Giant Salvinia

“Salvinia Molesta” is the scientific name of the invasive plant taking over the waterway.  Scientist, Lee Eisenberg has agreed to answer questions and give accurate information in his spare time.  Mr. Eisenberg is raising a weevil that helps control the Salvinia in Caddo Lake.

Lee Eisenberg 1  TMS Sept 2014 (153)

Salvinia  Salvinia 1

The Scientist answering most questions here, will be Lee Eisenberg.


  • Frequently Asked Question

    What kind of plant is giant salvinia, and where did it come from?

  • Frequently Asked Question

    How did it get here?

    • Giant salvinia was brought in as an ornamental plant for aquariums and water gardens. After escaping from cultivation, it has mostly been spread between water bodies by people with boats. A small piece of giant salvinia wedged on a trailer can-and often does-cause an infestation in a new water body.

  • Frequently Asked Question

    What makes giant salvinia so harmful to water bodies?

    • Giant salvinia out competes native plants and forms a floating mat (up to three feet thick) that can prevent oxygen from entering the water. It also blocks sunlight to aquatic plants and photosynthesizing microbes that make up a large part of the food chain. Nor can carbon dioxide get out of the water (at least not efficiently), so water quality is quickly degraded. Wildlife habitat is destroyed and recreational activities greatly reduced. Local economies suffer as a result.

  • Frequently Asked Question

    What makes giant salvinia so hard to control?

    • First of all, it only takes a very small piece of salvinia to start an infestation. A ramet, which consists of two leaves joined to a root, can survive and reproduce. Giant salvinia can grow very rapidly. It has been demonstrated experimentally that, under ideal (lab) conditions, a single ramet of giant salvinia could grow enough to cover 40 sq. miles of water in three months (Creagh, 1991). Ramets can easily break off of a mat, drift with the wind and waves, and start a new infestation.

      • How can we eradicate giant salvinia?

        • In order to do so, you would have to kill every last ramet- a nearly impossible task, considering its growth rate and the inaccessible backwater areas that it often infests. Management is a much more realistic goal.

          • Why can’t we just spray giant salvinia with herbicides and get rid of it?

          • Herbicides can kill a lot of salvinia in a short time in open water. However, regrowth takes place rapidly, and there are hundreds, if not thousands of acres of cypress groves and other inaccessible areas that the spray boats can’t reach. Even if you did manage to kill every last ramet in a water body, it only would take one ramet on an incoming boat trailer to re-infest a lake. It doesn’t take long for giant salvinia, with its’ exponential growth rate, to form a floating mat and cause big problems.

  • What other control methods are there?

    • Besides chemical control (herbicides) there are three control methods used for aquatic weeds are: mechanical, drawdowns and biological.
      – Mechanical control (or harvesting) simply isn’t fast or efficient enough, although it may be of value in a farm pond or tank. In a natural water body, with cypress groves and mud flats, etc., mechanical harvesting leaves too much of the salvinia population intact.
      -Drawdowns dry up much of a water body, and plants that are stranded on land dehydrate. However, it isn’t usually possible to kill all the salvinia by this method, so it re-infests the water body when flooded.
      Unfortunately, a substantial amount of salvinia can be washed out when the floodgates are open, which can infest backwaters downstream.
      -Biological control, using a tiny weevil, was developed by Australian scientists in the early 1980’s after chemical control efforts failed to stop the spread of giant salvinia. It has been employed successfully in about 15 countries since then, mostly in the tropics.

  • How can a tiny bug control such a rapidly growing weed?

    • It’s not the size of the weevil, but the numbers. A salvinia population can grow exponentially, but so can the weevil population. Females laid about 200 eggs in my lab study, so I’m estimating that, under natural condition, they can lay about half that many (100). So the number of weevils can jump up into the millions in a matter of months. The way the weevil population feeds is very destructive to the plant. Adults feed on buds and leaves, while the larvae bore in the rhizome. Also, there are two feeding stages: larvae and adult.

      • What kind of bug is a weevil?

        • A weevil is a type of beetle. Weevils are a very large group that feed on a wide variety of plants, but most of them feed on one or a small group of closely related plants. The boll weevil, for instance, only feeds on cotton.

  • Does the weevil eat anything else, like lettuce or raspberries? What if the weevils eat all the salvinia? Will they then start to attack other plants or crops?

    • The weevil only eats salvinia, and would starve if there was none available. This was demonstrated repeatedly during the testing required to import an “exotic” insect for use as a biological control agent. As part of this testing, weevils were given plants from 70 different families to feed on, but they were unable to survive or reproduce on anything except giant salvinia. The salvinia weevil has never been shown to do any harm to crops or other plants anywhere in the world. It has been used for biological control of giant salvinia for over 30 years. Biological control agents are not introduced casually. They undergo rigorous testing to be sure they are specific and effective.

  • But these kind of things always seem to go wrong and create more problems, don’t they?

    • Quite the opposite, actually. The vast majority of biocontrol agents that are released are effective and harmless to other organisms. And that is all the more impressive, given that biocontrol is often used as a last resort against a pest that can’t be controlled with chemicals.

      • But the love bug was brought in to fight the pine beetle, and look what a pest it turned out to be!

        • This is a very common urban legend (and we all know those don’t belong in the country!), but there is no truth to it. The love bug is native-it’s been here a lot longer than people- and it has nothing to do with the pine beetle. The adults that you seem swarming don’t even feed. They have a very short adult life. They reproduce and die shortly afterward, which is very common among insects.

  • My friend said the weevil could mutate, grow much larger, and then decide to attack crops or even people. Is that true?

    • Your friend watches the Sci Fi channel a lot, I’d like to bet. Most insects are small because the way their exoskeleton is constructed can be very strong on a small scale. It wouldn’t work at all on a larger scale. Insects don’t have any bones.

  • Has the salvinia weevil been used successfully in the U. S?

    • The weevil has been used very successfully in southern Louisiana by researchers from Louisiana State University, who developed a way to raise the bugs in ponds.

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