Fishermen, boaters, swimmers and others often question the need for, and safety of, weed control programs. Following are a few of the most frequently asked questions and answers regarding Florida's aquatic plant management program.
All herbicides should be handled with great care, especially in their concentrated form. Once diluted according to label instructions, aquatic herbicides are considered safe for the specified use by the U.S. Environmental Protection Agency and the Florida Department of Agriculture and Consumer Services, who ensure that they meet the most stringent safety standards under federal and state regulations. They are permitted for use in Florida waters by the Florida Fish and Wildlife Conservation Commission (FWC). “Restricted use” herbicides are not permitted for use in water.
No – only the amount of herbicide needed to adequately control aquatic weeds is allowed for use. Herbicides are extremely expensive so the least amount necessary for the desired level of control is always used. Less herbicides are needed and less money is spent when aquatic weeds are kept at low levels under maintenance control programs.
Research throughout the state has consistently shown that harvesters alone are ineffective for large-scale control of these fast-growing exotics. During periods when harvesters were used instead of herbicides, the weeds grew out of control. It is also far more cost-effective to use herbicides instead of mechanical harvesters. One crew applying herbicide can cover approximately 10-15 acres per day, whereas a crew operating a harvester can generally clear only 1-2 acres per day. In addition, harvesters are not selective; they remove non-target plants and animals that are in their paths. Repeated use of mechanical harvesters can significantly reduce the number of fish and other animals living among invasive plants. Finally, the use of harvesters can accelerate the spread of invasive plants. Harvesters fragment invasive plants and these fragments can float away and quickly take root in other parts of the waterbody; And since invasive exotic plants usually grow back much faster than native plants, only the invasives may remain after just a few cuttings.
No. Large muck deposits are created in shallow fish spawning areas by dense, crowded plant coverage. All plants contribute to the muck layer by the natural shedding of leaves and other plant material. When waters are stagnant or water levels are stabilized for flood control purposes, plants can become densely packed in the shallow areas. Occasional floods are prevented that would flush plant material into surrounding wetlands and uplands. Stabilization also prevents water bodies from periodically drying out, which would expose the muck to air and sunlight and allow it to dry, decompose, and consolidate or burn. Muck removal projects designed by the FWC help offset the problem of large muck deposits.
No. Although it sometimes appears to be the case, native plant species are not targeted for control unless they have become a problem. Under the maintenance control program, even small patches of floating weeds are sprayed to keep them from growing into large infestations. Sometimes native plants are mixed in with these weeds and are unintentionally sprayed. Selective herbicide formulations are used which kill the exotics, but only temporarily damage the natives. Spray crews are directed to minimize, as much as possible, exposure of native plants to herbicides. Sometimes the natural browning of aquatic plants due to seasonal change or insect and disease damage is confused with the effects of herbicides.
Herbicides do not kill fish when used according to the label. The FWC and the University of Florida have done studies on Florida waterbodies which indicate that spraying does not affect the catchability of fish1, or adversely affect "bedding" fish. Isolated fish kills are often attributed to the spraying of herbicides but they occur naturally due to low levels of dissolved oxygen in the water. Low dissolved oxygen can occur due to the natural stratification of water and weather events. It can also occur when large masses of plants are killed but not because of the herbicides. When dead plants begin to decay, the organisms that break down the plant material increase in numbers and use more dissolved oxygen in the water, decreasing the dissolved oxygen available to fish. To avoid fish kills, the FWC requires that dissolved oxygen in the water be measured before treating. Keeping invasive plants at low levels, controlling plants in smaller increments, and planning most large-scale plant management during cooler months when dissolved oxygen levels in the water are higher, helps ensure against adverse effects to fish.
No. 2,4-D is sometimes confused with "Agent Orange," the military's name for a defoliant mixture that was created and used during the Vietnam War. During the manufacture of Agent Orange, it became contaminated with a cancer-causing dioxin, tetrachlorodibenzo-p-dioxin, known as TCDD. While 2,4-D was one of the components of Agent Orange, it is not Agent Orange, nor does it contain TCDD, nor has it been shown to cause cancer. After numerous lifetime feeding studies in rats and mice, the EPA recently classified 2,4-D as Class D – Not Classifiable as to Human Carcinogenicity – Agents without adequate data either to support or refute human carcinogenicity. Breakdown of 2,4-D in water occurs microbially and through photolysis and its half-life in water ranges from 7-48 days.
When stocked in high enough numbers, grass carp can be extremely effective. However, grass carp have an uncanny ability to outsmart virtually every type of fishing technique. Nets, hooks-and-lines, electro-shocking, and poison baits are minimally successful, especially after the first attempt. Once released, grass carp are nearly impossible to remove. Once grass carp consume their preferred plants, they can eat every plant in a waterbody, including submersed, emersed, and floating plants. They’ve even been observed wriggling out of the water to eat grasses along the shoreline. If grass carp are stocked in high enough numbers, there is the potential for a lake to change from a clear-water, plant-dominated system to a murky, algae-dominated system. View the grass carp page for more information.
1 Does diquat effect movement and catchability of largemouth bass? by Sweatman,J., Moyer,E., Hulon,M., Hujik,B., And Buntz,J. 1993. Aquatics 15(3):4, 6, 7-10.