Contrary to popular belief, there is no “one size fits all” model for aquatic invasive plant management. There are only a handful of aquatic invasive plant control methods suitable for the management of hydrilla in Kerr Lake, Lake Gaston, and other reservoirs in our area. Last week we talked about some other means of aquatic plant control that aren’t necessarily suitable for a large reservoir system. There are, however, a few methods of control that we use quite frequently on such large expansive, man-made bodies of water. Some of these you may be familiar with, and some, maybe not so much. This week we will discuss what is available for treatment of problem weeds, like hydrilla, in Kerr Lake and Lake Gaston.
Grass Carp
The grass carp (Ctenopharyngodon idella) is a very frequent tool utilized in aquatic plant management, particularly that of Hydrilla. These fish are native to China and were originally distributed world wide as a food source. In 1963, the large herbivorous (plant eating) fish was brought to the United States for assessment as a biocontrol to address the simultaneously expanding populations of invasive plants. In order to be used, grass carp must be hatchery produced and certified sterile to prevent natural reproduction and spread in our waters. Grass carp will eat just about any and all vegetation in a system, but prefer more palatable species like hydrilla, southern naiad, and duckweed. These fish can live up to 25 years old, thus they can provide effective, long term control of problem plants when stocked at appropriate rates. Grass carp eat over 150% of their body weight daily in the early part of their lives and continue to eat up to 30% of their weight as they grow to maturity. This can be a very substantial number given that these fish can grow to 80+ pounds! Grass carp are extremely effective in ponds and large, man-made impoundments (reservoirs) where little native vegetation existed prior. They are not a good option for natural bodies of water where large amounts of native vegetation are required to drive the food web. Over 8,000 grass carp are currently stocked into Lake Gaston annually to address the lake’s Hydrilla problems.
Chemical Control
The well known method of control in Lake Gaston is the chemical control/ herbicide use for controlling aquatic invasive plants. Unlike terrestrial systems, there are only a few herbicides approved for aquatic use. These herbicides are strictly regulated by the EPA and are held to the same standards as pesticides used on the food crops we eat. Aquatic herbicides can be separated into two distinct groups based on how they act on a plant for control: Contact and Systemic.
Much as their name implies, contact herbicides kill plants directly by making contact with plant tissues. On the other hand, systemic herbicides are actually absorbed by the target plant and disrupt biological processes in the plant. While contact herbicides may be quicker to kill target plants than systemic herbicides, they often only burn down hydrilla biomass rather than killing the entire plant. Thus, contact herbicides often require multiple treatments to obtain control. Some common contact herbicides include copper based products, Diquat and Endothall. Systemic herbicides require much longer contact time with a target plant but can often affect all portions of the plant including the roots and rhizomes. Fluridone is the most commonly used herbicide for hydrilla control in our area. This herbicide is the only EPA registered herbicide that is only used in aquatic systems. Fluridone is a bleaching herbicide that targets enzymes which protect the chlorophyll in hydrilla. Hydrilla must be exposed to fluridone for a fairly long period of time so applicators often administer sequential treatments to keep the herbicide at sufficient levels for control.
Revegetation
The last, and probably least known of the control methods used for aquatic invasive species in our reservoirs is the planting of native vegetation to compete with invasive plants. Some of you might be wondering: Why plant MORE plants to get rid of others? Revegetation of native species ensures the colonization of non-nuisance plants that don’t grow rapidly and expand like invasive plants. By filling a voided niche with native species, invasive plants like hydrilla have less of an area in which to colonize and expand. Revegetation is often used in areas of low human traffic where other methods (like herbicides) might not be as desired. Not only does revegetation help to fight the battle against invasive species, but they also provide valuable habitat and food for other wildlife.
All of the control methods mentioned above are frequently utilized in reservoirs throughout the southeast, including Lake Gaston and Kerr Lake. For more information on these methods, please visit the AERF’s best management practices handbook for the control of Aquatic Plants posted in the “web links for additional information” section. Stop by next time when we will begin talking about hydrilla and its controversial role in fishing, particularly in the pursuit of Largemouth Bass.
Photo of Grass Carp provided by NCWRC
Web Links For Additional Information:
AERF Best Management Practices Handbook
NCSU Hydrilla Page
If you have questions please contact your Aquatic Extension Associate, Brett M. Hartis, at (919)-515-5648 or email at bmhartis@ncsu.edu.
