Langeland 1996). 295

Although hydrilla may improve waterfowl habitat, native species such as eelgrass (Vallisneria 296

americana Michx.) also serve as important food and habitat sources without creating the 297

severe negative impacts. Hydrilla is also host to the cyanobacterium Aetokthonos hydrillicola, 298

which causes avian vacuolar myelinopathy (AVM), a neurological disease affecting waterfowl 299

and their predators (Wilde et al. 2005, Wilde et al. 2014). AVM has been previously 300

documented in North Carolina (Augspurger et al. 2003). 301

Biological control options involve the release of specific organisms for control of a 302

specific target pest. Host specific biological controls have been evaluated for dioecious hydrilla 303

in warm climates, however, these organisms have never been documented to overwinter on 304

monoecious hydrilla. This is likely due to the herbaceous perennial nature (shoots dying off 305

15

over winter) of monoecious hydrilla and the lack of surface growth for a large portion of the 306

year. 307

Triploid grass carp (Ctenopharyngodon idella Val.) are an effective biological control for 308

hydrilla and have been recommended for aquatic macrophyte management in many water 309

bodies throughout the Southeast US. Grass carp were first imported to the US in 1963 from 310

Malaysia (Chilton and Muoneke 1992). Grass carp are naturally a diploid (2n) species with high 311

fecundity that reach maturity in 2-4 years (Chilton and Muoneke 1992). Research conducted in 312

the 1980s led to the production of sterile triploid (3n) grass carp. Triploidy can be induced by 313

heat or cold shock, chemical exposure, and the preferred method of hydrostatic pressure 314

shock. Triploid grass carp have extremely low fertility with 99.999994% of sperm sterile and low 315

rated of fertilization success with diploid individuals. Additionally, survival of diploid x triploid 316

cross is extremely low (Chilton and Muoneke 1992). Blood tests can be used to confirm fish 317

ploidy level. Grass carp grow rapidly with abundant food sources with a mean weight of 20-25 318

pounds and recorded weights of over 40 pounds. The apparent life span of grass carp is 319

approximately 10 years (Sutton et al. 2012). Grass carp can tolerate near-freezing temperatures 320

but rarely feed at temperatures below 3 °C. Feeding peaks at 20-26 °C and temperatures above 321

38 °C are lethal to the fish (Chilton and Muoneke 1992). Grass carp are tolerant of high 322

salinities, with reports of carp withstanding salinities up to 100 g L-1 for several days (Liepolt and 323

Weber 1969). 324

Triploid grass carp are non-specific grazers with a preference for the soft tips of tender 325

young plants. (Sutton et al. 2012). Consumption rates vary based on water temperature, 326

oxygen, food availability, water chemistry and preference (Chilton and Muoneke 1992). Oxygen 327

16

levels below 4 ppm can result in a 45% reduction in consumption, with no consumption 328

occurring below 2 ppm oxygen (Colle 2014). Consumption rates as high as 300% body weight 329

day-1 have been reported, with more consistent findings of 50-120% body weight day-1 (Jensen 330

1986, Opuszynski 1972, Chilton and Muoneke 1992). Grass carp exhibit a strong preference for 331

hydrilla, however, muskgrass (Chara spp.), Southern naiad (Najas guadalupensis [Spreng.] 332

Magnus), and Brazilian elodea (Egeria densa [Planch.]) are also among the species highly 333

preferred by grass carp (Sutton et al. 2012). 334

Grass carp stocking rates vary between states and water systems, and it is generally 335

recommended that grass carp should not be stocked in open systems to reduce the likelihood 336

of the grass carp leaving the stocked water body (Colle 2014). NCWRC has developed a stocking 337

model (cohort analysis) in collaboration with NCSU and NCDEQ to determine the necessary 338

number of fish to be stocked. This population model is needed to maintain a desired rate under 339

specific variables and provide various management options. Input variables for the stocking 340

model include: number of fish in the system, the number of fish to be stocked, amount of 341

standing hydrilla present, tube bank acreage, target stocking rate for stand hydrilla, and target 342

stocking rate per tuber bank acre. This is combined with a grass carp population estimate that 343

assumes 30% initial mortality and 20% in subsequent years. From this data a stocking 344

recommendation is produced based on the total target density for previous year hydrilla 345

acreage and the number of fish needed to maintain target density. In ponds, a standard grass 346

carp stocking rate would be 15 fish per surface area (Richardson and Getsinger 2014). Larger 347

systems are more complex and require consideration of many variables, but a stocking rate of 348

15 grass carp per acre of target vegetation is a common starting point. Stich et al. (2013) 349

17

recommend stocking with older fish for increased impact on hydrilla biomass. Stocking water 350

bodies with grass carp is often used in conjunction with herbicide applications, although timing 351

is critical (True-Meadows et al. 2016). Grass carp stocking is a multiyear approach to hydrilla 352

management, however there are concerns over the direct and indirect impacts the fish have of 353

the water bodies being stocked. Decrease in water quality, increase in chlorophyll, phosphorus 354

and nitrogen are commonly observed as the herbivory by grass carp shifts the stocked water 355

body from a plant-based community to a system dominated by phytoplankton and algae (Colle 356

2014). 357

Although stocking triploid Grass carp is an effective tool for managing submerged 358

aquatic vegetation, there are negative impacts that must be considered before determining if 359

stocking is an appropriate tool. Concerns with Grass carp stocking include habitat alterations, 360

negative impacts to water quality, impacts to native fish communities, and migration of Grass 361

carp out of the desired management area (Bain 1993). One of the main concerns is the ability of 362

Grass carp to eliminate large amounts of native and beneficial aquatic vegetation resulting in 363

the complete elimination of all submersed species (Bonar et al. 2002; Hoyer et al. 2005). This 364

large scale macrophyte removal has been shown to alter fish community structures, decrease 365

the abundance of aquatic macroinvertebrates, and deteriorate waterfowl habitat (Gasaway and 366

Drda 1977; Bettoli et al. 1993). Large scale removal of submersed aquatic vegetation also has 367

the potential to alter the entire trophic structure of an ecosystem by negatively impacting both 368

fish and invertebrate communities (Bain 1993). These negative impacts are not confined to the 369

area that is being actively managed by Grass carp stocking. Reports of large scale movements 370

by Grass carp have heightening the concern of these fish migrating out of the desired 371

18

management area and moving downstream into sensitive estuarine plant communities that 372

provide critical nursery habitat (Bain et al. 1990; Kirk et al. 2001). Other possible negative 373

effects of Grass carp stocking include impacts to water quality through increased turbidity and 374

nutrient concentrations, and changes in dissolved oxygen concentrations (Bain 1993; Bonar et 375

al. 2002). 376

Traditional stocking strategies for Grass carp are designed to eradicate submersed 377

vegetation in a short period of time and rates are based solely off a desired number of fish per 378

surface acre of nuisance vegetation. However, these stocking rates produce variable results 379

including re-colonization of hydrilla after successful control was achieved (Hanlon et al. 2000; 380

Kirk et al. 2000) and are not designed to mitigate potential negative impacts from grass carp. 381

Kirk et al. (2014) found indirect evidence that suggests that once hydrilla has been completely 382

eliminated from a system there is often insufficient vegetation to support a Grass carp 383

population large enough to effectively manage regrowth from tuber banks. Grass carp present 384

in systems that are void of vegetation and provide little to no food likely experience high 385

mortality from starvation (Kirk et al. 2014). One issue with traditional stockings is that the 386

model does not take into account biological factors such as Grass carp growth, mortality rates, 387

and overall lifespan, all of which influence the overall effectiveness of Grass carp stockings 388

(Stich et al. 2013; Kirk et al. 2014). Managers began developing stocking models that 389

incorporate mortality and growth rates, but these factors vary not only from system to system 390

but also within a single Grass carp population. Kirk et al. (2014) evaluated Grass carp 391

populations from four reservoirs within a single river system and found variable growth and 392

erratic survival not only across systems but among year-classes. Stich et al. (2013) also found 393

19

age-specific growth and mortality rates for Grass carp in Lake Gaston, Virginia‒North Carolina. 394

Due to this high variability among age classes, stocking models are now incorporating the 395

effects of different cohorts within a system to be able to better predict the appropriate Grass 396

carp stocking rate for their specific management objectives. 397

Mechanical control, or the use of power-driven equipment is useful in the management 398

of some aquatic plant species, but the utility is limited in submersed species. Mechanical 399

harvest of hydrilla is complicated by fragmentation and the low impact on the tuber bank (True-400

Meadows et al. 2016). There are several distinct disadvantages to mechanical harvest 401

treatments. Mechanical harvesting was estimated to result in loss of 18% of fish biomass in 402

Orange Lake, FL (Haller et al. 1980) and is costly ($455/acre; Haller and Jones 2012). Site-403

specific management with mechanical harvests is possible, however this is non-selective and 404

will result in the removal of desirable macrophytes. Mechanical harvesting generally provides 405

short term control only, similar to mowing a yard. For monoecious hydrilla in North Carolina, 406

mechanical harvesting is generally a poor option. Our reservoirs are impounded rivers, thus 407

hydrilla fragments can be transported significant distances to form new colonies. Further, many 408

reservoirs have submersed stumps and rocks that would either cause extensive damage to 409

mechanical units or prohibit effective operation. 410

There are a variety of other methods for control of hydrilla and other aquatic plant 411

species. Benthic barriers provide localized control of aquatic macrophytes and are most useful 412

on dense pioneer infestations. Barriers are most commonly made of materials such as PVC, 413

fiberglass, and nylon, although fabrics can be used when fastened and anchored properly 414

(Bellaud 2014). Large installations are not practical due to cost and the need to remain in place 415

20

for one to two months to ensure control. This method is non-selective and impacts fish and 416

benthic organism communities. Hand pulling is a simple and widely used method of control and 417

be performed from a boat, wading from shore, or with SCUBA in deeper waters. Hand-pulling is 418

highly selective and useful when target species are in extremely low densities. Cost, labor 419

intensity, and time required are major components to consider when implementing hand 420

pulling. Nutrient inactivation and sediment manipulation is another approach for control that 421

provides mixed results but have been largely unsuccessful for monoecious hydrilla (Bellaud 422

2014, Spencer et al. 1992). Shading via use of EPA-registered dyes limits light penetration and 423

restricts the maximum depth of plant growth. This method is limited to small, highly controlled 424

water bodies and is non-selective. 425

Drawdowns, or the lowering of water levels are an effective method of controlling many 426

aquatic invasive macrophytes. This practice is most commonly used in systems with gate valve 427

or flashboard systems in dams (Bellaud 2014). Siphoning or pumping can be used but is time 428

consuming and costly. Drawdowns are most commonly performed in northern states where the 429

plants are exposed to freezing and drying conditions. Drawdown conditions should be 430

maintained for six to eight weeks for exposure. Drawdowns are often most effective in fall to 431

reduce stranding amphibians and benthic organisms. Poovey and Kay (1998) investigated the 432

potential for summer drawdown to control monoecious hydrilla and determined that one-week 433

drawdowns in sandy soils and two-week drawdowns in silt loam were needed to reduce tuber 434

numbers and prevent significant regrowth. In most North Carolina waterbodies a summer 435

drawdown is not feasible due to usage. Additionally, Hodgson et al. (1984) investigated the 436

potential for drawdown treatments on hydrilla in North Carolina and found them to be 437

21

ineffective due to the presence of an organic detrital layer over the clay substrate most tubers 438

are found in. Drawdowns may be effective when used in conjunction with herbicide treatments 439

to further reduce tuber sprouting (Spencer and Ksander 1999). Winter drawdowns should not 440

be expected to be effective on monoecious hydrilla due to its herbaceous perennial nature and 441

copious turion production in fall. 442

Chemical control of monoecious hydrilla infestations may also be an effective 443

management practice. There are currently nine herbicides registered with the US 444

Environmental Protection Agency for hydrilla control: bispyribac-sodium, copper, diquat, 445

endothall, florpyrauxifen-benzyl, flumioxazin, fluridone, imazamox, and penoxsulam. Of these, 446

fluridone has been used most frequently in NC for hydrilla management. Fluridone is a 447

bleaching herbicide that targets a plant-specific photosynthetic enzyme responsible for 448

photosynthesis in plants, resulting in bleaching of new growth (Netherland 2014). Fluridone is 449

particularly efficacious on submersed species, with low use rates but long exposure 450

requirements to provide season-long control. Selectivity is commonly achieved through the use 451

of low treatment rates (Netherland 2014). Fluridone labels indicate a minimum of 45 days are 452

necessary for exposure, however studies have indicated there may be more flexibility in 453

exposure requirements than previously thought (Netherland et al. 1994, Netherland and 454

Getsinger 1995, Netherland 2015). Monoecious hydrilla is highly sensitive to fluridone, with 455

significant reductions in biomass at rates of 1.5 µg L-1 and typical use rates of 5-30 µg L-1 provide 456

sufficient control (Langeland and Pansecreta 1986, Nawrocki et al. 2016, Netherland 2015). 457

Fluridone was successfully used on the Eno River to control monoecious hydrilla with target 458

22

rates of 2-4 µg L-1 and has been documented to reduce tuber density in North Carolina 459

(Nawrocki et al. 2016). 460

Endothall inhibits respiration and protein synthesis in plants and has traditionally been 461

used for spot treatments of small target areas to non-selectively control submersed plants 462

(Netherland 2014). Endothall should be applied early in the season (early to mid-June) when 463

hydrilla is more manageable and reduce impact on desirable species that grow later in the 464

season (Langeland and Pesacreta 1986, Netherland 2014). Hydrilla treatment with fast-acting 465

herbicides such as endothall when water temperature is cooler and biomass has not yet peak 466

can reduce the oxygen depletion that occurs in warmer months as biomass degrades. Repeat 467

applications in August may be necessary if regrowth is apparent (Langeland and Pesacreta 468

1986). Endothall efficacy is directly correlated to concentration exposure time (CET), with 469

appropriate CET providing excellent control of hydrilla biomass (Hodson et al. 1984, Netherland 470

et al. 1991, Langeland and Pesacreta 1986, Poovey and Getsinger 2010). Granular applications 471

of endothall may provide more effective hydrilla control in areas of high water exchange 472

(Langeland and Pesacreta 1986). Poovey and Getsinger (2010) observed that endothall 473

concentrations as low as 1 mg L-1 were effective in reducing hydrilla biomass grown from 474

stem fragments with 96 h exposure but increased concentrations were needed to control 475

hydrilla sprouted from tubers. Endothall can be combined with other herbicides such as diquat 476

and copper for hydrilla control (Pennnington et al. 2001, Skogerboe et al. 2004). 477

Diquat interferes with photosynthesis and is fast acting on susceptible floating and 478

submersed aquatic macrophytes, with exposure requirements of hours to days depending on 479

water exchange (Netherland 2014). Diquat has similar timing restrictions to endothall and 480

23

should be applied before hydrilla biomass has peaked. Diquat rapidly kills hydrilla at rates of 481

0.25-25 ppm but is not persistent in the water column and rapid regrowth is possible 482

(Langeland and Pesacreta 1986, Van et al. 1987). Caution should be used when diquat is applied 483

to enclosed areas because oxygen depletion due to macrophyte and algae degradation can 484

cause fish kills (Langeland and Pesacreta 1986). Diquat can be used to enhance endothall 485

treatments for increased control and slower biomass recovery in areas of low water exchange 486

(Blackburn et al. 1969, Skogerboe et al. 2004). Pennington et al. (2001) observed 95% control of 487

hydrilla when 1 mg L-1 endothall was combined with 0.5 mg L-1 diquat in laboratory trials. 488

However, more variation in control was observed in field trials, with biomass reduction for 12 489

weeks after treatment but regrowth 12 months after treatment when 3 mg L-1 endothall and 490

0.2 mg L-1 diquat were applied (Skogerboe et al. 2004) 491

Copper is commonly used as an algaecide, but is also utilized as for fast-acting control 492

of aquatic macrophytes with short exposure requirements of hours up to one day. Copper 493

sulfate is the most widely used copper product, however it is corrosive and affected by water 494

alkalinity (Netherland 2014). Copper chelates were developed in the 1970s to address these 495

problems. Copper is typically applied at rates of 0.2-1.0 mg L-1 and is commonly used in 496

conjunction with diquat or endothall to increase uptake and longevity of control (Blackburn and 497

Weldon 1969, Pennington et al. 2001, Skogerboe et al. 2004, Sutton et al. 1970, Sutton et al. 498

1972). Copper has been indicated to increase uptake of diquat, as well as diquat increasing the 499

uptake of copper (Sutton et al. 1970, Sutton et al. 1972). Copper/diquat combinations of 0.37 500

mg L-1 diquat and 0.5-1 mg L-1 copper provide 68-100% reduction in hydrilla biomass for 4 501

weeks after treatment (Turnage et al. 2015). Similarly, combinations of 3 mg L-1 endothall with 502

24

0.5 mg L-1 copper provide hydrilla biomass reduction for 12 weeks after treatment with 503

regrowth occurring 12 months after treatment (Skogerboe 2004). Concerns over the use of 504

copper for chemical control of aquatic invasive species are commonly related to lack of 505

biodegradation and regular use can increase sediment copper residues. 506

Flumioxazin is another fast-acting herbicide that affects the plant enzyme 507

protoporphyrinogen oxidase resulting in membrane destruction and tissue necrosis (Netherland 508

2014, Glomski and Netherland 2013). Flumioxazin is a relatively new herbicide for aquatic 509

invasive macrophyte control, so use patterns are still being investigated and developed. Hydrilla 510

is sensitive to flumioxazin with electrolyte leakage and biomass reduction occurring at 200-400 511

µg L-1 (Glomski et al. 2013, Mudge and Haller 2010, Mudge et al. 2010). Flumioxazin has 512

potential for selective hydrilla applications since some native species including Vallisneria 513

americana, longleaf pondweed (Potamogeton nodosus Poir.), and Najas guadalupensis tested 514

have not exhibited sensitivity (Glomski and Netherland 2013). Water pH can strongly impact 515

treatment efficacy because flumioxazin degrades rapidly at pH of 8 or higher, altering the 516

concentration and exposure time (Glomski and Netherland 2013, Mudge and Haller 2010, 517

Mudge et al. 2010, Netherland 2014). Diurnal pH fluctuations and plant matter density should 518

be considered when using flumioxazin for hydrilla control. 519

The recent registration of the auxin-mimic herbicide florpyrauxifen-benzyl provides a 520

new option for aquatic plant management. Florpyrauxifen-benzyl is an arylpicolinate, a new 521

class of synthetic auxin herbicides, with a different binding affinity than that of other registered 522

auxin-mimic herbicides (Bell et al. 2015, Lee et al. 2013). Florpyrauxifen-benzyl has shown 523

activity on several aquatic macrophytes including hydrilla, crested floating heart (Nymphoides 524

25

cristata [Roxb.] Kuntze), and Eurasian watermilfoil (Myriophyllum spicatum L.) (Beets and 525

Netherland 2018, Netherland and Richardson 2016, Richardson et al. 2016). In small-scale 526

studies significant reductions in hydrilla biomass were observed with 12 µg L-1 static exposure 527

and 24 µg L-1 florpyrauxifen-benzyl 24 hr exposures (Beets and Netherland 2018). Since this 528

herbicide is new to aquatic plant management, application rates and exposure times are still 529

being investigated for hydrilla control. 530

There are several other slow-acting systemic herbicides that control hydrilla via 531

inhibition of enzyme activity including topramezone, imazamox, penoxsulam and bispyribac. 532

These herbicides are effective at rates of less than 100 ppb, and provide one to two years of 533

control (Netherland 2014). The slow decay of plant matter minimizes oxygen depletion and risk 534

of fish mortality. These herbicides commonly require whole-lake treatments or treatment in 535

areas with low water movement and potential for dilution is low. One critical factor to consider 536

with chemical control of hydrilla is the potential for herbicide resistance to develop in a 537

population. Repetitive treatment of fluridone in Florida led to fluridone-resistant dioecious 538

hydrilla (Michel et al. 2004). The exclusive use of a single herbicide mode of action is not 539

recommended due to the potential for mutations conferring resistance (True-Meadows et al. 540

2016). While herbicide resistance has not been reported in monoecious hydrilla, it is important 541

to monitor treated hydrilla for possible resistance development. 542

543

Selected Hydrilla Management Plants in NC. Integrated management programs have been 544

implemented in both the Tar River Reservoir and Lake Gaston. Approximately 135 acres of 545

hydrilla were discovered in 2005 on the Tar River Reservoir (Nawrocki et al. 2016). Annual 546

26

fluridone treatments were applied from 2007 to 2012, with a dewatering due to drought 547

conditions in summer 2007. Triploid grass carp were not a precious option for this system due 548

to concerns about escapement and potential downstream habitat impacts. In 2013, triploid 549

grass carp were introduced at a very low stocking rate of ~1.5 fish per tuber bank acre where a 550

tuber bank acre is the total acreage of waterbody previously infested with hydrilla. Core 551

sampling was performed regularly to observe shifts in hydrilla tuber density. Tuber density 552

decreased 74% in the first year of treatment, with a 19% further decrease in year 2 (Figure 1; 553

Nawrocki et al. 2016). By year 7 tuber density had decreased by 98.6%, indicating miniscule 554

gains in tuber bank attrition in the last 5 years of management. Previous studies with dioecious 555

hydrilla using similar management regimes indicate that herbicide treatment followed by 556

stocking of grass carp can lead to eradication of hydrilla (Sutton 1996). 557

Hydrilla was first discovered in Lake Gaston in the mid-1980s and by 2011 an estimated 558

1,530 acres were infested (Remetrix 2012). Lack of a cohesive management plan in the 1990’s 559

resulted in hydrilla spreading to up to 3,500 acres of the lake. During this time, management 560

practices were based primarily on immediate visibility impacts to stakeholders rather than long-561

term considerations. In the mid-2000’s a technical advisory team was formed which refined the 562

management program to include the best scientific practices for long term management with 563

some consideration for immediate impact where appropriate. This adaptive management plan 564

combined a specific and modeled grass carp stocking rate with selective herbicide use to meet 565

the stakeholder stated goal of hydrilla reduction while maintaining some vegetation in the 566

system. The greatest target rate for grass carp stocking in Lake Gaston was approximately 18 567

27

fish per acre of established hydrilla (Nawrocki et al. 2016), although this has transitioned to 568

stocking based on tuber bank acres as hydrilla declined. 569

Large scale fluridone treatments were also used to control hydrilla on Lake Gaston. 570

Originally, these treatments rotated yearly between sites which allowed hydrilla to regrow in 571

off years and replenish the tuber bank. In the 2000’s, the plan shifted to maintain herbicide 572

treatment in most sites for multiple years in order to deplete the tuber bank, while a lesser 573

acreage of treatment was rotated to the most impacted areas not under long-term 574

management. Consecutive treatment of sites on Lake Gaston resulted in more than a 60% 575

reduction in tuber densities, with minimal replenishment of tubers. In contrast, sites that were 576

not treated consecutively resulted in biomass resurgence and a 22% reduction in tuber density 577

(Nawrocki et al. 2016). This highlights the need for consecutive year treatments to maintain 578

hydrilla populations and Nawrocki et al. (2016) suggest at least five years of consecutive 579

herbicide management for effective hydrilla control in North Carolina Reservoirs. Adaptive and 580

consistent management practices have ultimately led to a decline in hydrilla coverage in Lake 581

Gaston can serve as a successful example of integrated management practices. Herbicide 582

treatments followed by grass carp stocking can provide North Carolina systems with cost 583

efficient and lasting control of nuisance aquatic weeds such as hydrilla. 584

There are a variety of management options available for hydrilla control. Prevention is 585

often overlooked as the most important step in reducing risk of invasion by aquatic 586

macrophytes but through education and signage, risk and spread of invasion can be greatly 587

reduced. Mechanical and physical practices are often non-selective, costly and labor intensive, 588

but can augment other methods of control. Grass carp can provide long-lasting but non-589

28

selective control of aquatic macrophytes and are a particularly useful method of hydrilla control 590

when used in conjunction with herbicides. There are several herbicides available for hydrilla 591

control, with some providing selectivity and minimizing or avoiding non-target damage to 592

desirable native species. Herbicides can be used for localized, spot treatments or whole-lake 593

control. When using herbicides for hydrilla control, concentration and exposure time are vital 594

for effective hydrilla control. The current largest gaps in knowledge are management practices 595

for monoecious hydrilla and the efficacy of florpyrauxifen-benzyl as a management tool for 596

monoecious hydrilla in North Carolina. Most hydrilla research has been performed on the 597

dioecious biotype and further investigation into the best management practices for 598

monoecious hydrilla are warranted. Florpyrauxifen-benzyl is a new herbicide registered for 599

aquatic research and has shown potential as a valuable new tool for aquatic macrophyte 600

management. Further research on the concentration and exposure requirements and field 601

efficacy would be particularly valuable to aquatic plant management. 602

603

Developing a Management Plan 604

It is important for resource managers to have a long-term aquatic macrophyte 605

management plan, even before invasion by exotic species. Madsen (2014) outlines the major 606

focal points of a successful management plan. This plan should establish protocols for 607

prevention, as well as early detection and rapid response. This can identify problems at an early 608

stage and reduce costs so new invasions can be quickly managed. Management plans for North 609

Carolina water systems should identify potential resources and stakeholders so that 610

cooperative groups can be formed to aid in aquatic invasive species management. Current 611

29

information should be used to identify gaps in knowledge about the infestation and then 612

communicate the need for management as well as provide a rationale for management. Site-613

specific management plans should focus on eight main components: prevention, problem 614

assessment, project management, monitoring, education, management goals, site-specific 615

management and evaluation (Madsen 2014). 616

Prevention through education and quarantine of nuisance species combined with early 617

detection and rapid response are pivotal to management. Introductions most often occur as a 618

result of human activity and prevention can reduce the risk of further spread of nuisance 619

aquatic macrophytes. Utilization of state and federal legislation and enforcement in conjunction 620

with education, outreach, and signage are powerful prevention methods with minimal costs. 621

Early detection and rapid response will also minimize costs since it is much more likely to 622

eradicate small populations than large, well-established populations. 623

Problem assessment requires acquiring information about the problem and identifying 624

groups and stakeholders that should provide input. Maps and plant distribution data are pivotal 625

to acquiring the information necessary to manage the problematic infestation. Proper project 626

management increases the likelihood of success. Planning and asset management including 627

detailed expense records are essential, especially when government funding is being used. 628

Monitoring is a necessity to assess the effects that management activities are having on the 629

aquatic system. Periodic monitoring efforts can be augmented with citizen monitors, especially 630

for water quality assessments (Madsen 2014). Education and outreach are not only important 631

as a method of prevention, but can also help include the public in the decision-making process, 632

30

explain possible solutions, and inform the public of progress made. It is important to be open 633

about management activities and can help garner public support for management projects. 634

Proper understanding of the distribution and abundance of invasive, nonnative, native, 635

threatened, and endangered species is necessary for a management plant in order to 636

understand the needs and possible restrictions on any management actions. Qualitative data 637

should be used to inform management decisions whenever possible. Qualitative data allows for 638

statistical analyses and reduces the risk of ineffective techniques being used in management. 639

Specific management goals need to be formulated, with milestones established to measure the 640

success of the program. Specific goals will be easier to attain, and progress can be relayed to 641

stakeholders to show progress. Management techniques vary between water systems, water 642

conditions, can change over time, and are influenced by environmental, economic and 643

efficiency requirements (Madsen 2014). Site-specific management techniques are selected 644

based on the needs of a specific location, balanced with environmental and regulatory 645

constraints. Factors to consider are the identity and density of the infested area, water flow 646

characteristics and water use restrictions. These factors can eliminate certain herbicides or 647

management techniques from use in a management plan, and inform a decision on 648

management techniques with the highest chance of success in a given water system. It is also 649

important to consider management techniques may change over time due to successes and 650

failures of the initial techniques implemented. Integration of several techniques, such as 651

herbicide applications followed by grass carp stockings or hand-pulling can greatly increase the 652

chance of a successful management plan. Finally, a quantitative assessment of the 653

31

effectiveness of aquatic weed management practices should be made to provide stakeholders 654

with an analysis of environmental impacts and economic costs (Madsen 2014). 655

656

Conclusions 657

Aquatic macrophyte management is of vital importance for the proper maintenance and 658

function of North Carolina water systems. North Carolina water systems, particularly reservoirs, 659

natural lakes, and rivers are at risk of hydrilla infestation. Existing partnerships between state 660

agencies are important for prevention, education and outreach, and management of hydrilla. 661

Each agency has a role in aquatic macrophyte management. Interagency cooperation and 662

partnership should be maintained to increase the likelihood of success of weed management 663

efforts. There are a variety of management options available for hydrilla control. Management 664

options differ between lakes, with economic and environmental factors greatly influencing the 665

management plan implemented in a specific aquatic system. Integrated management programs 666

can provide long-term hydrilla control and are often the most successful, but need a detailed 667

and well-developed management plan. 668

669

670

32

II. Operational Guidance 671

Aquatic Vegetation Surveys. Standard surveys for most North Carolina lakes and reservoirs 672

should consist of an evaluation of both submersed and emergent vegetation within the littoral 673

zone and around the perimeter of the waterbody. These surveys should be conducted in late 674

summer or fall when peak hydrilla growth would be expected to be present. Standard practices 675

include the use of a point intercept evaluation to determine specific species present in 676

combination with a hydroacoustic (SONAR) survey to estimate the density of submersed 677

species present. Surveys should be repeatable in time so that year to year comparisons can be 678

made. 679

A point intercept survey establishes GPS referenced points located in the littoral zone of 680

the reservoir and spaced uniformly around the waterbody in areas habitable by vegetation 681

(Table 3A). Point separation distances may vary depending on the size of the waterbody and 682

degree of precision desired, common distances include 200 ft (60 m) to 1200 ft (365 m). At 683

each sample point a double sided throw rake is used to sample submersed vegetation for both 684

species identity and plant density, common practice would be two “rake tosses” at each point 685

on opposite sides of the boat. Samples should be taken in water depths at which vegetation 686

would be expected to occur. In addition, a visual assessment of shoreline vegetation species 687

and other observations should be taken “in the vicinity” of the point, commonly observations 688

within 20 to 50 ft (6 to 15 m) on either side of the point. GPS coordinates of each point will 689

allow repeatable surveys over time to evaluate management efforts, or catch invasions of 690

unwanted species. 691

33

A hydroacoustic survey (also SONAR or fathometer) consists of transects between points 692

where the active hydroacoustic signal is recorded and GPS referenced. This survey can be 693

conducted concurrently with a point intercept study for maximum efficiency. Hydroacoustic 694

data is then processed to generate graphic representations of vegetation density (heat maps; 695

Table 3B). Transects should be conducted between 4 and 15 feet of water depth as return 696

signals in shallow water are generally of poor quality and hydrilla is unlikely to grow in water 697

depths greater than 15 feet unless water is extremely clear. 698

699

Hydrilla Management. Operational hydrilla management plans should be developed with a 700

specific goal as part of an IPM plan. These plans should generally be made for 5 to 10 years in 701

length due to recorded tuber longevity in hydrosoil. With the specific goal in mind, the most 702

appropriate management practices should be selected for implementation. In general, 703

management in reservoirs will be primarily driven by the need to reduce hydrilla to non-704

nuisance levels, while the primary driver of natural systems will be to maintain ecosystem 705

integrity and ensure that the system is not degraded by hydrilla or the management practices. 706

For reservoir management, triploid grass carp will likely be the primary management 707

technique (in terms of acres managed) due to ease of implementation and relative lower cost. 708

Reservoirs also typically have less native vegetation that might be negatively impacted by grass 709

carp. In these situations, grass carp should be stocked on a similar manner across reservoirs so 710

that knowledge gains can be used to refine stocking practices. The current system of stocking 711

carp based on standing acres of hydrilla for immediate control (first 1-3 years) and stocking 712

based on hydrilla tuber bank acres for long term control (years 4-10) is working well in multiple 713

34

systems and the cohort analysis can be modified over time as additional scientific data becomes 714

available. Herbicides can be used to compliment grass carp to speed initial management 715

results, control hot spots that grass carp may be avoiding, or as a primary control technique in 716

reservoirs where grass carp cannot be used. 717

In natural systems, herbicides will likely be the primary management technique due to 718

the selective nature of herbicides. In these systems, native vegetation serves as important 719

habitat for many other organisms and negative impacts to native vegetation may have cascade 720

effects. Selective herbicides such as fluridone can control hydrilla while less sensitive vegetation 721

is not killed. Over time this results in hydrilla depletion in the system while some native 722

vegetation remains. Herbicide use might result in additional expense compared to other 723

methods, but the protection of ecosystem integrity has great value which cannot be maintained 724

by using relatively non-selective measures such as triploid grass carp or drawdowns. Herbicides 725

have been used successfully in this manner for control at Lake Waccamaw and in the Eno River. 726

727

728

729

35

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731

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remote sensing technologies. Masters’ Thesis. 789

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submersed plant biomass. Aquatic Botany 155: 45-51. 792

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Practices Handbook. Third Ed. Aquatic Ecosystem Restoration Foundation, Marietta GA. 827

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Madsen JD, Wersal RM. 2018. Proper Survey methods for research of aquatic plant ecology and 829

management. J. Aquat. Plant Manage. 56s: 90-96. 830

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dissertation. North Carolina State University. 832

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40

Mudge CR, Haller WT. 2010. Effect of pH on submersed aquatic plant response to flumioxazin. J. 838

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dependent hydrolysis on the efficacy of flumioxazin for hydrilla control. J. Aquat. Plant 841

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various management regimes on four North Carolina reservoirs. J. Aquat. Plant Manage. 851

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control of Hydrilla verticillata. J. Aquat. Plant. Manage. 39:56-58. 881

42

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carp incrementally stocked for hydrilla control. North Am. J. Fish. Manage. 33(1):14–25. 912

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Canal, Florida. Aquat. Bot. 53:121–130. 914

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by hydrilla. Weed Sci. 20(6): 581-583. 916

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Sci. 18(6): 703-707. 918

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tubers. J. Aquat. Plant Manage. 30:15-20. 920

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and other aquatic weeds in Florida. University of Florida IFAS Extension BUL 867. 6 pp. 922

True-Meadows S, Haug EJ, Richardson RJ. 2016. Monoecious hydrilla-A review of the literature. 923

J. Aquat. Manage. 54:1-11. 924

44

Turnage G, Madsen JD, Wersal RM. 2015. Comparative efficacy of chelated copper formulations 925

alone and in combination with diquat against hydrilla and subsequent sensitivity of 926

American lotus. J. Aquat. Plant Manage. 53: 138-140. 927

Van TK, Steward KK, Conant Jr. 1987. Responses of monoecious and dioecious hydrilla (Hydrilla 928

verticillata) to various concentrations and exposures of diquat. Weed Sci. 35(2):247–252. 929

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gen. et sp. nov.: epiphytic cyanobacteria on invasive aquatic plants implicated in avian 931

vacuolar myelinopathy. Phytotaxa 181(5):243–260. 932

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cyanobacterial species. Environ. Toxicol. 20:348–353. 935

45

Table 1. NCDEHNR (1996) estimate of potential hydrilla colonization in North Carolina lakes and reservoirs by geographic 936

area. 937

938

46

939

Table 2. Summary of major management options for hydrilla in North Carolina. 940

Management Option Stage of Invasion Longevity of Control Selectivity Expense Summary

Prevention Pre-invasion Indefinite N/A $$ to $$$$ Requires new infrastructure

Hand-weeding Early Short to moderate Moderate $$ Difficult to conduct in water

Drawdown Early to late Short to moderate None 0 to $ Winter drawdowns ineffective.

Summer drawdowns not practical.

Mechanical harvesting Mid to late Short None $$ to $$$$ Short term and expensive.

Rocks and stumps hinder practicality.

Biological: host specific Mid to late None in NC High 0 to $ No evidence they will establish in NC

Biological: triploid grass carp Early to late Long Poor $ Inexpensive and long term.

Do not stay in one area.

Will feed on desirable vegetation.

Herbicides Early to late Moderate Poor to High $$ to $$$$ More flexible than other options.

Very expensive over long term.

941

47

Table 3. A) Point intercept survey established for Lake Waccamaw. Due to shallow water depth, 942

points are evenly distributed across the entire lake. B) Hydroacoustic survey results for a small 943

cove on Lake Gaston. Red colors indicated high plant density while blues represent little to no 944

standing plants. 945

A B

946

48

Figure 1. Observed and predicted tuber bank attrition in the Tar River Reservoir. 947

948

949

49

Figure 2. Tuber replenishment in between alternate years of management and when 950

management stopped after 3 consecutive years of management (Lyons). 951

952

953

50

Appendix 1 : Lake Gaston Volunteer Survey Overview 954

In fall of each year, an annual aquatic plant survey is conducted at Lake Gaston as a 955

collaborative effort between North Carolina State University (NCSU) and volunteers from the 956

Lake Gaston Association (LGA) to document all aquatic vegetation around the entire 350 mile 957

shoreline of Lake Gaston. The Lake Gaston Extension Associate (NCSU) coordinates activities 958

with members of the LGA Environmental Committee. This committee recruits volunteers, 959

schedules training sessions, and coordinates sampling equipment allocation among volunteers. 960

NCSU conducts volunteer training sessions, develops appropriate press releases, coordinates 961

survey sites for each volunteer, and samples any areas that are not covered by volunteers. 962

NCSU is also responsible for analyzing the data collected and presenting it to the Technical 963

Advisory Group that determines the upcoming year’s treatment plan. 964

The Lake Gaston shoreline is broken up into 3 mile survey sections and volunteers are 965

allowed to select areas that they would prefer to survey. Of course, volunteers tend to prefer 966

sites close to their homes (or familiar territory). NCSU provides coordination to ensure that 967

effort is not duplicated and that non-preferred areas are also adequately surveyed. Volunteers 968

are trained to sample every 200 feet with two rake tosses per point and record the vegetation 969

present. 970

During the survey, the Environmental Committee assigns coordinators to arrange the 971

pick-up and drop-off of the surveying equipment to the volunteers. Volunteers are issued a 972

handheld tablet to collect data and sampling rake to sample submerged vegetation. Volunteers 973

are also provided with a plant ID book that covers most of the plants that will likely be 974

encountered at Lake Gaston, a “How-To” instructional sheet, and back-up battery pack. NCSU 975

51

continuously monitors the shoreline sites that are being surveyed and coordinates with any 976

new volunteers that are interested in joining the survey. Also, NCSU maintains the data 977

collection program, GIS Cloud, which is used for the survey. GIS Cloud is a user friendly, GPS 978

based program that allows for data collection without access to cellular data. Currently, the 979

survey has 14 active licenses that are distributed among the volunteer tablets. 980

Post survey, NCSU analyzes all the data collected during the survey to identify the extent 981

of nuisance species as well as beneficial species such as water willow and eel grass. The data is 982

also used to identify and estimate the acreage of those areas that may require herbicide 983

treatments in the upcoming treatment plan. Results are shared with appropriate agencies and 984

the Lake Gaston Weed Control Council. 985