Pulling the Plug
Grass carp and chemicals used to control aquatic weeds
By Danielle Supercinski
Demonstrations to control non-native aquatic weeds have been instituted with collaborating irrigation districts in the Lower Rio Grande Valley to unclog and manage weed-infested irrigation canals and conserve water.
Species such as hydrilla (Hydrilla verticillata), water lettuce (Pistia stratiotes) and water hyacinth (Eichornia crassipes) are most commonly found in the Lower Rio Grande Valley, and they cause numerous problems in irrigation canals.
According to Dr. Michael Masser, professor and Texas Cooperative Extension fisheries specialist in the Department of Wildlife and Fisheries Sciences at College Station, hydrilla is the most problematic of these aquatic weeds because it severely plugs canals. Therefore, irrigation districts and producers have to pump more water so it will lay down the hydrilla to push water over it, resulting in increased pumping costs, Masser said. In addition, there is more percolation and seepage losses.
These aquatic weeds create other issues as well. They form small pools of water in the vegetation that then serve as breeding grounds for mosquitoes, which is problematic with the West Nile Virus situation, he said. In addition, water hyacinth is also known to cause an increase in water evaporation, known as evapotranspiration, causing a higher loss of irrigation water.
There is a myriad of treatment options. “Historically, there was just physical or mechanical removal of aquatic weeds plugging canals. Now we’ve come along with (triploid) grass carp and herbicides,” Masser said.
To control and manage these aquatic weeds, as well as conserve water, Masser has sought effective controls of these plants.
In April 2002, Masser began his first demonstration under the “Environment, Ecology and Water Quality Protection” task of the Rio Grande Basin Initiative. The first demonstrations utilized triploid grass carp to control hydrilla at Brownsville Irrigation District and Hidalgo County Irrigation District No. 1.
In 2005, Masser started additional hydrilla projects with Santa Cruz Irrigation District No. 15 and Hidalgo County Irrigation Districts No. 1 and No. 2. Currently, approximately five grass carp demonstrations are ongoing.
“The bottom line from these demonstrations is that grass carp readily consume hydrilla and offer long-term (5 to 7 years), economical control,” he said.
Triploid grass carp have been scientifically proven to be the most effective biological control for hydrilla. As “triploid,” genetically altered grass carp, their life expectancy is 10 to 12 years with an estimated 20 percent annual mortality rate, he said. They also are infertile and do not reproduce.
“We see losses every year past about year three,” Masser said. “We’ve only had to do a little supplemental restocking because of districts draining canals and canals rupturing; of course there is always a loss of fish when there is no water. Also, by year five we have to do some restocking – the fish are getting really large and not eating much causing the cumulative mortality issue.”
At other locations, hyacinth and lettuce have been controlled using chemical herbicides.
“Other researchers have tried biological control on hyacinth, but that is equivalent to treating fleas on a dog; it slows the hyacinth down but doesn’t control it. You can see damage (to hyacinth) everywhere, but it’s not controlling it,” Masser said. “There is no good biological control for water hyacinth or lettuce; therefore, we use herbicides to control them.”
Water lettuce demonstrations with herbicide control began in 2003 and continued into 2004 with Cameron Irrigation District No. 6. Reward™ herbicide was used, and all plants contacted by the herbicide were killed within three days.
“This represents a water savings by reducing plant transpiration of 30 percent,” Masser said. “In this case, this would represent a water savings of approximately 20,000 gallons per day.”
Prior to using grass carp and herbicides, irrigation districts removed these aquatic weeds by mechanical methods. Mechanical control proved to be very expensive – several thousand dollars an acre – and labor intensive, whereas biological control is much cheaper.
“Mechanical control is like mowing the grass; (weeds) keep coming back, so you don’t get good control,” Masser said. “Compared to grass carp on a per acre basis, we’re not spending $200 an acre.”
Irrigation districts have saved in excess of $500,000 per year from on-going demonstrations of the biological control of submerged aquatic weeds in canals and resacas, he said. Removal of submerged aquatics has reduced labor costs, pumping costs and water loss from evapotranspiration and percolation/seepage from the canals.
“For example, Hidalgo County Irrigation District No. 1 was spending more than $130,000 per year on mechanical control,” Masser said. “After one stocking of grass carp, the hydrilla was under control in about three months.”
Masser sends an annual newsletter to irrigation districts with recent demonstrations and their progress and results. As far as planned demonstrations in the future, “it is up to the districts and depends on whoever takes advantage of it,” he said.
“I plead with the irrigation districts to please utilize me; that is what I’m here for,” Masser said. “I make several trips a year to the Lower Valley to work with the people and irrigation districts to help with their aquatic weed problems. I enjoy working in the Valley and would be glad to take on some more demonstrations. As an Extension specialist, I’m available to everyone.”
Masser has also built the AQUAPLANTS Web site that is available for anyone who is unsure of what aquatic weed is in their canal. By looking on the Web site and working with Masser, irrigation district managers or landowners can make sure they know what plant they’re dealing with and identify how best to manage it. The pages can easily be printed off to use as a reference. Visit the AQUAPLANTS Web site at http://aquaplant.tamu.edu.
