Hygrophila Polysperma Plant
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Weed Risk Assessment for Hygrophila United States polysperma (Roxb.) T. Anderson Department of (Acanthaceae) – Miramar weed Agriculture Animal and Plant Health Inspection Service January 28, 2015 Version 1 Top: Submerged Hygrophila polysperma plant. Bottom: Emergent H. polysperma plants (source: Winterton, 2014). Agency Contact: Plant Epidemiology and Risk Analysis Laboratory Center for Plant Health Science and Technology Plant Protection and Quarantine Animal and Plant Health Inspection Service United States Department of Agriculture 1730 Varsity Drive, Suite 300 Raleigh, NC 27606 Weed Risk Assessment for Hygrophila polysperma Introduction Plant Protection and Quarantine (PPQ) regulates noxious weeds under the authority of the Plant Protection Act (7 U.S.C. § 7701-7786, 2000) and the Federal Seed Act (7 U.S.C. § 1581-1610, 1939). A noxious weed is defined as “any plant or plant product that can directly or indirectly injure or cause damage to crops (including nursery stock or plant products), livestock, poultry, or other interests of agriculture, irrigation, navigation, the natural resources of the United States, the public health, or the environment” (7 U.S.C. § 7701- 7786, 2000). We use weed risk assessment (WRA)—specifically, the PPQ WRA model (Koop et al., 2012)—to evaluate the risk potential of plants, including those newly detected in the United States, those proposed for import, and those emerging as weeds elsewhere in the world. Because the PPQ WRA model is geographically and climatically neutral, it can be used to evaluate the baseline invasive/weed potential of any plant species for the entire United States or for any area within it. As part of this analysis, we use a stochastic simulation to evaluate how much the uncertainty associated with the analysis affects the model outcomes. We also use GIS overlays to evaluate those areas of the United States that may be suitable for the establishment of the plant. For more information on the PPQ WRA process, please refer to the document, Background information on the PPQ Weed Risk Assessment, which is available upon request. Hygrophila polysperma (Roxb.) T. Anderson – Miramar weed Species Family: Acanthaceae Information Synonyms: Justicia polysperma Roxb. (Weber, 2003). Common names: miramar weed, East Indian swampweed, Indian swampweed, hygro, Hygrophila, miramar weed (Langeland and Burks, 1998; NGRP, 2014). Botanical description: Hygrophila polysperma is an aquatic annual or perennial herb that can grow either emerged or fully submerged. It grows 10-20 cm tall and has scrambling or erect stems. The leaves of H. polysperma are opposite, 2-8 cm long, and lanceolate to ovate in shape (Les and Wunderlin, 1981; Zhengyi et al., 2014). Initiation: PPQ received a market access request for Hygrophila corymbosa, H. difformis, H. pinnatifida, and H. polysperma aquatic plants for propagation from the Ministry of Food, Agriculture and Fisheries of the Danish Plant Directorate (MFAF, 2009). These Hygrophila species are not native to the United States (NGRP, 2014) and may pose a threat to U.S. natural and agricultural resources. Although H. polysperma is already regulated as a U.S. Federal Noxious Weed and did not need to be evaluated, the PERAL weed team evaluated it for comparison with the other Hygrophila species. Foreign distribution: Hygrophila polysperma is native to India, Bangladesh, Bhutan, Nepal, Pakistan, Myanmar, Vietnam, and China (Bailey and Bailey, 1976; Langeland and Burks, 1998; NGRP, 2014). It is naturalized in Mexico Ver. 1 January 28, 2015 1 Weed Risk Assessment for Hygrophila polysperma (Kasselmann, 2003; Mora-Olivo et al., 2008) and reported from Germany (Nault and Mikulyuk, 2009). U.S. distribution and status: In the United States, H. polysperma is naturalized in Florida, Texas (NGRP, 2014), Alabama (Kartesz, 2014), Kentucky, and South Carolina (EDDMapS, 2014). It has also been reported from lakes in Virginia (NRCS, 2014) but this species is probably not naturalized there (Les and Wunderlin, 1981). Hygrophila polysperma is listed as a U.S. Federal Noxious Weed and as a state noxious weed in Alabama, California, Florida, Indiana, Massachusetts, Minnesota, North Carolina, New Hampshire, Oklahoma, South Carolina, and Vermont (NGRP, 2014). WRA area1: Entire United States, including territories. 1. Hygrophila polysperma analysis Establishment/Spread Hygrophila polysperma was detected in Lee County, Florida in 1979 (Les and Potential Wunderlin, 1981) and rapidly spread to dozens of other public water bodies in Florida (Langeland and Burks, 1998). It has also spread to South Carolina, Alabama (Kartesz, 2014), and Texas (Angerstein and Lemke, 1994). Hygrophila polysperma can grow at low light levels that occur at up to two meters underwater (Spencer and Bowes, 1985) as well as at the water surface, where it forms dense mats of vegetation (Doyle et al., 2003). This species can quickly spread by vegetative stem fragments that readily root to produce new plants (Angerstein and Lemke, 1994; Les and Wunderlin, 1981; van Dijk et al., 1986). Hygrophila polysperma is a popular aquarium plant (Les and Wunderlin, 1981) and may have been initially introduced into waterways through dumping of aquaria (Angerstein and Lemke, 1994). We had a moderate amount of uncertainty for this risk element. Risk score = 21 Uncertainty index = 0.18 Impact Potential In bodies of water in natural areas, H. polysperma can occupy the entire water column (ISSG, 2014), outcompeting and displacing native vegetation (Cuda and Sutton, 2000; Doyle et al., 2003; Weber, 2003). In urban and suburban areas, it forms large floating mats that clog and interfere with the functions of culverts, pump stations, and other water control structures (Cuda and Sutton, 2000; Duke et al., 2000; Sutton, 1995; van Dijk et al., 1986) and may increase desirable habitat for mosquito development (Cuda and Sutton, 2000; Nault and Mikulyuk, 2009). Hygrophila polysperma is listed as a U.S. Federal Noxious Weed (NRCS, 2014) and is controlled in natural areas, and urban and suburban settings (Duke et al., 2000; IFAS, 2012). We had a moderate amount of uncertainty for this risk element. Risk score = 3.6 Uncertainty index = 0.13 1 “WRA area” is the area in relation to which the weed risk assessment is conducted [definition modified from that for “PRA area”] (IPPC, 2012). Ver. 1 January 28, 2015 2 Weed Risk Assessment for Hygrophila polysperma Geographic Potential Based on three climatic variables, we estimate that about 40 percent of the United States is suitable for the establishment of H. polysperma (Fig. 1). This predicted distribution is based on the species’ known distribution elsewhere in the world and includes point-referenced localities and areas of occurrence. The map for H. polysperma represents the joint distribution of Plant Hardiness Zones 7-13, areas with 0-100+ inches of annual precipitation, and the following Köppen-Geiger climate classes: tropical rainforest, tropical savanna, steppe, desert, Mediterranean, humid subtropical, and marine west coast. Note that in this weed risk assessment it was not clear if H. polysperma occurs in Plant Hardiness Zone 7. Because H. polysperma has been reported surviving in a pond in Richmond, Virginia for 15 to 20 years (Cuda and Sutton, 2000), we assumed it could occur in Plant Hardiness Zone 7 for this prediction. The area estimated is likely conservative since it only uses three climatic variables. Other environmental variables, such as soil and habitat type, may further limit the areas in which this species is likely to establish. Hygrophila polysperma is an aquatic plant that can grow either emerged or fully submerged (Les and Wunderlin, 1981; Spencer and Bowes, 1985). It occurs in freshwater lakes, ponds, and in riparian habitats (Weber, 2003). Entry Potential We did not assess the entry potential of H. polysperma because it is already present in the United States (Kartesz, 2014; Les and Wunderlin, 1981). Ver. 1 January 28, 2015 3 Weed Risk Assessment for Hygrophila polysperma Figure 1. Predicted distribution of H. polysperma in the United States. Map insets for Alaska, Hawaii, and Puerto Rico are not to scale. 2. Results Model Probabilities: P(Major Invader) = 95.2% P(Minor Invader) = 4.7% P(Non-Invader) = 0.1% Risk Result = High Risk Secondary Screening = Not Applicable Ver. 1 January 28, 2015 4 Weed Risk Assessment for Hygrophila polysperma . Figure 2. Hygrophila polysperma risk score (black box) relative to the risk scores of species used to develop and validate the PPQ WRA model (other symbols). See Appendix A for the complete assessment. Figure 3. Model simulation results (N=5,000) for uncertainty around the risk score for Hygrophila polysperma. The blue “+” symbol represents the medians of the simulated outcomes. The smallest box contains 50 percent of the outcomes, the second 95 percent, and the largest 99 percent. Ver. 1 January 28, 2015 5 Weed Risk Assessment for Hygrophila polysperma 3. Discussion The result of the weed risk assessment for H. polysperma is High Risk. Hygrophila polysperma shares traits in common with other major invaders (Fig. 2) used to develop and validate the PPQ WRA model. One hundred percent of the simulated risk scores received a rating of High Risk, indicating that our result is very robust. The results of this weed risk assessment support H. polysperma’s status as a U.S. Federal Noxious Weed and a State Noxious Weed in at least 11 states (NGRP, 2014). Control of existing infestations is challenging; mechanical control can disperse vegetative pieces that re-root to form new plants (Angerstein and Lemke, 1994; Les and Wunderlin, 1981; van Dijk et al., 1986). In experiments, H. polysperma stem fragment regrowth surpassed regrowth of even Hydrilla fragments (Spencer and Bowes, 1985). Hygrophila polysperma can also be difficult to control with herbicides (Cuda and Sutton, 2000), but grass carp can help control infestations in conjunction with herbicide applications (Sutton, 1995). 4. Literature Cited 7 U.S.C. § 1581-1610. 1939. The Federal Seed Act, Title 7 United States Code § 1581-1610.