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.
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 strategy 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. Allowing the accumulation of large masses of invasive plants like water hyacinth has significant impacts on habitat and dissolved oxygen in the water. Dense rafts of floating plants exclude light and seal off the recharge by atmospheric oxygen. Subsequent large-scale herbicide spraying reduced already low oxygen levels in the water, periodically leading to 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.
Aquatic plant control using herbicides represents a very small market compared to production crop management. The U.S. Environmental Protection Agency (EPA) implemented herbicide registration initiatives in the early 1970s to ensure that chemical applications do not present unreasonable risks to humans or the environment. Some chemicals that had been applied to Florida waters did not meet new stringent requirements. In other cases, companies cited low economic return, significant registration costs, and a relatively short patent life after registration for not seeking continued aquatic use. Chemical compounds must pass a lengthy and rigorous process to be registered by EPA, especially for use in natural area aquatic sites. The more than 140 required health and environmental tests can cost $40-60 million to register for aquatic use and take 8-10 years for full registration. Few new compounds were registered prior to 2002 and companies allowed many existing off-patent registrations to expire. Consequently, in 2001, there were only six herbicide compounds registered for use in Florida natural area aquatic systems.
This scenario emphasizes the need for multiple herbicides with different modes of action, efficacy for target plants, and selectivity to conserve non-target vegetation. After spreading to more than 40% of the state’s public waters in the 1970s, the submersed invasive plant hydrilla began to fill the water columns and cover the surfaces of nearly 100,000 acres of Florida’s largest and most important lakes. Mechanical and biological controls cannot keep pace with this kind of invasive growth. Four of the six remaining herbicides registered in the 1900s provided differing levels of hydrilla control, but the technology was available only for fluridone for large-scale hydrilla management.
Fluridone is active on a single gene in hydrilla, inhibiting photosynthesis and slowly causing plant death. Hydrilla only reproduces asexually via fragments and buds, so sexual genetic recombination leading to resistance was not perceived to be an issue with repeated fluridone use. However, we now understand that there may be hundreds of different hydrilla clones in Florida waters. Some clones are as much as five to ten times more tolerant than the susceptible population that dominated most Florida waters in the mid-1980s when fluridone was first registered in Florida. Repeated fluridone applications quickly selected for the more tolerant strains. Fluridone became too expensive to use at the higher doses, and more importantly, was not as selective in not controlling non-target native plants that grew among hydrilla. By 2000, this increased tolerance was confirmed by a number of research institutions, leaving Florida aquatic plant managers effectively with no large-scale, economical, or selective hydrilla management options.
Researchers at the University of Florida convened a Summit in 2001 among aquatic plant managers, regulatory agency personnel, herbicide industry and research institute representatives to discuss strategies for invasive plant control in the presence of fluridone-tolerant hydrilla. Managers revised prevention, early detection/ rapid response (EDRR) and biological control initiatives. New application strategies for existing herbicides integrated with other control techniques were evaluated along with testing new herbicide compounds.
Hundreds of alternative herbicides with differing mechanisms of action were evaluated. From these, dozens of compounds were vetted in laboratory tests for hydrilla efficacy and non-target plant selectivity. These efforts ultimately resulted in nine new herbicide registrations since 2003 representing five different mechanisms of action. Although most were screened for hydrilla control, many have added benefits in that they are active on other invasive plants like water hyacinth and water lettuce via in-water or foliar applications.
All herbicides registered for aquatic use since 2003 are single site plant enzyme inhibitors, therefore they present a much lower risk to humans and the environment than broad spectrum contact type herbicides. They are used at much lower doses, measured in parts per billion concentrations vs. parts per million, lowering the amount of herbicide applied to the environment. The newly registered herbicides are also from classes of compounds in which resistance has been documented in terrestrial applications. Following are key concepts incorporated when developing invasive plan and herbicide resistance management programs.
Herbicide Resistance Management Strategies Employed in Aquatic Sites