The Practical Guide to Lake Management in Massachusetts
TREATMENT WITH FLURIDONE
How it Works
Fluridone is a systemic herbicide that comes in two general formulations, an aqueous suspension and a slow release pellet, although several forms of pellets are now on the market. This chemical inhibits carotene synthesis, which in turn exposes the chlorophyll to photodegradation. Most plants can be damaged by sunlight in the absence of protective carotenes, resulting in chlorosis of tissue and death of the entire plant with prolonged exposure to a sufficient concentration of fluridone. When carotene is absent the plant is unable to produce the carbohydrates necessary to sustain life. Some plants, including Eurasian watermilfoil, are more sensitive to fluridone than others, allowing selective control at low doses.
For susceptible plants, lethal effects are expressed slowly in response to treatment with fluridone. Existing carotenes must degrade and chlorosis must set in before plants die off; this takes several weeks to several months, with 30-90 days given as the observed range of time for die off to occur after treatment. The slow rate of plant die-off minimizes the risk of oxygen depletion. Fluridone concentrations should be maintained in the lethal range for the target species for at least 6 weeks, preferably 9 weeks, and ideally 13 weeks. This presents some difficulty for treatment in areas of substantial water exchange.
If the recommended contact time can be achieved, the use of the liquid formulation of fluridone in a single treatment has been very effective. Where dilution is potentially significant, the slow release pellet form of fluridone has been applied, but in highly organic, loose sediments a phenomenon termed 'plugging' has been observed, resulting in a failure of the active ingredient to be released from the pellet in a predictable manner. New pellet formulations are intended to avoid this problem. Multiple sequential treatments with the liquid formulation can be used in areas with extremely soft sediments and significant flushing. It may also be possible to sequester a target area with limno-curtains to reduce dilution effects in the target area.
The selectivity of fluridone for the target species depends on the timing and the rate of application. Early treatment (April/early May) with fluridone effectively controls overwintering perennials before some of the beneficial species of pondweed and naiad begin to grow. Variability in response has also been observed as a function of dose, with lower doses causing less impact on non-target species. However, lesser impact on target plants has also been noted in some cases, so dose selection involves balancing risk of failure to control target plants with risk of impact to non-target species. Maximum label application rates are 8 lb per acre-foot and 0.4 quarts per acre-foot for the Sonar SRP and Sonar AS formulations, respectively.
The maximum concentrations of fluridone expected would be 0.15 ppm, but since the mid-1990s it has been extremely rare to have a target concentration greater than 0.02 ppm. With target levels as low as 0.006 ppm, control of the target species is not always achieved, and only the most sensitive non-target vegetation is impacted. At application rates more certain to kill milfoil, damage to many non-target plants has been observed, but recovery of native plants within 1-3 years is typical.
Fluridone is considered to have low toxicity to invertebrates, fish, other aquatic wildlife, and mammals, including humans. The USEPA has set a tolerance limit of 0.15 ppm for fluridone or its degradation products in potable water supplies, although some state restrictions are lower. Substantial bioaccumulation has been noted in certain plant species, but not in animals. The LC50 for sensitive fish species is 7.6 ppm, which is 50 times higher than the expected maximum concentration and about 500 times higher than typical doses used Fluridone is a systemic herbicide; it is translocated throughout the plant and kills all parts except seeds and certain winter buds. It is used at relatively low doses (now routinely <20 ppb) and can be used selectively through adjustment of dose, application timing, and duration of exposure. today. Fluridone was not found to impact non-target organisms at concentrations of 0.1 to 1.0 Rat LD50s are >10,000 mg/kg.
Complete kill of susceptible vegetation
Can be used selectively on certain major invasive species at low doses
Slow death of plants minimizes oxygen demand and nutrient release
Minimal risk of any direct impacts on fauna
Acts slowly in the aquatic environment; exposure time of up to 90 days needed
Highly diffusive; dilution will limit effectiveness in areas of high flushing activity
Information for Proper Application
Knowledge of water intake locations if dose is to be >20 ppb
Knowledge of system hydrology and detention time; need to provide adequate contact time
Mapping of aquatic vegetation with accurate identification of all species and general appraisal of relative abundance and overall cover/biomass
Inventory of aquatic biota with emphasis on sensitive species
Treatment plan to include dose, areas treated, expected alteration of plant community, and follow-up activities
Tracking of concentration over intended exposure period
Monitoring program for assessing effectiveness and impacts
Factors Favoring the Use of this Technique
Complete kill of targeted submergent vegetation is desired
High selectivity for susceptible species is desired
Long exposure time can be maintained
Essential to eliminate potential direct impacts on fauna
Treatment is within a drinking water supply
Map plant community and note density and distribution of target and non-target species; presence of protected species may limit treatment
Application must be performed by licensed applicators
Apply fluridone product in accordance with label instructions and restrictions; justify dose, location and timing of treatment
Control flushing in the lake or target areas to maximize exposure time
Track fluridone levels and add more herbicide as necessary to achieve the needed combination of dose and exposure
Monitor plant community features before and after treatment
WPA permit through local Conservation Commission/DEP
Review by NHESP (further action if protected species are present)
License to Apply Chemicals from DEP
Management Techniques: Fluridone Fluridone is very diffusive and require: extended contact tim (40-90 days, dependii on dose and species) flushing cannot be controlled, slow relea pellet forms may pros the desired combinat of dose and exposure time.
Fluridone has not bee found to be toxic to animals at any field concentrations.
An enzyme limited immuno-sorbent assa (ELISA) has been developed that allows tracing of the concentration of fluridone. Biochemica tests for potential and actual impact have ale been developed, allowing much more sophisticated use of tl herbicide.
Impacts Specific to the Wetlands Protection Act
Protection of public and private water supply Generally neutral, but may have detriment at high doses (prohibition within one quarter mile of drinking water intakes at dose >20 ppb) Protection of groundwater supply Generally neutral (no significant interaction)
Storm damage prevention Neutral (no significant interaction)
Prevention of pollution Generally neutral (no significant interaction)
Protection of land containing shellfish Generally neutral (no significant interaction)
Protection of fisheries Possible benefit (habitat enhancement) and possible detriment (food source alteration, loss of cover)
Protection of wildlife habitat Possible benefit (habitat enhancement) and possible detriment (food source alteration, loss of cover)
Fluridone treatments typically cost $500 to $1000 per acre for single treatments with the liquid form. Costs rise to $1000 to $2000 per acre for sequential treatments. For partial lake treatments in which a portion of the lake is sequestered, an additional cost of about $10 to $20 per linear foot of sequestering curtain is to be expected. The cost of application with the pelletized form is usually $800 to $1200 per acre.
Sequestered treatment of parts of Shoecraft Lake in Washington with fluridone. An average dose of 20 ppb for about 55 days resulted in virtual elimination of Eurasian watermilfoil with no discernible impact on the remainder of the lake. (Photos provided by Remetrix)