Make Waves With Effective Aquatic Weed Control

The control of aquatic vegetation is more than the control of invasive aquatic plant species. Native, naturalized and invasive weeds need to be controlled to maintain open waterways, free movement of water, aesthetics, and recreational uses.

A Brief History

The water hyacinth was introduced to Florida in 1884. By 1898 the plant was causing major navigational disruptions. The River and Harbor Act of 1899 empowered the Army Corp of Engineers to construct and operate vessels and log booms for the removal and containment of waterhyacinth in navigable waters of Florida and Louisiana.

By 1902 Congress had authorized extermination and removal of waterhyacinth by any mechanical, chemical, or other means. Several chemical controls were tried and some were effective but toxic to cattle and humans. Due to this toxicity, in 1905 Congress prohibited the use of any chemical which was injurious to cattle or man. This left only mechanical means which were inefficient and temporary at best, and a losing battle was waged for the next 50 years. The tide changed in this fight in 1947 when several agencies started spraying 2,4-D, after its introduction in 1946. This chemical is still being used today. Not only does it control waterhyacinth, it is not toxic to fish, cattle, or humans.

In 1964 the first biological control of an aquatic weed started with the introduction of Agasicles hygrophila, the alligator flea beetle, on an introduced weed, alligatorweed. This was after it had been determined that this beetle was host specific and would not feed on other plants. This bio-control has been and still is effective on alligatorweed.
Yet another introduced species, hydrilla, was discovered in Florida in 1959. By the late 1960s it reached a level of infestation that was causing serious interference with the use of the state’s waters. Hydrilla is still a major aquatic weed today.

All three of these weeds are still present in Florida’s waters and cause problems requiring some means of control. These real examples should give all of us the desire to keep new weeds out of our water bodies.

Once we have the weeds it does not matter how they got here. They now have to be controlled. What do we do? There are several ways to clean up the waters and remove the weeds. Physical removal, water level manipulation, reducing light penetration, nutrient limitation, biological control, and chemical are all available measures for control.

Maintenance control is used to minimize the detrimental impacts that aquatic vegetation has on water uses. A coordinated manner of using the various control techniques is utilized on a continuous basis in an attempt to maintain the plant population at the lowest feasible level.

Aquatic Regulations

The U.S. EPA regulates the registration, manufacture, transportation, use, and marketing of all pesticides in the United States. The EPA does this through the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). In Florida pesticides and restricted-use pesticide licenses are regulated by the Florida Department of Agriculture and Consumer Services (FDACS). The Florida Pesticide Law (Chapter 487 FS) is the law that gives FDACS its authority to regulate pesticides within Florida.

Anyone who applies or supervises the application of restricted use pesticides (RUP) to an aquatic site must be licensed. Not all pesticides labeled for use on an aquatic site are RUPs. Many agencies performing aquatic weed control require applicators of any product, not just RUPs, to be licensed in the Aquatic Pest Control category.

Anyone that applies RUPs must keep records of the application for at least two years. FDACS has a suggested recordkeeping form. If their form DACS 13340 is filled out correctly, it will have all of the information needed to meet the requirements of the law.

Any person applying any organo-auxin herbicide, such as 2,4-D, must follow the Organo-Auxin Herbicide Rule. This rule has its own recordkeeping requirements. FDACS has a suggested recordkeeping form, DACS 13328 , for this as well.

The Florida Fish and Wildlife Conservation Commission (FWC) is the agency in charge of aquatic plant management in this state. A permit must be obtained from the FWC to control, eradicate, remove, or otherwise alter any aquatic plants in waters of the state. The FWC (Invasive Plant Management Section) directs the control of and disburses the funds to manage the aquatic plants in the waters of the state.

There are three weed lists to be aware of. One is the United States Department of Agriculture’s (USDA) Federal Noxious Weed List . Without a permit from the USDA, movement of any plant on this list into or through the United States is prohibited. There is also a Florida Noxious Weed List and the Prohibited Aquatic Plant List . Plants on this list may not be possessed, moved, or released except by permit from FDACS.FDACS has the authority to regulate plants on both of these lists.

Aquatic Herbicides

Currently there are only 14 active ingredients registered for use on aquatic sites in the US. Only 12 are registered for use in Florida waters with one other having a Special Local Need Permit (SLN). The registered active ingredients are listed below with their means of uptake or translocation and their mode of action.

Active Ingredient Uptake Mode of Action
Carfentrazone Contact Enzyme Inhibitor
Copper Contact Photosynthesis inhibitor (and other effects)
Diquat Contact Photosynthesis inhibitor
Diuron (SLN) Systemic Photosynthesis inhibitor
Endothall Contact Cell membrane disruptor
Fluridone Systemic Photosynthesis inhibitor (pigment synthesis)
Glyphosate Systemic Enzyme inhibitor
2,4-D Systemic Growth regulator (organo-auxin)
Imazamox Systemic Enzyme inhibitor
Imazapyr Systemic Enzyme inhibitor
Penoxsulam Systemic Enzyme inhibitor
Hydrogen peroxide Contact Cell membrane disruptor
Triclopyr Systemic Growth regulator (organo-auxin)

These herbicides are either contact or systemic. The contact herbicides must be evenly distributed to give good coverage of the weed since they diffuse very little after they are absorbed by the plant. The systemic herbicides are translocated through the plant to the site where they are active in controlling the weed.

The mode of action is how the herbicide kills the plant. The enzyme inhibitors interfere or inhibit, the action of one or more enzymes in the plant. This is turn causes the plant to not function properly and it dies. Photosynthesis inhibitors affect the electron capture or transfer the plant uses to obtain energy and manufacture the food it needs to survive.

The cell membrane disruptors cause the cell membrane to leak. When this happens the cell cannot function and dies. The growth regulators, organo-auxin compounds, mimic natural plant hormones. The plants are not able to survive due to the abnormal tissue development and abnormalities of other plant functions.

Understanding The Label 

What is the label? This is the material printed on or attached to the container of pesticide as it comes from the dealer. You must read the label, it is the law. Florida Pesticide Law states “It is unlawful for any person to use any pesticide, including a restricted-use pesticide, or to dispose of any pesticide containers in a manner other than as stated in the labeling or on the label or as specified by the Department or the United States Environmental Protection Agency.”

It is imperative that you read and understand the labels for the products you use. This is true not only for aquatic sites but for all pesticides that you use at work and at home. The label is the way that the manufacturer and EPA provide the information you need to know about the pesticide. This includes where it can be used, warnings, what you need to wear when using the product, how to remediate exposure to the pesticide, environmental concerns, what rates to use, and storage and disposal.

The label will have warnings for you, others near your work area, and the environment. The first warning will be whether or not it is a RUP. RUPs are restricted due to the hazards they present to the handlers or the environment. Only licensed applicators can purchase and use RUPs (or supervise the use of).

The signal word will be the next warning you will see on the label. The signal word is on the label to let you know the toxicity of the pesticide with which you are working. Caution is the lowest warning meaning it is relatively non-toxic to slightly toxic. Warning is the next level higher indicating that the product is moderately toxic. The highest level of warning is danger or danger/poison; these products are considered highly toxic. This toxicity level is based on the entire product that you purchase, both the active and inert ingredients.

Along with the warnings there will be various sections of a label that contain further cautions and use directions of the product. These will include:
• Statement of Practical Treatment
• Child Hazard Warning
• Hazards to Humans and Domestic Animals
• Personal Protective Equipment
• Environmental Hazards
• Use Classification
• Brand (Trade) Name
• Ingredient Statement
• Net Contents
• EPA Registration Number
• EPA Establishment Number
• Name and Address of Manufacturer
• Formulation
• Physical of Chemical Hazards
• Limited Warranty and Disclaimer
• Directions for Use
• Storage and Disposal

You should be familiar with these different sections and the information they contain. They are on the label so that you can use the product safely and correctly. By reading and following the label applicators are doing their best to apply the pesticide in a manner that will give them the desired control.

Non-herbicide Methods of Aquatic Plant Management

There are several ways to control weeds in aquatic situations. One of the simplest and often overlooked is preventing introduction from outside sources. If you have a clean water body, keep it clean. Work to prevent the introduction of weeds that might be brought in. Maintain vigilant control of any movement of equipment between water bodies. If equipment must be brought in, make sure it is clean and does not bring in any new weeds.

If you have aquatic vegetation you need to control, what do you do? Other than chemical methods of control there are a few other methods that you may be able to use. Different non-herbicidal methods include physical removal, water level manipulation, reducing light penetration, nutrient limitation, and biological control.

Physical removal is used in varying degrees. It can be anything from hand removal to draglines and specialized aquatic weed harvesters. With hand removal you can be very selective but it can be back breaking work. With the other mechanical means the work is easier but you lose the selectivity. Any physical removal means you will dirty up the water and most likely leave roots and other plant parts that will rapidly regenerate. This is a fast means for ridding the water body of unwanted vegetation usually in smaller areas without using chemicals. It can be used with chemical control as well.

Water level manipulation refers to either raising or lowering the water level to control aquatic vegetation. In Florida the lowering of water levels, drawdown, is more common. Drawdown has been used for many years to help control vegetation as well as oxidize and consolidate flocculent sediments and to alter fish populations. To be able to effectively use this method a water control structure is needed.

There may be limitations on the water-use and water rights for the water body. Will the drawdown affect the recreational or agricultural use of the water body? In Florida the drawdown is typically done in the winter to get the added effect of a freeze if the weather cooperates. This is not a foolproof method. The advantages are that this can be a fairly inexpensive method and you can remove some of the sediment from the basin’s bottom.

Some disadvantages include not getting the control you desire, removing desirable species, moving the unwanted vegetation to the deeper part of the water body, and the loss of valuable water if insufficient water is available to refill the basin.

Reducing light penetration can be accomplished by a few different methods. Special aquatic dyes, fertilization, or special fabric bottom covers may be used. The dyes are not pesticides, but are formulated and approved for use in water. The dyes produced for aquatic use block the light that plants need for photosynthesis. Ponds should be deeper than 3 feet and have very little to no flow for dyes to be effective.

Fertilization is another way to reduce light penetration into pond or lake waters. Phytoplankton feed on the fertilizer and their increased numbers reduce the light penetration. Fertilization on water bodies can cause problems through reduced oxygen and weed growth if the practice is stopped.

Special materials designed to be placed on the bottom of water bodies help prevent the growth of rooted aquatic plants. They require some maintenance to prevent sediment accumulation on top of the cover. They are usually used in ornamental ponds, swimming areas, or around boat docks.

Another way to help eliminate or prevent weed growth is to limit nutrients that plants need in the water. Phosphorus and nitrogen are usually found naturally in high enough amounts to allow for weed growth. Extra amounts of these nutrients from pollution can exacerbate the weed problem. Reduction of nutrients should be a goal to help reduce the growth of free floating plants in water bodies.

Biological control of aquatic weeds has been used since 1964, when the alligatorweed flea beetle was released, to reduce aquatic weeds in our waters. There are several biological controls that are used in Florida waters. These controls are herbivorous fish, insects, pathogens, and other organisms such as snails, manatees and others.

There are two herbivorous fish used in Florida to help maintain control of aquatic weeds, grass carp and blue tilapia. The grass carp used in Florida are sexually sterile. A permit from the FWC is required to release them in any water bodies in Florida. Blue tilapia is used to control filamentous algae. Permits from the FWC are required for certain parts of Florida. Other tilapias are not allowed to be released. Always check with the FWC before releasing new fish in Florida waters.

Several insects have been released to control aquatic weeds including the alligatorweed flea beetle, alligatorweed stem borer, alligator thrips, waterhyacinth weevils, waterhyacinth borer, waterlettuce weevil, hydrilla tuber weevil, hydrilla stem borer, the Australian leaf mining fly, and the Indian leaf mining fly.

The best biological control has been observed on alligatorweed. Of the insects used against alligator weed, the flea beetle has been the most effective. The insects released to control waterhyacinth have not yet reduced the need for waterhyacinth control programs. The waterlettuce weevil has impacted waterlettuce populations but the impact is not predictable. Of the insects released for hydrilla control, the Australian leaf mining fly, and the Indian leaf mining fly are the only two to become established. None of these insects have had a measurable impact on hydrilla populations.

Pathogens may one day be formulated and applied similar to an herbicide. They tend to be environmentally sensitive and can be made ineffective by extremes of temperature and humidity. No pathogen products are currently available.

Snails have been studied as a biological control for aquatic plants. The possible problems with snails have caused the interest to be subdued. They can be host for fish parasites and are not specific in what they eat.

Manatees have been tested as a means to control weeds in canals. In cold winters, heaters had to be used to keep them warm. The US Fish and Wildlife Service found that ten times the normal amount of manatees in an area were needed to consume the existing hydrilla.

Other biological controls suggested or used in specialized instances, have been ducks, geese, crayfish, nematodes, viruses, and water buffalo. None of these have proven practical yet in Florida.


If we practice maintenance control with the existing aquatic weeds, we should be able to manage to keep them under control so that they do not adversely affect the water body. We should prudently use the available resources in our efforts to control these weed problems. Be safe, always read and follow the label.

Langeland, K. A. and Fishel, F. M. 2010 Aquatic Pest Control

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