Q. Will the herbicides referenced here kill my fish?
- A. No, not if used at labeled rates. It is very important to read and understand the herbicide labels. The label is the LAW! Used at label rates the registered aquatic herbicides are not toxic to fish and would not have been register by U.S. EPA (Environmental Protection Agency) if they were.
- A good example is diquat (Reward). Diquat is labeled to be applied at a maximum rate of 2 gallons per surface acre (even if the water was as shallow as a foot deep that is only a concentration of approximately 6 ppm) while the 96-Hour LC50 toxicity to bluegill sunfish is 245 ppm. Therefore, there should be no toxicity problem. The most likely herbicide that has toxicities near labeled application rates are copper and copper compounds. Application of copper products should be carefully applied based on water alkalinity. (see: SRAC #410 – Calculating Treatment for Ponds and Tanks (PDF)).
Q. Why do fish often die after herbicide treatments?
- A. Fish often die 2 to 5 days after an herbicide treatment because of an oxygen depletion. The decomposing/rotting plants (actually the billions of bacteria and fungi consuming them) simply take up most of the available dissolved oxygen, and the fish suffocate. This problem is the reason that most treatments recommend only applying the herbicide to 1/4 or less of the pond at a time and allowing 10 to 14 days between applications. The limited application area and extended time period allows the vegetation to rot without reducing dissolved oxygen concentrations below critical levels to the fish and other aquatic organisms. For more information, see “Dissolved Oxygen“.
Q. Do I have to have a pesticide applicator’s license to purchase and apply aquatic herbicides?
- A. Yes and No. Most of the registered aquatic herbicides can be purchased without a pesticide applicator’s license. The exception is 2,4-D compounds which require a license to purchase and apply in Texas (in other states check with the Department of Agriculture). Habitat (active ingredient imazapyr) is also an exception because “by label” it can only be applied by a person with an “Aquatic Applicator’s” license. On one’s own property it can be purchased and applied by anyone with a general applicator’s license.
Q. Where can I purchase aquatically registered herbicides?
- A. First, try local farm supply or feed and seed stores. If that fails, contact your local County Extension Agent or try searching the World Wide Web under the trade name of the herbicide.
Q. Does it matter what time-of-day or time-of-year these aquatic herbicides are applied?
- A. Yes, it is best to apply these herbicides in the morning on sunny, low wind days (to reduce possible chemical drift). Sunny mornings augment uptake of the herbicide because the plant is actively metabolizing and therefore increases its effectiveness.
- It is best to apply herbicides early in the growing season (spring of the year) while the plants are actively growing (they uptake the herbicide better) and before the biomass of vegetation has built up. Many types of submerged vegetation can reach biomasses as high as 10 to 15 tons per acre. Once summer temperatures set in and vegetation biomass has peaked out it is extremely risky to treat the vegetation because of the likelihood of an oxygen depletion.
Q. What is a surfactant and when are surfactant(s) needed in aquatic herbicide application?
- A. Short for ‘SURFace ACTive AgeNT’ – a surfactant is a molecule/compound that reduces the surface tension of water, thereby permitting it to penetrate a material more easily or to spread over the surface.
- Check the label. If the label states that surfactants are needed then one (or more) should be added. Make sure to use only an aquatic registered surfactant – many are available.