The use of chemicals for controlling aquatic weeds is highly effective, but just how safe are they for the environment and you? Each year chemical control of aquatic weeds, particularly hydrilla, occur on our local water bodies. This week we will talk, in depth, about the responsible use of aquatic herbicides.
Many of you have heard of aquatic herbicides being used in our local lakes to control aquatic weeds. Some of you may have even paid out of pocket for a licensed applicator to treat around your dock. But what exactly does this mean for the environment? Is the chemical being used potentially toxic? Does it remain in the water indefinitely? What about causing cancer? These are all concerns that each and every one of us should have prior to introducing anything into our environment. However, more often than not, the hesitation in using aquatic herbicides is based purely on the associated “fear of the unknown” and not on science, information, and in many cases… common sense. I will be referencing a number of concepts discussed in previous articles so feel free to go back and “refresh” your knowledge of aquatic plants should certain concepts seem hard to grasp.
Aquatic plants, whether phytoplankton or macrophytes, provide the very foundation of the aquatic food web (see last week’s article – “The Aquatic Food Chain”) but often, invasive species like hydrilla cause much more harm than good, thus calling for management. This management, as discussed in “Aquatic Invasive Plant Species Control”, can come in many forms including chemical control, or the use of aquatic herbicides. Aquatic herbicides are pesticides intended to kill target plants, and they do so by targeting various plant processes and properties that we will get into shortly. Unfortunately, many of us lump all different kinds of pesticides (insecticides, rodenticides, etc.) together. On the contrary, each type of pesticide acts to kill a specific organism, and is most often VERY different in how they do this.
OK, so how do herbicides target certain aquatic plants? To understand this, we first have to think about how we, and other animals, are different than plants. First off, plants make their own food through photosynthesis whereas we have to eat other plants or animals to survive. Second, plants make very specific amino acids that we as humans do not make ourselves. Amino acids are the building blocks of proteins that help us function and grow. Lastly, plants make certain hormones that we do not. It is through these various differences and processes that aquatic herbicides work on controlling target plants while not harming us in the process. Here are a few examples from some herbicides that may sound familiar to you:
Fluridone (often known by the trade name SONAR), acts by targeting carotenoid synthesis in plants. Carotenoids function to protect chlorophyll from the sun. When carotenoid pigments aren’t present, chlorophyll pigments are bleached, halting the very process that keeps plants alive: photosynthesis. There is sometimes damage to other plants nearby, however, the concentrations needed to control hydrilla are much lower than what most native plants can withstand, so collateral damage is often avoided. We as humans, do not produce carotenoids, therefore the process involved does not affect us or other animal life.
Glyphosate (often called Rodeo or roundup) is often used on emergent plants. This particular herbicide acts on the production of certain amino acids in plants, specifically inhibiting the production of an enzyme called ESP synthase. ESP synthase drives a reaction that creates certain amino acids in plants that are vital for plant survival. When Glyphosate inhibits this enzyme, plants cannot make specific amino acids, and consequently the proteins that are vital for survival. So what about animals, and humans? Well, animals and humans do NOT produce the amino acids associated with ESP synthase and can only obtain these from eating plants. Therefore, herbicides that inhibit certain enzyme production do NOT harm humans and other animals.
The above herbicides are considered “systemic” because they translocate in plants, often inhibiting certain processes that are vital to plant survival. But what about contact herbicides, like copper sulfate, that many of you associate with the “burning away” of plant material? Contact herbicides do not translocate in plant material, but rather destroy plant material like cell membranes. If they can do this, then what does that mean for animals and humans? Well contact and systemic herbicides could be harmful to humans and other animals, but the reason they aren’t is all about the concentration, dissipation and fate of the chemical. Toxicity of different herbicides is all about the amount being used. The toxicity of herbicides is based upon very concentrated amount of the chemical but when aquatic herbicides are applied, they become GREATLY diluted in water. Aquatic herbicides also only persist in the environment for a number of hours to days because they are quickly absorbed by soil particles, subject to microbial degradation or photodegradation. As for fate, most aquatic herbicides are extremely soluble and do NOT accumulate in fatty tissues in our bodies.
Now for an example: Copper sulfate is one of the most widely used aquatic contact herbicides on the market. Copper is an essential micromineral for healthy human diets. Many of you take a multivitamin every day, right? Have a look at how many milligrams of copper are present in your daily vitamin alone? Mine currently says 2 mg. Now, let’s think about the amount of copper in standard herbicide use. In order to get the same amount of copper into your body that is present in your multivitamin alone, you would have to consume several gallons of treated water every day. Don’t go looking in the lake for your daily vitamin uptake for long however, because copper products often only stay in the environment for 7 days before binding to the soil. So much for saving on your monthly vitamin bill!
To wrap things up, aquatic herbicides aren’t nearly as scary as they may have initially seemed. All pesticides (including aquatic herbicides) undergo nearly 150 tests for toxicity, carcinogenicity, mutagenicity, persistence in the environment and many others. Unlike their terrestrial counterparts numbering in the hundreds, there are only around a dozen herbicides approved for use in aquatic environments. Appropriate use of these herbicides at recommended concentrations should be of little concern unless you are a stem of hydrilla. In most cases, fish kills or other environmental impacts from aquatic herbicides are an indirect result of extreme over treatment beyond recommended use, not the herbicides alone. For example, if you over treat a pond, removing ALL of its plant life at once, you have depleted the amount of dissolved oxygen in that system almost instantaneously. Therefore, aquatic life can and will be affected. This drives home the most important point of aquatic herbicide use. Making anything into a poison is all about the concentration. Water itself can be a “poison” if drank at excessive amounts through water intoxication (sounds familiar to the copper example, huh?).
I hope this article has given many of you a better sense of aquatic herbicides and their responsible use. For more information, please visit Dr. Carole Lembi’s “Why Aquatic Herbicides Affect Aquatic Plants and Not You” in the web links for additional information.
Web Links For Additional Information:
Why Aquatic Herbicides Affect Aquatic Plants and Not You
If you have questions regarding this article or any other, please contact your Aquatic Extension Associate, Brett M. Hartis, at (919)-515-5648 or email at bmhartis@ncsu.edu.
