The Adventive Status of Salvinia minima and S. molesta in the Southern United States and the Related Distribution of the Weevil Cyrtobagous salviniae

Colette C. Jacono*, Tracy R. Davern**, and Ted D. Center**

* USGS, Florida Integrated Science Center
7920 NW 71^st St., Gainesville, Florida 32653

**USDA-ARS, Invasive Plant Research Laboratory
3205 College Ave., Fort Lauderdale, Florida 33314

ABSTRACT

     The recent introduction of Salvinia molesta constitutes a serious threat to aquatic systems in the warm temperate regions of the United States.  Salvinia minima, the only other member of the family Salviniaceae present in North America, is considered native by current floras.  Evidence is presented which suggests that S. minima was also introduced to North America, probably during the late 1920s and early 1930s. Likely sites of introduction and subsequent range expansions are identified.  The accidentally introduced salvinia weevil, Cyrtobagous salviniae, was found to occur widely on S. minima in Florida but was not found in other states. The disparate distribution of this herbivore may account for the reduced aggressiveness of S. minima in Florida as compared to its troublesome growth in Texas and Louisiana, where the weevil is not yet known.

INTRODUCTION

     The Brazilian floating fern, Salvinia molesta, is now widely distributed in tropical and subtropical regions, where it is notorious for dominating water bodies with slow moving fresh water (Forno and Harley 1979). Salvinia molesta was recently reported as naturalized in Texas and Louisiana (Jacono 1999) and has since been found in Alabama, Mississippi, Florida, Georgia and from the lower Colorado River, bordering Arizona and California.

     Salvinia minima ranges through Central and South America (Stolze 1983). In North America, it is listed for Florida, Georgia, Alabama and Louisiana, where it pre-dates S. molesta and is considered native (Nauman 1993).

     The salvinia weevil, Cyrtobagous salviniae has been used as a biological control for Salvinia molesta in many countries (Thomas and Room 1986).  This highly specialized insect tunnels through rhizomes and feeds on buds, greatly reducing infestations of S. molesta and maintaining low population levels.  It ranges naturally in South America (Calder and Sands 1985) and was accidentally introduced to Florida before 1960 (Kissinger 1966). To date, the North American distribution of C. salviniae and its impact on S. minima has not been determined.

OBJECTIVES

1. Develop a chronology for early records on Salvinia minima in order to evaluate native status.
2. Determine the geographic distribution of Salvinia minima.
3. Determine the distribution of Cyrtobagous salviniae associated with Salvinia minima.

METHODS

     Plant distribution was determined from herbarium voucher specimens, published data, state agency vegetation surveys, sightings by regional biologists and collection by the authors. Data were geographically referenced by aligning location within the USGS 8-digit Hydrologic Unit Code to illustrate occurrence according to natural drainage basins.

     Weevil distribution was determined by survey during this study and supplemented with label data from previously vouchered specimens.

RESULTS

1) Chronology of records on Salvinia minima

     The earliest specimen of Salvinia minima in North America dates to 1930 from the St. Johns River, Florida.  Plants were subsequently documented from widely separated regions of Florida and Georgia, likely the result of independent introductions.  All sites were near well-populated towns where the plant may have entered natural areas through flooding of water-gardens and ornamental ponds. By 1967 S. minima was known from 14 drainages in two states, 20% of the current coverage (Figure 1).

2) Distribution of Salvinia minima

The dramatic geographic expansion of Salvinia minima over the last 70 years is demonstrated by its present occurrence in 69 drainage systems of the southern United States (Figure 2).

     Landry (1981) documented the introduction of Salvinia minima west of the Mississippi River, into southern Louisiana.  There, the species quickly spread through waterways and swamps of coastal regions. New drainages in Louisiana continue to be affected as the species spreads north and eastward (Figure 3).  Salvinia minima was introduced to Texas in 1992. It is suspected as having been carried there from Louisiana on a "marsh buggy" during geologic exploration. The earliest record from Alabama was made in 1982.  Populations are common in creeks and bays of the Mobile Delta, however Alabama also hosts the most northern location of S. minima, at Auburn. Salvinia minima is new to Mississippi where it was collected in 1999 from a tributary of the Upper Leaf River. The suspected source of infestation is a recently landscaped lake. Along the Atlantic coast, S. minima extends north to the Broad River drainage of South Carolina.

Figure 3.  Distribution of Salvinia minima in drainage basins of the southern United States.

     Expansion in geographic distribution alone does not necessarily reflect the aggressiveness of a non-native plant.  In Texas and Louisiana, Salvinia minima typically occurs in abundant stands and is viewed as a very troublesome weed (Figure 4).  At Lacassine Bayou, Louisiana, densely packed plants extend 19.3 km down the bayou and 110 m across its width. However in Florida, S. minima is less vigorous and not identified as problematic.

Figure 4. Densely packed Salvinia minima at Lacassine Bayou, Jefferson Davis and Cameron Parishes, Louisiana. Area shown is approximately 37 X  24 cm.

3) Distribution of Cyrtobagous salviniae and possible effects on Salvinia minima

     The present study found the accidentally introduced salvinia weevil, Cyrtobagous salviniae, occurring widely in Salvinia minima populations in Florida. It was not found on either S. minima or S. molesta in other southern states. Cyrtobagous salviniae was found by the authors at 34 of the 46 Florida sample sites, corresponding to 23 drainages, or 77 percent of the drainage basins surveyed. Voucher collections contributed three additional drainages, resulting in a total distribution of 26 drainage basins where C. salviniae was recorded in association with S. minima (Figure 5).  This is equivalent to 68 percent of all Florida drainages known to contain S. minima.  The frequency of C. salviniae was greater in peninsular Florida and less in the northern and western regions of the state.

Figure 5.  Distribution of Cyrtobagous salviniae on Salvinia minima in Florida drainage basins.

   In other southern states, Cyrtobagous salviniae was not found at any of the 28 sites surveyed, that is within 80 percent of the known drainage basins having Salvinia minima outside of Florida (Figure 6).

   While many factors such as light, temperature, nutrients and hydrologic conditions may contribute to the differences perceived in the growth of Salvinia minima among geographic regions of the South (Cary and Weerts 1984), the possible influence of herbivory by Cyrtobagous salviniae must not be overlooked.  The disparate distribution of this well-known biological control agent may account for the lesser aggressiveness of S. minima in Florida as compared to Louisiana and Texas.

Figure 6.  Distribution of Cyrtobagous salviniae on Salvinia minima in drainage basins of the southern United States.

CONCLUSION

     Although present in North America for 70 years, Salvinia minima continues to appear in new drainage basins within the southern states.  Outside of the range of the salvinia weevil, S. minima can be predicted to be a robust and abundant weed.

     The recent introduction of Salvinia molesta also continues to involve new states, however it is too early to assess its status in many states where relatively small populations have been targeted for eradication.  Salvinia molesta is, however, well naturalized in Texas and Louisiana. Fourteen drainage basins in Texas contain infested water bodies, many of which are impoundments on tributaries that flow near federally protected wetlands. If continued introduction and spread are not curtailed, S. molesta may at least become established in all drainages currently populated by S. minima.

     If in fact the less aggressive nature of Salvinia minima in Florida results from suppression by Cyrtobagous salviniae, then the introduction of C. salviniae outside of Florida would likely mediate adverse effects of both Salvinia species.

REFERENCES

Calder, A.A. and D.P.A. Sands. 1985.  A new Brazilian Cyrtobagous Hustache (Coleoptera: Curculionidae) introduced into Australia to control Salvinia. J. Austral. Entomol. Soc.  24:57-64.

Cary, P.R. and P.G.J. Weerts.  1984. Growth of Salvinia molesta as affected by water temperature and nutrition. III. Nitrogen-phosphorus interactions and effect of pH. Aquatic Bot. 19:171-182.

Forno, I.W. and K.L.S. Harley. 1979.  The occurrence of Salvinia molesta in Brazil. Aquatic Bot. 6:185-187.

Jacono, C.C. 1999.  Salvinia molesta (Salviniaceae), new to Texas and Louisiana. Sida 18(3):927-928.

Kissinger, D.G.  1966. Cyrtobagous Hustache, a genus of weevils new to the United States fauna (Coleoptera: Curculionidae: Bagoini).  Coleopt. Bull. 20(4):125-127.

Landry, G.P. 1981.  Salvinia minima new to Louisiana. Amer. Fern J.  68:95.

Nauman, C.E. 1993.  Salviniaceae Reichenbach. p. 336-337.  In: Flora of North America Editorial Committee (eds.).  Flora of North America north of Mexico. Volume 2.  Pteridophytes and Gymnosperms. Oxford University Press, New York.

Stoltze, R.G.  1983. Ferns and Fern Allies of Guatemala. Part III.  Marsileaceae, Salviniaceae, and the fern allies. Fieldiana Bot. 12:10-13.

Thomas, P.A. and P.M. Room.  1986. Taxonomy and control of Salvinia molesta.  Nature 320:581-584.

ACKNOWLEDGEMENTS

     Appreciation is extended to Dr. Charles O'Brien for specimen identification and helpful discussions; to Dr. Daniel Ward for sharing insight into early literature; to the curators of the herbaria referenced for this paper and to others who contributed their collections and observations (especially Peter Schweizer and Dennis Reicke); to Rhandy Helton, Larry Hartmann, Don Morgan, James Hyde, Charles Dugas and Karl Mapes for field assistance; and to John Rodgers, Joe Hinkle, Joe Zolczynski, Eileen Pokorny, Willey Durden, and Jorge Leidi for help in sample collection and processing.

Full results of this study published in: Castanea, The Journal of the Southern Appalachian Botanical Society  66(3):214-226.