A Brief History of Aquatic Herbicide Use

Since the late 1880s, various chemicals have been applied to our freshwater systems to control the growth of invasive aquatic plants. Chemical development and use was spurred by the realization that human- and machine-power were not able to keep up with the explosive growth of certain invasive plant species such as water hyacinth (Eichhornia crassipes) and hydrilla (Hydrilla verticillata). Modern plant managers rely on research and guidelines that have been developed throughout Florida's more than one hundred years of experience in using aquatic herbicides to control invasive plants.

Aquatic Herbicides, 1900-1950

During the first half of the 20th century, aquatic plant management was characterized by extensive mechanical control operations and manual removal efforts. In 1902, the U.S. Congress amended the Rivers and Harbors Act, allowing for the “extermination” of water hyacinth by mechanical and chemical or any other means. Annual appropriations were allotted to the U.S. Army Corps of Engineers to clear waterways of weeds and maintain navigation. Army engineers used chains to dislodge submersed weeds as well as specialized equipment such as sawboats, choppers and cranes to remove floating water hyacinth from rivers and canals in the southern United States. Although old photos and documents often show blocked rivers, washed out bridges, and other problems created by water hyacinth and other aquatic weeds, these mechanical and manual efforts did attain a degree of success. However, they were not enough to keep waters clear of invasive aquatic plant problems.

The ineffectiveness and high cost of mechanical control, paired with the rapid regrowth of invasive plant populations, prompted engineers to experiment with methods such as steam, sulfuric/hydrochloric acid, copper sulfate, sodium arsenate and other inorganic contact "desiccants." Of these, only copper sulfate continues to be used today. The others were found to be ineffective and/or toxic to non-target plants, animals, and sometimes the people who applied them.  

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Aquatic Herbicides, 1950-1975

The discovery of the herbicidal properties of 2,4-D during the 1940s began an era of rapid development of new herbicides for agricultural and other uses, including aquatic weed control. By 1947, water hyacinth was proven to be particularly susceptible to 2,4-D. This herbicide also was effective against the submersed plant, Eurasian water-milfoil (Myriophyllum spicatum). Other herbicides were found to be effective against canal weeds (Potamogeton, Chara, etc.), even in rapidly flowing water.

Herbicides were relatively less expensive and more efficient than mechanical or manual methods because they could be applied to larger areas of weeds in a shorter time period, and could be applied to shallow areas inaccessible to large machines. The use of herbicides also provided longer periods of weed control.

By 1975, as many as 500 new pesticides had been discovered by the chemical industry and total pesticide use internationally was increasing dramatically. The publication of "Poison on the Land" in England by J. Wentworth Day in 1957 was followed by "Silent Spring" by Rachel Carson in 1962. These books clearly outlined the environmental and human health concerns of widespread pesticide use and mis-use, and called for greater regulation of pesticide manufacturing, distribution, labeling, and use. Public concern for the environment in the United States was so widespread that the U.S. Environmental Protection Agency (EPA) was formed in 1970 and given the responsibility for pesticide regulation. This responsibility was formerly held by the U.S. Department of Agriculture through the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) of 1947. FIFRA was amended in 1972, and has been amended numerous times since, greatly enhancing the registration, labeling and safety requirements for pesticides. Extensive studies on human exposure, carcinogenicity, effects on fish and wildlife, environmental persistence, evaluation of metabolites and other breakdown products and many other tests are now required to ensure that these chemicals will not generally cause any unreasonable risk to man or the environment.

Testing for registration of aquatic herbicides is much more stringent than that for terrestrial herbicides. Regulations often require additional testing and registration for additional uses. For example, in addition to the registration for aquatic weed control, diquat has a Food and Drug Administration registration for treatment of fish parasites in fish hatcheries.

During this period, the lack of a funded, sustained and organized management plan often resulted in weed problems becoming massive in size and scope before action was taken. Aerial spraying of thousand-acre water hyacinth infestations was not uncommon in the southeastern United States, including Florida. This large scale spraying periodically lead to reduced oxygen in the water and subsequent fish kills.

Although aquatic herbicides enabled more economical and efficient weed management, large scale use and public concern about potential environmental and human health problems prompted state and federal regulatory officials to more closely monitor aquatic plant management programs, demand additional testing for safety, and to support additional research. 

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Aquatic Herbicides, 1975-Present

By the end of the 20th century, a worldwide
increase in aquatic weed control efforts resulted in the development of planned programs for aquatic weed management. These programs include biological, mechanical, physical and chemical methodologies, combined into Integrated Pest Management programs. Still, the number of herbicides registered for use, particularly in the United States, has decreased dramatically as expensive studies required for EPA re-registration have increased. As of 2011, only fourteen general compounds are approved for use in Florida waters by the US EPA.

Read more about the specific herbicides approved for Florida waters here.

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Last updated: 28 November 2011