Giant Salvinia Weevils in Outdoor Ponds

How to Rear Giant Salvinia Weevils in Outdoor Ponds Charlie Wahl, Lori Moshman and Rodrigo Diaz Department of Entomology LSU AgCenter How to Rear Giant Salvinia Weevils in Outdoor Ponds Charlie Wahl, Lori Moshman and Rodrigo Diaz Department of Entomology, LSU AgCenter Background on Giant Salvinia and salvinia leaves are typically about half the size of giant salvinia leaves (Figures 3 and 4). Trichomes on giant the Salvinia Weevil salvinia leaves fuse together to form an egg-beater shape Giant salvinia, Salvinia molesta Mitchell, is a floating (Figure 5), while trichomes on common salvinia leaves do aquatic fern that has become one of the worst invasive not fuse together (Figure 6). weeds in the southeastern United States (Figure 1). This The salvinia weevil, Cyrtobagous salviniae Calder and plant is native to southern Brazil, Argentina and Uruguay Sands, is a South American beetle that is a specialist and is considered an invasive species in several tropical feeder on salvinia. The salvinia weevil was first intro- and subtropical countries (McFarland et al. 2004). Giant duced into Australia in the 1980s, and it has successfully salvinia is of great economic and ecological importance managed giant salvinia infestations (Room et al. 1981). in Louisiana because of its ability to form extensive mats Weevils collected from this Australian population were of vegetation in lakes, ponds, reservoirs and bayous. The released in Texas and Louisiana in 2001 and have result- dominant or tertiary growth stage of giant salvinia is a ed in successful establishment of the weevil in southern densely packed mat of individual plants that can be easily regions of the states (Flores and Carlson 2006). Salvinia spread through fragmentation (Figure 2). The proliferation weevil adults are shiny black and about 2 millimeters of giant salvinia in water bodies displaces native species long (Figures 7 and 8), and feed mostly on young leaves and changes the water quality in addition to inhibiting and buds of Salvinia spp. Female weevils lay eggs directly boating and other aquatic activities. Removal of giant on the plant. Young larvae feed on leaves, buds and roots salvinia from water bodies is difficult and extremely costly and later tunnel into the rhizome that supplies nutrients and often inadequate to prevent re-infestation and spread. to the plant (Figure 9; Sands et al. 1983; Knutson and Common salvinia, Salvinia minima, is another closely re- Mukherjee 2012). The tunneling action of the larvae ulti- lated species found throughout the southeastern United mately kills the plant by limiting new growth and causing States (Madeira et al. 2006). Common salvinia differs from the mat to sink (Sands et al. 1983). giant salvinia in size and trichome structure. Common Figure 1. Floating mat of giant salvinia. 2 How to Rear Giant Salvinia Weevils in Outdoor Ponds Figure 5. Trichomes on giant salvinia leaves. Figure 2. Tertiary stage of giant salvinia. Figure 3. Common salvinia leaves. Figure 4. Common salvinia leaves. Figure 6. Trichomes on common salvinia leaves. Figure 7. Tertiary stage of giant salvinia with weevil on the leaf surface. Figure 8. Salvinia weevil on giant salvinia leaf. 3 How to Rear Giant Salvinia Weevils in Outdoor Ponds B A C D Figure 9. Closeups of weevil larvae. (A) 3rd, 2nd, and 1st instar weevil lavae. (B,C) weevil larva feeding on rhizome. (D) Larva extracted from rhizome. To maintain control over giant salvinia infestations, it is temperatures do not reach below 25°F (-4°C) (Sanders et important to have local sources of good quality weevils al. 2012). In practice, the weevils have experienced high- that can be reared year-round, harvested and released in est winter survival rates in areas south of latitude 32°N areas where they are needed. Although indoor rearing (Mukherjee et al. 2014). facilities can be very effective, they are costly to install Location and construction method should be carefully and maintain (Eisenberg and Johnson 2012). Where the considered when creating a new rearing pond. Rearing climate is suitable, rearing the weevils in outdoor ponds ponds ideally should be sheltered from harsh elements can be done successfully and at a fraction of the cost, such as wind and heavy rain that can result in drift and often with low maintenance required once initial condi- flooding, which can negatively affect both plant and tions are set (Sanders et al. 2012). The goal of this publi- weevil survival. Excavating ponds near tree lines can cation is to describe the steps involved in rearing salvinia provide such shelter and also can moderate temperature weevils in open-earthen ponds. and light intensity during the summer. Leveed ponds with overflow channels require more effort and resourc- es to construct but can yield greater protection from Location, Type and drought and flooding (Figure 10; Sanders et al. 2012). When designing a new pond, the dimensions should aim Dimension of Pond to maximize growing area and facilitate pond mainte- In the United States, establishment of the salvinia weevil nance (Figure 11). The ideal pond depth is between 18 is mostly determined by winter temperatures (Owens and 36 inches, deep enough to withstand water evapora- and Smart 2010). Weevils have successfully overwintered tion during the dry summer months and shallow enough in rearing ponds in parts of southern Louisiana and Texas, for wading and weevil harvest (Sanders et al. 2012). Ponds but they cannot survive the winter in northern parts of that are too deep or too large require greater fertilization the states without some kind of cold-weather protection to meet desired water nutrient levels, which is expen- (Eisenberg and Johnson 2012). A general guideline is to sive and adds extra labor. For most purposes the ideal build rearing ponds only in areas where minimum winter pond size is ¼ to ½ of an acre and less than 25 feet wide. 4 How to Rear Giant Salvinia Weevils in Outdoor Ponds Figure 10. Leveed salvinia weevil rearing pond with overflow and stand pipes. Figure 12. Colorimeter water quality testing instrument. Design of Leveed Rearing Ponds 40 ft. 40 ft. 165 ft. 40 ft. Pond Dimensions: • Waterbody: 165 ft long x 40 ft wide x 3 ft deep. Water overow control • Pond Separation: 40 ft using PVC stand pipe Figure 11. Dimensions of designed leveed rearing ponds. 5 How to Rear Giant Salvinia Weevils in Outdoor Ponds Bigger ponds may require sectioning off to maintain the vinia’s root-like leaves. Commercially available fertilizers desired volume of giant salvinia. The design must include include ammonium sulfate, Miracle-Gro Water Soluble enough space around the pond to allow vehicle access All Purpose Plant Food (various N-P-K formulations) and for maintenance and harvesting. Scotts Turf Builder Lawn Food. Be careful to avoid “weed and seed” formulations, which may contain herbicides. Water testing should be done on a regular basis during Water Quality and the growing season (e.g., monthly or bimonthly) to mon- Pond Fertilization itor the pond’s pH, dissolved oxygen, conductivity and nitrogen. Water samples may be collected and sent to Water level and pond depth are very important to pond a testing lab, or water quality test kits (Figure 12) can be maintenance, and water availability must be carefully purchased for personal use. Ideal water quality parame- considered when deciding on pond location. A water ters for giant salvinia are pH 5.5-7.5 (Owens et al. 2005), source should be accessible to supplement rearing ponds nitrogen content 7-16 milligrams/liter, iron concentration in the event of drought. Water can be pumped from wells 3 milligrams/liter and water conductivity <1,500 microsie- or gravity fed by larger water bodies. Ponds located in mens/centimeter (Knutson 2012). heavily agricultural areas may be subject to contamina- tion by herbicides or insecticides from surface runoff. Well water or ground water should be tested to ensure that it contains acceptable levels of salts and possible contaminants. Giant salvinia grows best in acidic to neu- tral water (Owens et al. 2005). Rainwater-fed lakes and ponds tend to stabilize at a pH around 6 to 7. The frequency of pond fertilization depends on water and plant quality and time of the year. In winter, plant and weevil activity decline and fertilization during this time is generally discouraged. During the spring and summer months of active growth, adequate fertilization is important for the formation of new buds that support feeding by weevil adults and larvae. An advantage of rearing giant salvinia in ponds is that naturally occurring sediments and other biological inputs provide necessary Figure 13. Dissolving ammonium sulfate in water to make a concentrated minerals and micronutrients that can be absorbed by sal- fertilizer liquid. Figure 14. Dispersing concentrated fertilizer liquid throughout the pond. 6 How to Rear Giant Salvinia Weevils in Outdoor Ponds During the growing season (April to October), LSU Ag- Pest Control In and Center ponds are fertilized every six weeks with ammonium sulfate to reach a desired nitrogen concentration of Around the Pond ~3 mg/L (Knutson 2012). Using a 5-gallon bucket and a shovel, the ammonium sulfate is dissolved in pond water Samea Moth to make concentrated batches of fertilizer (Figure 13). Larvae of the samea moth, Samea multiplicalis (Guenée), Small volumes from the bucket are then poured at many feed on giant salvinia and could become a serious pest locations within the pond (Figure 14). Before pouring if not properly controlled (Figures 15 and 16; Julien and the concentrated fertilizer liquid, be sure to clear away Bourne 2009, Wang et al. 2016). Samea may be detected the salvinia and create an open patch of water. This will by observing adults on leaves and by extracting larvae ensure the fertilizer makes it into the water column and from plant material using Berlese funnels.

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