Management Information
Preventative measures: H. verticillata is on the United States Federal Noxious Weed List, but aquarium supply sales continue through the Internet. It has been classified as a Nationally Banned Plant List species in New Zealand. A Risk assessment of
Hydrilla verticillata for Australia was prepared by Pacific Island Ecosystems at Risk (PIER) using the Australian risk assessment system (Pheloung, 1995). The result is a score of 20 and a recommendation of: reject the plant for import (Australia) or
species likely to be a pest (Pacific).
Physical: Harvesting and use of motorised boats is not recommended in partially infested lakes or where uncontamainated waterbodies occur nearby, because this can chop the plants and facilitate spread of shoot fragments (NIWA, 2003). In ponds and small lakes, water draw-downs, which expose and kill the plants, have been found effective. Weed mats in public access sites have been used to contain spread,and signage to increase public awareness are some of the containment methods adopted (NIWA, 2003).
Chemical: Aquatic herbicides are effective at temporarily controlling the weed but do not kill the tubers, turions (overwintering structures that detach and geminate in the spring), and seeds. Some of the herbicides which have been used are Fluridone and endothall (dipotassium).
Biological: Biological controls include Chinese grass carp (Ctenopharyngodon idella), tuber-feeding weevils, and leaf-eating flies. Chinese grass carp have been found effective, but these fish are vegetative generalists, so they should be used with care so as not to destroy native aquatic vegetation. Tuber-feeding weevils and leaf-eating flies are still under evaluation for their effectiveness. The tuber-feeding weevil (Bagous affinis) only attacks the tuber when the plant is not submerged beneath the water. Leaf-eating flies, such as Hydrellia pakistanae , attack the weed by feeding on it as larva.(NIWA, 2003).
Integrated management: An integrated approach of fish, mechanical, and manual methods to eradication has been found to achieve maximum success.   
Location Specific Management InformationAustralia
A biological control programme for Hydrilla verticillata was undertaken simultaneously with programmes for the other aquatic invaders Eichhornia crassipes, Pistia stratiodes and Salvinia molesta, to avoid the increase of other invasive species as one was controlled. Southern Florida
Hydrilla verticillata has been controlled in canals with a combination of herbicides and grass carp (Ctenopharyngodon idella). However, its decline has coincided with the increase of the equally invasive Hygrophila polysperma.
Management Resources/Links
4. Champion, P.D.; Clayton, J.S. 2001. Border control for potential aquatic weeds. Stage 2. Weed risk assessment. Science for Conservation 185. 30 p.
Summary: This report is the second stage in the development of a Border Control Programme for aquatic plants that have the potential to become ecological weeds in New Zealand. Importers and traders in aquatic plants were surveyed to identify the plant species known or likely to be present in New Zealand. The Aquatic Plant Weed Risk Assessment Model was used to help assess the level of risk posed by these species. The report presents evidence of the various entry pathways and considers the impact that new invasive aquatic weed species may have on vulnerable native aquatic species and communities.
Available from: http://www.doc.govt.nz/upload/documents/science-and-technical/SFC185.pdf [Accessed 13 June 2007] 6. Effects of Grass Carp on the Aquatic
Vegetation in Lake Conway, Florida
Leslie J Andrew., et al.
Florida Department of Environmental Protection
(March 1994) 7. Gee II, David E., pers. comm. 2006. Wildlife Biologist, Guam Division of Aquatic & Wildlife Resources and Guam team member of the Pacific Invasives Learning Network (PILN). 9. National Pest Plant Accord, 2001. Biosecurity New Zealand.
Summary: The National Pest Plant Accord is a cooperative agreement between regional councils and government departments with biosecurity responsibilities. Under the accord, regional councils will undertake surveillance to prevent the commercial sale and/or distribution of an agreed list of pest plants.
Available from: http://www.biosecurity.govt.nz/pests-diseases/plants/accord.htm [Accessed 11 August 2005] 10. New Zealand Plant Conservation Network, 2005. Unwanted Organisms. Factsheet Hydrilla verticillata Results Page: 1
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