Texas Parks & Wildlife Magazine   


Hungry Hordes to the Rescue

Lilliputian insects may restrain giant salvinia better than humans can.

By Larry D. Hodge

Opinions about aquatic vegetation differ among anglers and other recreational users. Anglers generally like vegetation, because it provides habitat for fish. Young fish hide among plants and feed on them and insects living there, and larger fish use plants as hiding places from which they can prey on smaller fish. Casting a lure into a stand of hydrilla or other plants often results in a strike.

However, too much aquatic vegetation can block boat lanes and interfere with water skiers. Swimmers have drowned after becoming entangled. Out-of-control introduced species such as water hyacinth, hydrilla and giant salvinia can harm fishing, too.

Howard Elder is the Texas Parks and Wildlife Department’s aquatic vegetation biologist, and it’s his job to keep invasive aquatic species from taking over Texas reservoirs. “Ultimately we are concerned about losing the shallow water habitat that many fish, including bass and sunfish, use to spawn,” he says. “These exotic plants have the potential to cover that up.” A total takeover of the surface by the invading plants would block sunlight from reaching native plants and result in a body of water with little or no fish or other life. Boaters and swimmers would be unable to use the lake as well.

Control methods include physical removal of plants by machine or by hand, manipulation of water levels to kill plants by dehydration or freezing, using booms to block the spread of plants, chemical spraying and biological controls.

All the methods have drawbacks, but biological control in the form of insects that eat undesirable plants may prove to be the best in the long run. Water hyacinth weevils, alligator weed flea beetles and water lettuce weevils have proved effective in other countries.

“I would rather be putting beetles and weevils out there than spraying herbicides,” Elder says. “Results will not come as quickly, but in the long run, I think it will be more effective. We don’t think the weevils will eliminate giant salvinia, but we hope they will reduce the amount to a point that is more manageable.”

Until the bugs have time to do their job, boaters will play a significant role in the battle — for good or for bad. One of the main ways invasive aquatics spread from one body of water to another is by being carried on boats and boat trailers. Just one tiny fragment of a plant can multiply into thousands in a very short time. Giant salvinia is widespread on Toledo Bend Reservoir, and it would be very easy for it to spread to nearby Sam Rayburn. TPWD has placed signs at all boat ramps on Toledo Bend informing boaters of the danger and requesting their help in preventing the spread by thoroughly cleaning their boats, trailers and vehicles of all vegetation before leaving the ramp area.

The results of a recent study support Elder’s belief in the tiny bugs. Using a weevil that eats only giant salvinia, TPWD and U.S. Department of Agriculture scientists have been able to control infestations in test plots. “One 100-acre impoundment was 80 percent covered, and in two years the weevils reduced that to less than 1 percent,” Elder says. “

As the beetles chomp their way through their food supply and deplete it, they move to new areas. “We’re very optimistic,” Elder says. “This year we found evidence of weevils in an area more than a quarter of a mile from the nearest release site. I feel that’s a strong indication the weevils are moving around with the plant and are establishing in areas we are not able to reach with herbicide applications. That’s what we need.”

The old adage “An ounce of prevention is worth a pound of cure” was never more true than when applied to controlling invasive aquatic vegetation. Cleaning bits of vegetation from your equipment when you leave a lake takes time, but doing so will help ensure that lake will be usable the next time you want to go fishing, boating or swimming.

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