Invasive Exotic Plant Tutorial

Species Management and Control Information

Hydrilla
Hydrilla verticillata (L.f.) Royle

Source: The following information is taken from a number of sources and those sources are identified at the beginning of the referenced information. The intent of this resource is to provide the user with as much of the information that exists for management and control of this species as is practical. It is important to note that new and improved methods are added regularly which will require you to visit the websites provided for updates on this information. Bibliographies and resources referenced by these sources are not included here, but are included at the websites from which this information was extracted.

READ THIS FIRST!

Before administering any of the following management and control options, it is imperative that you are familiar with the background information provided under the General Management and Control Section.

For additional, and potentially more current, information on management and control of this species, use the Mid-Atlantic Exotic Pest Plant Council (MA-EPPC) listserve or any of the other listserves identified in the Resource - Listserve section of this tutorial. You will find directions for subscribing to the list serve there. The MA-EPPC listserve has an archives feature that saves past discussions (beginning in 1999) about specific species control. These messages are at: http://groups.yahoo.com/group/ma-eppc/messages.

GENERAL MANAGEMENT CONSIDERATIONS:

National Park Service and U.S. Fish and Wildlife Service - Plant Invaders of Mid-Atlantic Natural Areas - Hydrilla - http://www.nps.gov/plants/alien/pubs/midatlantic/hyve.htm - 2002

Physical, chemical and biological controls have all been used on hydrilla. Each control method has advantages and liabilities. Water level drawdowns have generally been ineffective in our area. Mechanical aquatic weed harvesters provide temporary relief and open boating lanes, but resulting plant fragments can help spread the vegetation faster. Contact herbicides provide temporary control, but systemic herbicides provide more long-term control. Herbivorous fish such as sterile grass carp have been used for hydrilla control where allowed by law. Other biological controls are still being investigated. Each control method has its drawbacks and liabilities. On the Potomac River and other parts of the Chesapeake Bay system, resource managers who are struggling with hydrilla also recognize the beneficial impacts of submerged aquatic vegetation (including hydrilla) for water quality and fish and shellfish habitat.

The Nature Conservancy - Invasive Species Initiative - Species Management Summary (ESA or Element Stewardship Abstract): Hydrilla - http://tncweeds.ucdavis.edu/esadocs/hydrvert.html - Author: Michael S. Batcher - 2000.

H. verticillata has an unusually high level of reproductive vigor, and is also highly adaptable to different habitats. There are currently no known important native insect pests of H. verticillata. In wildlands, the application of herbicides can kill other aquatic plants and animals. Unless and until biological controls become biologically and economically feasible, the potential for large-scale restoration of wildlands infested with H. verticillata is probably very low.

MANUAL AND MECHANICAL CONTROL:

Delaware River Invasive Plant Partnership - Invasive Plant Fact Sheets - Hydrilla - http://www.paflora.org/DRIPP.html - Authors: Ann F. Rhoads and Timothy A. Block - March 2004

Mechanical removal is of limited utility due to the potential for stem fragments to spread and for tubers to remain behind in the lake bottom mud.

Southeast Exotic Pest Plant Council - Invasive Plant Manual - Hydrilla - http://www.se-eppc.org/manual/HYVE.html -2003

Harvesting: Harvesting may manage small initial populations. Where possible, remove all of the plant parts from the water and dispose of properly. Plant fragments as small as one whorl can potentially start a new infestation. Consistent monitoring for several growing seasons is required to control new or missed plants. For larger infestations in high value areas, specialized harvesters are available, although the cost may be prohibitive at as much or more than $1000 or more per acre. Up to six harvests per year may be necessary due to hydrilla's rapid growth.

Drawdowns: When growing in bodies of water that have control structures, hydrilla may be controlled by seasonal drawdowns. Drawdowns are most effective while the tubers are developing in the fall and before regrowth occurs in the spring. Tubers may remain dormant and viable in the soil even after the lake or pond has been drained, so drawdowns are limited in effectiveness.

Specialized machines are used for mechanically removing H. verticillata. This is not a widespread practice because of the high cost involved (often over $1000 per acre) and because of logistical constraints. Up to six harvests may be required annually due to the rapid growth rate of H. verticillata (McGehee 1979). Mechanical removal is used for H. verticillata management only in areas that are in close proximity to domestic water supply intakes, in rapidly flowing water, or when immediate removal is necessary. The high cost of harvesting H. verticillata, and its low nutrient value, greatly restrict hydrilla's value as a forage plant (Easley & Shirley 1974; Bagnall 1978).

Drawdowns can be an effective mode of hydrilla control if the drawdown is performed while subterranean turions are developing in the fall, and prior to regrowth in the spring (Haller et al. 1976). Drawdowns for aquatic plant management are restricted to only those lakes or ponds that have water control structures, and have hydrologic characteristics that permit water levels to be controlled. Additionally, the drawdown must not negatively impact other primary water uses, such as domestic or irrigation supplies, navigation, or hydrologic power. Even in drained lakes and ponds, subterranean turions may remain dormant and viable in organic substrates (Haller and Shireman 1983).

University of Florida - Center for Aquatic and Invasive Plants and Sea Grant - Non-native Invasive Aquatic Plants in the United States - Hydrilla verticillata - http://aquat1.ifas.ufl.edu/hyvepic.html - Author: Victor Ramey - 2001.

In some cases, lake drawdowns may help manage hydrilla by letting the exposed plants die and decompose. The action of mechanical harvestors and chopping machines remove hydrilla from the water and transport it to disposal on shore; chopping machines, unfortunately, fragment the hydrilla plants and may actually increase the plant's distribution.

BIOCONTROL:

Delaware River Invasive Plant Partnership - Invasive Plant Fact Sheets - Hydrilla - http://www.paflora.org/DRIPP.html - Authors: Ann F. Rhoads and Timothy A. Block - March 2004

Surveys for natural enemies of hydrilla have been conducted in the U.S. and in Africa, Asia, and Australia. More than 20 species of insects or pathogens have been considered as potential biological control agents. While evaluations of many of these are still going on, 4 insects have been released - 2 leaf-mining flies, a stem-boring weevil, and a tuber-attacking weevil.

Southeast Exotic Pest Plant Council - Invasive Plant Manual - Hydrilla - http://www.se-eppc.org/manual/HYVE.html - 2003

Triploid Grass Carp: Sterile grass carp or white amur (Ctenopharyngodon idella) may be an option in areas with adequate control structures to ensure retention of released fish. Grass carp will eat a variety of vegetation including native species. This method is an option only in areas where impact to all of the vegetation is acceptable. The stocking rates for grass carp have not been established. Local and state laws should be checked before release.

Since 1981, the U. S. Department of Agriculture, U. S. Army Corps of Engineers and the University of Florida-IFAS have conducted worldwide surveys for other biological control agents. Over forty insect species have been identified as feeding on hydrilla and are currently being evaluated. The larvae of a weevil (Bagous affinis) discovered in India and Pakistan feeds on the subterranean tubers of hydrilla. This insect is only effective in areas of intermittent wet and dry periods or where periodic drawdowns are possible. It has not become established. A leaf-mining fly (Hydrellia pakistanae) has been released in Florida but it has had no impact. Hydrellia balciunasi was released in 1989 but had limited success due to variation in hydrilla populations and parasitism by native wasps among other factors. The accidental release of an aquatic moth (Parapoynx diminutalis) was shown to defoliate plants but leave viable stems.

Grass carp or white amur (Ctenopharyngodon idella Val.) is a biological control agent that effectively controls H. verticillata (Van Dyke et al.1984). Possession of this fish species, however, is illegal in many states because of the potential environmental damage that could result if escaped fish were to establish breeding populations. Sterile, triploid grass carp are available and legal by permit in some states in the U.S. Not all triploid grass carp are sterile, however, and every individual released needs to be genetically checked. Grass carp is recommended for small ponds or lakes and canal systems where the fish can be retained within the water body and where the removal of all vegetation is acceptable. There is no adequate method of recapture. Since Chinese grass carp prefer food other than H. verticillata, a reduction in the overall abundance of native aquatic plants, and the potential reduction in food and habitat for invertebrates, other fish, and waterfowl, are to be expected. Stocking rates for partial control have not been established.

Worldwide surveys for natural H. verticillata enemies were begun in 1981 in a cooperative study undertaken by the University of Florida-IFAS, United States Department of Agriculture, and U.S. Army Corps of Engineers. Snails consume large amounts of H. verticillata when they are present in high densities in enclosed experimental areas, but not in natural settings. Plant pathogens effective against H. verticillata under experimental conditions have been ineffective in the field (Charudattan and Lin 1974; Charudattan and McKinney 1978). Several pathogens have been identified from Asia, but their effectiveness has yet to be tested (Shearer 1997).

Over 40 species of insects have been found that feed on H. verticillata. Several are being evaluated as potential H. verticillata biocontrols in the United States. Other insects from Australia are also under consideration (Balciunas et al.1996). Bagous affinis Hustache is a weevil that was discovered in Pakistan and India. Adults lay eggs on rotting wood and other organic matter. After hatching, the larvae burrows through the sediment until it encounters a H. verticillata subterranean turion, which it then feeds on and destroys (Buckingham and Bennett 1994). This insect is useful only where there are periodic lake drawdowns or intermittently wet and dry shorelines. Another Bagous species has been released in the U.S. but has not become established. Hydrellia pakistanae Deonier is a leaf-mining fly that is very promising as a H. verticillata biosuppressant (Buckingham et al.1989). H. pakistanae is established in Florida, but its impact on H. verticillata has yet to be determined. H. balciunasi, released in 1989, has had limited establishment, apparently due to several factors including competition with other biological agents, parasitism by native wasps, genetic differences in H. verticillata types, and possible inbreeding depression (Grodowitz et al. 1997). 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 H. verticillata, though usually not until late in the growing season. Large areas may be defoliated but viable stems remain and the plant remains a problem.

USDA Forest Service - Biological Control of Invasive Plants in the Eastern United States, - http://www.invasive.org/eastern/biocontrol/7Hydrilla.html - Publication FHTET-2002-04 - Authors: R. Van Driesche, et al. - 2002.

Many of the natural enemies identified during overseas surveys still have not been fully evaluated to judge their safety as potential biological control agents for hydrilla. Only four hydrilla insects have been released in the United States: The tuber attacking weevil Bagous affinis Hustache (Coleoptera: Curculionidae) and the leaf mining fly Hydrellia pakistanae Deonier (Diptera: Ephydridae) were both released in 1987; another leaf-mining fly H. balciunasi Bock (Diptera: Ephydridae) was released in 1989; and the stem-mining weevil B. hydrillae O'Brien (Coleoptera: Curculionidae) was released in 1991 (Buckingham, 1994).

The leaf-mining flies have been the most extensively released species. Hydrellia pakistanae has been released at more than 50 sites in Alabama, California, Florida, Georgia, Louisiana, and Texas (Center et al., 1997). About 1.2 million individuals were obtained, mainly from greenhouse colonies maintained at the U.S. Army Engineer Research and Development Center in Vicksburg, Mississippi and various USDA, ARS facilities, along with an additional two million insects from a Tennessee Valley Authority pond-based rearing facility (Grodowitz and Snoddy, 1995). These releases ended in 1995. Recently (September 2000), releases resumed using Hydrellia-containing hydrilla obtained from ponds at the Lewisville Aquatic Ecosystem Research Facility, Lewisville, Texas with more than 300,000 immatures being released in Lake Raven in Huntsville State Park, Texas.

Although considerably less effort went into the release of H. balciunasi, still close to one million individuals were released at 11 sites in Florida and Texas only (Grodowitz et al., 1997). Bagous affinis was extremely difficult to maintain under mass-rearing conditions. This was due primarily to the high demand of tubers for larval feeding. However, over 10,000 individuals were released in three states (i.e., California, Florida, and Texas) at more than 10 locations (Godfrey et al., 1994; Grodowitz et al., 1995).

A larger effort went into the release of the stem-feeding weevil, B. hydrillae. For example, close to 300,000 individuals have been released in four states (Florida, Texas, Georgia, and California) at more than 15 locations (Grodowitz et al., 1995).

No overseas pathogens have yet been approved for release to control hydrilla.

Biocontrol fish and insects have been introduced to control hydrilla. The herbivorous (plant-eating) biological control fish, the Chinese grass carp, has a good preference for hydrilla (Cassani). Check the website http://plants.ifas.ufl.edu/guide/grasscarp.html to learn about problems associated with Chinese grass carp.

Other biological control work has been done for this species, including tuber-eating weevils and leaf-eating flies.

CHEMICAL CONTROL*:

Delaware River Invasive Plant Partnership - Invasive Plant Fact Sheets - Hydrilla - http://www.paflora.org/DRIPP.html - Authors: Ann F. Rhoads and Timothy A. Block - March 2004

Several herbicides have been used to control hydrilla including copper sulfate and endothal, which are contact poisons and the systemic chemicals fluridone and bensulfuron methyl.

Southeast Exotic Pest Plant Council - Invasive Plant Manual - Hydrilla - http://www.se-eppc.org/manual/HYVE.html -2003

Fluridone: Fluridone is a selective herbicide depending on application rates, contact times, and timing of application. Application rates depend on site-specific factors such as infestation size, water depth and chemistry, and water flow rates. It is intended for use on whole ponds and large-scale infestations (greater than 2 ha). It has been used successfully with minimal long-term effects to native plants. Fluridone is intended to reduce but not eliminate Hydrilla. Refer to manufacturer's label for specific information and restrictions regarding use.

Several herbicides have been used to control hydrilla. Most effective have been the contact poisons copper sulfate (brand name Komeen and others) and endothal (brand name Aquathol and others), and the systemic herbicides fluridone and bensulfuron methyl. For both contact and systemic herbicides, concentration in the water column and exposure time are key variables determining effectiveness. Copper sulfate and endothal are non-selective herbicides, and copper sulfate is highly toxic to fish. Fluridone has been used to control H. verticillata in Lake Okeechobee in Florida with minimal to no long-term impact on native aquatic plants (Langeland 1996). Application rates vary according to a number of factors, including water depth, water chemistry, whether the water is still or moving, and the size of the infestation. Getsinger & Netherland (1997) report that the following formulations have been effective: for endothal, 2.0 mg ae/L for 48 hours or 3.0-5.0 mg ae/L for 24 hours; for fluridone, 15-30 ug/L for 20-40 days (minimum of 4 ug/L); and for bensulfuron methyl, 25 ug/L and higher for in excess of 42 days. The use of plant growth regulators such as fluridone and bensulfuron methyl is relatively recent, and is intended to reduce, but not to necessarily eliminate, H. verticillata. Less vigorous remnant plants may perform useful functions such as providing oxygen, stabilizing sediment loads, and creating habitat (Lembi and Chand-Goyal 1994).

Acetic acid in concentrations of 9-26 mmol/L (which is less concentrated than commercial vinegar) for 24 hours reduced growth by 50% in laboratory studies (Spencer and Ksander 1995). The use of compounds from native aquatic plant species with allelopathic properties has not been shown to be an effective control for H. verticillata (Jones 1995).

Registered aquatic herbicides do provide temporary control of hydrilla. According to the University of Florida Aquatic Weed Management Guide, Vandiver 1999, copper, diquat, endothall and fluridone; As always, comply with federal law by following the herbicide label instructions, permissible sites and application rates.

* Mention of pesticide products in this document does not constitute endorsement of any particular material.