Biological Control of Common Salvinia
(Salvinia minima) in Louisiana

Cooperators:
Philip W. Tipping, Ph.D. and Ted D. Center, Ph.D.
USDA-ARS Invasive Plant Research Laboratory, Ft. Lauderdale, FL.

Bill Hulslander, David Muth
National Park Service
Jean Lafitte National Historical Park and Preserve, New Orleans, LA.
 

Click here for an Overview

Research Reports:

2004

• Nov - Thanks to last minute funding, the common salvinia work in Lafitte, Louisiana will continue another year
• June - Favorable weevil reproduction and consistent salvinia damage observed
• May - Weevils released at Jean Lafitte, LA field nursery site
• Mar - Biological Control of Common Savinia in Louisiana - Common salvinia mats found broken and scattered; forest sites are eliminated as spring efforts focus on natural area research site

2003

• Oct -  More weevils are released during the last trip of the year at the Louisiana research plots
• Sept - Weevil releases continue on Salvinia minima as differences in biomass remain significant between release and control sites
• Aug - Summertime weevil releases begin on Salvinia minima at Louisiana swamp site
• Jul   - Lone brown weevil heralds augmentation in June releases on Salvinia minima
• Mar - Cyrtobagous weevil on Salvinia minima in Louisiana

2002

• Dec - 2002 Final Report - Jean Lafitte National Historical Park and Preserve in Louisiana
• Aug - A second release of Cyrtobagous salviniae was made on August 13, 2002
• Jun -  New Release Project Initiated at Jean Lafitte National Historical Park and Preserve

June 2002

• New Release Project Initiated at Jean Lafitte National Historical Park and Preserve

The first releases of the Florida type of Cyrtobagous salviniae were made on June 21, 2002 by the USDA-ARS Invasive Plant Research Laboratory in cooperation with the National Park Service. A total of eight sites were established in the Barataria Preserve, four of which each received 150 adult weevils.  The remaining four sites will serve as control sites. The condition of the common salvinia and weevils in these sites will be monitored regularly and more weevils will be released throughout the summer and fall of this year.

August 2002

• A second release of Cyrtobagous salviniae was made on August 13, 2002

A second release of the Florida ‘type’ of Cyrtobagous salviniae was made on August 13, 2002 by the USDA-ARS Invasive Plant Research Laboratory in cooperation with the National Park Service.  A total of 675 adult weevils were released at four sites with another four sites serving as controls. Minor salvinia damage was noted from the first release in June and 4 adult weevils were recovered at one site after hand-searching 50 plants. No weevils were found at the other 3 release sites or at any of the controls. Most the sites are located in flooded hardwood forests under shaded conditions, which results in very small salvinia plants. This small size, coupled with the very small size of the Florida weevils (less than 1 mm in length) makes their detection difficult. More efficient but destructive sampling methods may disrupt establishment of the weevil and, therefore, will not be used at this stage.  The condition of the common salvinia and weevils in these sites will be monitored regularly and more weevils will be released throughout the summer and fall of this year.

December 2002

• 2002 Final Report - Jean Lafitte National Historical Park and Preserve in Louisiana

Project: Biological Control of Common Salvinia at Jean Lafitte National Historical Park and Preserve in Louisiana. We sampled our research sites during the second week of December and set up two new sites in the open marsh areas.  Despite high water levels experienced in October from a hurricane, the sites remained intact and an adult weevil was found at one site after hand-searching 50 plants within the study square.  There was clear weevil damage at two other sites. Overall, there was a significant difference between control and release sites in the percent difference between fresh biomass within the squares and adjacent to it.  We interpret these data as an indication that the weevils are reducing biomass in the squares but have not spread outside of it.  Those that did were probably carried away by October flooding.  In the flooded forest locations, common salvinia biomass actually increased since August as the leaves dropped off the trees and formerly shaded areas received much more light.  There are now ten study areas at Jean Lafitte: five control and five release sites.  Field research will resume in March 2003.  17 Dec.

March 2003

• Cyrtobagous weevil on Salvinia minima in Louisiana

The year’s first visit to common salvinia (Salvinia minima) research plots in Louisiana occurred March 20–21, 2003. No new weevils (Cyrtobagous salviniae) were released. The trip was made to assess the extent of over wintering populations. Despite releases in December 2002, no adult weevils were found at any release site. While the reasons for this are unknown, it is possible that the weevils have dispersed out of the study squares because of the relatively low densities of common salvinia. 

Common salvinia biomass in open marsh and flooded forest sites at both release and control plots had declined since December 2002.  Cold winter temperatures and/or higher salinity levels may have been the cause. Low winter temperatures resulting in higher levels of mortality may have caused the absence of weevils.  However, this seems less likely since weevils have successfully over wintered farther north, on giant salvinia, for the past two winters.

Our goal in 2003 is to establish the salvinia weevil in the research plots by use of monthly releases and to continue to evaluate all plots throughout the year.  We will also be establishing a field nursery site in an old cut-off canal that has a history of high densities of common salvinia. This will permit the weevils to establish in a sheltered area far removed from experimental evaluation areas and begin to suppress the weed. The next sampling date will be in June 2003.

July 2003

• Lone brown weevil heralds augmentation in June releases on Salvinia minima

Research plots at Jean Lafitte National Historical Park and Preserve, New Orleans, Louisiana were visited and data collected during June 26-27, 2003. A total of 500 salvinia weevils (Cyrtobagous salviniae) were released on common salvinia (Salvinia minima) during this trip.  The current release follows another 500 that were released three weeks previously.

All weevils were tested and found free of any pathogens prior to release.  About 300 adults from the two shipments were released at a canal containing high densities of common salvinia.  This will serve as a nursery site for establishment and the initiation of weed suppression in the field, outside of and far removed from the experimental area.

Eight of the ten research sites are located in deep shade at flooded forest locations, where low light levels result in very small common salvinia plants.  Two additional sites are located in the open floating marsh in full sun. Here the salvinia is much larger.  It is expected that the open marsh sites may facilitate the establishment of the weevils, which seem to prefer larger plants growing in full sun.

On this trip, one adult weevil was found.  It was collected at a shaded site.  Although solitary in number, the observed weevil was brown in color, indicating it was a new generation from a reproducing population.  There were no significant differences found in plant damage or biomass between release and control sites. Monthly releases and visits are planned through the remainder of 2003.

August 2003

• Summertime weevil releases begin on Salvinia minima at Louisiana swamp site
  
  The research plots in Louisiana were visited and data collected during August 28-29, 2003. A total of 600 weevils were released on this trip. About 100 adults from this shipment were released in a field nursery site in a canal with high densities of common salvinia.  Eight of the ten study sites are located in deep shade at flooded forest locations, resulting in very small common salvinia plants. Two additional sites are located in full sun out in the floating marsh area. Here the salvinia is much larger and this may facilitate the establishment of the weevils, which seem to prefer larger plants growing in full sun.  We found weevils at two of the five release sites including one at a heavily shaded site on very small plants. At one site with larger plants (partial sun) there was significant larval damage evident with reduced biomass in the study square as compared to the surrounding biomass. We also found four adults by hand-searching 100 plants.  Another full-sun site yielded 14 adults, including brown ones which indicate that a new generation was produced. Overall, there was a significant difference in plant damage and biomass between release and control sites. Monthly releases and visits are planned through the remainder of 2003.

September 2003

• Weevil releases continue on Salvinia minima as differences in biomass remain significant between release and control sites
  
  The research plots in Louisiana were visited and data collected during Sept. 25-26, 2003.  A total of 1200 weevils were released on this trip.  About 200 adults from this shipment were released at a field nursery site in a canal with high densities of common salvinia.  Eight of the ten study sites are located in deep shade at flooded forest locations, resulting in very small common salvinia plants. Two additional sites are located in full sun out in the floating marsh area. Here the salvinia is much larger and this may facilitate the establishment of the weevils, which seem to prefer larger plants growing in full sun.  As in August, we found weevils at two of the five release sites including a partial shaded and full sun site.  The partial shaded site yielded 3 adults and the open marsh site (full sun) yielded 10 adults.  Again the difference in biomass between release and control sites was significant although percent coverage was the same.  Monthly releases and visits are planned through the remainder of 2003.

October 2003

• More weevils are released during the last trip of the year at the Louisiana research plots
  
  The research plots in Louisiana were visited and data collected during Oct. 23-24, 2003. A total of 600 weevils were released on this trip. About 200 adults from this shipment were released in a field nursery site in a canal with high densities of common salvinia.  Eight of the ten study sites are located in deep shade at flooded forest locations, resulting in very small common salvinia plants. Two additional sites are located in full sun out in the floating marsh area. Here the salvinia is much larger and this may facilitate the establishment of the weevils, which seem to prefer larger plants growing in full sun.  Despite this, we found weevils at three of the shaded sites, including 12 at one site. Our open marsh site yielded 9 adults from hand searching.  Again the difference in biomass between release and control sites was significant although percent coverage was the same. This was our last visit this year and we plan to resume sampling next March.

March 2004

• Biological Control of Common Salvinia in Louisiana – Salvinia mats found broken and scattered; forest sites are eliminated as spring efforts focus on natural area research site
  
  The research plots at Jean Lafitte National Historical Park and Preserve, Louisiana were visited and data collected during March 24-25, 2004. No weevils were released this trip.  The original ten sites in open marsh and flooded forest have been reduced to six with the deep shade sites eliminated.  Currently, we now have two shaded sites, two partially shaded sites, and two full sun sites (floating marsh sites).  There appeared to be extensive winter kill in the more exposed marsh sites and no weevils were found there, despite the large number of adults recovered there last fall. Generally, the mats had been broken up and scattered by fluctuations in water level and wind (and maybe weevils).  However, very few buds were damaged, indicating that regrowth is coming as soon as temperatures increase. The average fresh weight biomass found in release and in no weevil check sites was 3.7 and 4.2 tons per acre, respectively.We will be releasing weevils on our next monthly trip and plan to continue evaluating this project through 2004.

May 2004

• Weevils released at Jean Lafitte, LA field nursery site
  
  The research plots in Louisiana were visited and data collected during April 22-23, 2004.  We released 800 weevils on this trip. The original ten sites have been reduced to six with the deep shade sites eliminated.  Currently, we now have two shaded sites, two partially shaded sites, and two full sun sites.  All sites showed an increase in coverage and biomass, except for the deep shade sites where water is very low and the salvinia is existing on muck. Conditions were very favorable however, for the partial and no-shade sites and the condition of the salvinia was perfect for weevil releases. Unfortunately, we found no weevils again this trip indicating that populations are too low to be detected using non-destructive methods.  Fresh weight biomass of common salvinia ranged from 2.4 tons per acre in shaded sites to 18.7 tons in full sun locations.
  
  We also released weevils at the field nursery site which is primarily full sun.  Our next shipment of weevils will be in May and our next visit (with weevils in tow) will be in June.

June 2004

• Favorable weevil reproduction and consistent salvinia damage observed

The research plots at Jean Lafitte National Historical Park and Preserve in Louisiana were visited and data collected during June 10-11, 2004.  We released 358 weevils on this trip. The original ten sites have been reduced to six with some of the deep shade sites eliminated.  Currently, we now have two shaded sites, two partially shaded sites, and two full sun sites. All sites showed an increase in salvinia coverage and biomass and conditions remain very favorable for weevil releases. We found weevils at 2 of the 3 release sites, including some brown adults, indicating that weevil populations are reproducing.  Also, the majority of buds examined in the release sites were damaged by weevils. Fresh weight biomass of common salvinia ranged from 7.8 tons per acre in shaded sites to 21.2 tons in full sun locations.

We also released weevils at the field nursery site which is primarily full sun.  Our next shipment of weevils will be in July and our next visit will be in September or October.

November 2004

• Thanks to last minute funding, the common salvinia work in Lafitte, Louisiana will continue another year.

The research plots in Louisiana were visited and data collected during October 21-22, 2004.  We did not release weevils on this trip. The original ten sites have been reduced to five with some of the deep shade sites eliminated. Currently, we now have two shaded sites, two partially shaded sites, one full sun site, and an insect nursery site which is also in full sun. The other full sun research site was vandalized and not replaced.  All sites showed a decrease in biomass with a greater decline present in the release sites. We found weevils at every release site including 18 adults at our full sun, open marsh site. These adults were collected from only a few plants.  We estimate conservatively that this site is home to hundreds of weevils. The common salvinia mat has been reduced to small, brown plants typical of sites in south Florida where populations of the weevil regulate the weed. While biomass amounts are lower, coverage is still high in the other sites indicating that weevil densities are still too low.  However, results like the marsh site indicate that, under the proper conditions, weevil populations can increase and negatively impact the plant.  Fresh weight biomass of common salvinia in release and control plots was 3.4 and 4.3 tons per acre, respectively.

We also visited our field nursery site which is primarily full sun.  Most of the mat is now supporting dense populations of sedges and other plants. One weevil was found after searching for a few minutes indicating that weevil populations are relatively high.  We expect that parts of the mat will sink and take the sudd islands of sedges and composites with it by our next visit in November.

Biological Control Program by Philip Tipping, Ph.D., Research Entomologist,
USDA-ARS, Invasive Plant Research Laboratory
Updated: 02 November 2004

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