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FAQs on Biological Control Program
What is the Story of the Salvinia Weevils from Florida and the ARS Biological Control Effort?
The weevil in Florida is recognized as Cyrtobagous salviniae, the same South American species that was used to control giant salvinia in other parts of the world. This insect was first detected in Florida in 1960 where it is now widespread on common salvinia, Salvinia minima. The most conservative approach to the giant salvinia problem, therefore, was to simply transfer the insect from Florida to Texas and Louisiana. This approach reduced the risk of introducing parasites or diseases that might have come with the weevil from Australia, which was originally from Brazil. However, in May of 2000, after comparing gene sequences between “Florida” weevils and “Australia” weevils, we found slight differences. The biological significance of these differences is unknown but it was decided to discontinue releases of the “Florida” weevil because even the slightest uncertainty about their identity was not acceptable. Virtually all the published information on this insect is based on the “Australia” weevil so we decided to work only with this insect.
How Can We be Sure the Weevils will not Attack Other Plants?
The range of plants that insects can attack is predictable through what is called host-range testing. Generally, most insects are specialists, able to feed and reproduce on only a few closely related plant species. This close relationship between plants and insects is thought to be the result of a selective process that has taken place over millions of years and is not easily changed. There are certainly more generalist feeders like gypsy moth or corn earworm but this type of insect is never considered as a suitable biological control agent and is screened out during rigorous host range testing. There is only one documented case out of 117 introductions of insects in the U.S. where the insect fed on plants not related to the target weed. In that early case, flawed testing suggested the species was a specialist, but further testing revealed it as a generalist feeder.
What Will the Weevils Eat After They Have Eaten All the Salvinia?
The successful biological control of a weed will result in reduction, not eradication, of the target weed. Therefore, biological control agents are unlikely to completely eliminate the plant, except perhaps in isolated locations. In cases of local eradication, the weevils will disperse to find new salvinia or starve. Overall, as the population of the weed declines, so will the population of the insect. The result will be an equilibrium between weed and insect, hopefully at the level where no further weed controls are necessary. Although giant salvinia will remain as part of the flora of the United States, it will no longer dominate and damage the local ecosystems as it once did.
Did the “Florida” Weevil Control Any Salvinia When It Was First Released?
Most of the original sites where the weevil was released were destroyed by floods, droughts, saltwater intrusion, or mechanical or chemical control by landowners. One site showed promise initially, but was destroyed by the landowner. In laboratory and tank tests, the “Florida” weevil completed development on giant salvinia, did not prefer common salvinia over giant salvinia, and has suppressed giant salvinia growth. At one site in western Florida where giant salvinia had invaded a nursery pond, the “Florida” weevil was found heavily attacking the weed. Further testing should provide insight on how the performance of the “Florida” weevil compares to the “Australia” weevil on both giant and common salvinia.
What is the Status of Permits for “Florida” and “Australia” Salvinia Weevils?
Currently, there is a permit for release of the “Florida” salvinia weevil anywhere in Texas and Louisiana. However, a permit for the “Australia” weevil is valid only in east Texas and western Louisiana. Therefore, any additional releases of either weevil will require additional regulatory action by APHIS (Animal Plant Health Inspection Service).
How Long Should it Take Before the Weevils Control the Giant Salvinia?
This is difficult to predict because of many factors, and often the factors themselves are unknown or poorly understood. The bottom line is that although you can predict the safety of a biological control organism, you cannot predict its effectiveness. However, a major point in the weevil’s favor is its success in other parts of the world where, in tropical areas, it controlled over 90% of the giant salvinia within 2 years. Control in temperate areas took longer but was still considered successful. Generally, a biological control agent that has worked in other places is usually successful when deployed in new areas with a similar climate. It should be noted that the majority of biological control agents do not successfully control the target weed. However, when an agent is successful, the benefits usually overwhelm the costs of previous failures.
Biocontrol article Attack on Giant Salvinia from the USDA Agricultural Research magazine.
FAQs on Biological Control Program by Philip Tipping, Ph.D., Research Entomologist,
USDA-ARS, Invasive Plant Research Laboratory
Updated: 00 Aug. 2002
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