Common Name: Hydrilla
Scientific Name: Hydrilla verticillata (L. f.) Royle
Hydrilla is an aquatic plant and member of the Hydrocharitaceae or Frog’s-bit family. Its appearance is variable depending on the growing conditions. It may be monoecious or dioecious. It was sold as an aquarium plant, which may have been the origin of its initial introduction into natural water systems. However, it is now listed by the federal government, and most states, as a noxious weed. As a result, its sale in recent years has been limited.
Origin and Distribution
It is believed that Hydrilla is native to Asia and/or India. It was discovered in Florida in the 1960’s and by 1988 it had spread to an estimated 20,000 ha of water. It is believed that at least two separate introductions occurred since two different forms of hydrilla occur. Boats and trailers are the primary means it is spread to new water systems. Current distribution includes all of the southeastern states, Connecticut, Delaware, Maryland, Virginia, Alabama, Texas, Arizona, California, and Washington. It has been listed as a Federal Noxious Weed and is prohibited or restricted in eleven states.
Hydrilla resembles the native elodea (Elodea canadensis) and the non-native egeria (Egeria densa). The main distinguishing feature is that Hydrilla has leaves arranged in whorls of four to eight and the other two have whorls of three and four to five, respectively.
Hydrilla is an obligate submerged perennial aquatic plant and can be found in a variety of aquatic habitats such as reservoirs, lakes, ponds, springs, rivers, and tidal zones. It can tolerate a wide range of water chemistry conditions including lakes and ponds of high and low nutrient concentrations. It is rarely found in fast moving water systems. Hydrilla will tolerate salinities as high as 7% and a wide range of acidity, although optimum growth is seen at a pH of 7. It will grow in very low light conditions, 1% of full sunlight, allowing it to colonize deeper depths than many native plants.
Harvesting: Harvesting may manage small initial populations. Where possible, remove all of the plant parts from the water and dispose of properly. Plant fragments as small as one whorl can potentially start a new infestation. Consistent monitoring for several growing seasons is required to control new or missed plants. For larger infestations in high value areas, specialized harvesters are available, although the cost may be prohibitive at as much or more than $1000 or more per acre. Up to six harvests per year may be necessary due to hydrilla’s rapid growth.
Drawdowns: When growing in bodies of water that have control structures, hydrilla may be controlled by seasonal drawdowns. Drawdowns are most effective while the tubers are developing in the fall and before regrowth occurs in the spring. Tubers may remain dormant and viable in the soil even after the lake or pond has been drained, so drawdowns are limited in effectiveness.
Fluridone: Fluridone is a selective herbicide depending on application rates, contact times, and timing of application. Application rates depend on site-specific factors such as infestation size, water depth and chemistry, and water flow rates. It is intended for use on whole ponds and large-scale infestations (greater than 2 ha). It has been used successfully with minimal long-term effects to native plants. Fluridone is intended to reduce but not eliminate Hydrilla. Refer to manufacturer's label for specific information and restrictions regarding use.
Triploid Grass Carp: Sterile grass carp or white amur (Ctenopharyngodon idella) may be an option in areas with adequate control structures to ensure retention of released fish. Grass carp will eat a variety of vegetation including native species. This method is an option only in areas where impact to all of the vegetation is acceptable. The stocking rates for grass carp have not been established. Local and state laws should be checked before release.
Since 1981, the U. S. Department of Agriculture, U. S. Army Corps of Engineers and the University of Florida-IFAS have conducted worldwide surveys for other biological control agents. Over forty insect species have been identified as feeding on hydrilla and are currently being evaluated. The larvae of a weevil (Bagous affinis) discovered in India and Pakistan feeds on the subterranean tubers of hydrilla. This insect is only effective in areas of intermittent wet and dry periods or where periodic drawdowns are possible. It has not become established. A leaf-mining fly (Hydrellia pakistanae) has been released in Florida but it has had no impact. Hydrellia balciunasi was released in 1989 but had limited success due to variation in hydrilla populations and parasitism by native wasps among other factors. The accidental release of an aquatic moth (Parapoynx diminutalis) was shown to defoliate plants but leave viable stems.
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