7 HYDRILLA J. K. Balciunas1, M. J. Grodowitz 2, A. F. Cofrancesco2, and J. F. Shearer2 1 USDA-ARS, Exotic and Invasive Weed Research Unit, Western Regional Research Center, Albany, California, USA 2 U.S. Army Engineer Research and Development Center, Waterways Experiment Station, Vicksburg, Mississippi, USA Nature of Damage PEST STATUS OF WEED Economic damage. In the United States, hydrilla of- Hydrilla verticillata (L.f.) Royle (hereafter, referred ten dominates aquatic habitats causing significant eco- to as “hydrilla”) (Fig. 1) is a submersed, rooted nomic damage (Fig. 2). Hydrilla interferes with a wide aquatic plant that forms dense mats in a wide variety variety of commercial operations. Thick mats hinder of freshwater habitats (canals, springs, streams, ponds, irrigation operations by reducing flow rates by as lakes, rivers, and reservoirs) (Langeland, 1990). Plants much as 90% (CDFA, 2000a) and impede the opera- grow from the substrate to the water’s surface in both tion of irrigation structures (Godfrey et al., 1996). shallow and deep water (0-15 m in depth) (Langeland, Hydroelectric power generation also is hindered by 1990; Buckingham, 1994). This plant is listed on the fragmented plant material that builds up on trash 1979 federal noxious weed list (USDA-NRCS, 1999) racks and clogs intakes. During 1991, hydrilla at Lake and also is identified in the noxious weed laws of Moultrie, South Carolina shut down the St. Stephen Florida (FDEP, 2000), Louisiana (LDWF, 2000), powerhouse operations for seven weeks resulting in Texas (TPWD, 2000), California (CDFA, 2000a), $2,650,000 of expenses due to repairs, dredging, and South Carolina (SCDNR, 2000), North Carolina fish loss. In addition, during this repair period, there (NCAWCA, 2000), Oregon (OSDA, 2000), Wash- was an estimated $2,000,000 loss in power genera- ington (WSDA, 2000), and Arizona (ERDC 2001b). tion for the plant (letter from Charleston District In addition, the states of Alabama, Georgia, Mary- Engineer to Commander, South Atlantic Division, land, Mississippi, Tennessee, and Virginia, have pro- dated March 8, 1993). grams for the control of this invasive plant (Eubanks, 1987; Earhart, 1988; Zattau, 1988; Bates, 1989; Henderson, 1995; Center et al., 1997). Figure 2. Heavy infestation of hydrilla at Rodman Reservoir (August 2, 1978). The rapid underwater growth “pushes” a Figure 1. Growing tip of hydrilla. Note portion of the mat above the water, giving crowded internodes at tips. the reservoir a field-like appearance. 91 Biological Control of Invasive Plants in the Eastern United States Boat marinas have been reported closed for ex- can reproduce from very small fragments (Langeland tended periods on the Potomac River, Virginia; Lake and Sutton, 1980). Apparently, recreational boaters Okeechobee, Florida; Santee Cooper Reservoirs, and fishermen quickly spread hydrilla to new loca- South Carolina; and Clear Lake, California. Propel- tions when fragments of hydrilla are transported on ler driven boats are hampered by thick mats of boats, motors, and trailers. Once an aquatic site is hydrilla that form at the water’s surface, requiring infested, eradication of hydrilla is very difficult. It frequent cleaning to progress short distances. The produces specialized asexual, reproductive ‘buds’ on fragmented plant material removed from the propel- stems (referred to as turions) and on the underground lers can easily colonize new areas. In the late 1980s, stolons (tubers). These tubers and turions assist hydrilla populations at Lake Guntersville, Alabama hydrilla in reinfesting a site after a drought, or after increased rapidly. Henderson (1995) examined the application of herbicides. Langeland (1990) reported economic impact of aquatic plant control programs that the annual control cost to manage 7,600 ha of on recreational use of this lake between 1990 and hydrilla in Florida exceeds $5 million. The U.S. Army 1994. He found that the greatest economic value for Corps of Engineers spends more than one million recreation ($122 million annually) occurred when dollars per year to suppress hydrilla populations in vegetation levels were 20% of the total lake area, and the Jacksonville District and more than $400,000 an- that revenue declined as hydrilla acreage increased. nually to treat infestations of this plant at Lake Semi- Although California does not consider hydrilla nole, a 30,000-acre lake located on the borders of established, the state has, for decades, aggressively Florida, Alabama, and Georgia. Since 1989, millions pursued an eradication program that seeks to rap- of dollars have been spent to introduce the triploid idly eliminate new infestations as they are discovered. grass carp into the Santee Cooper Reservoirs (70,000 California officials have stated that if infestations are ha) for the management of more than 17,000 ha of not contained and treated promptly, hydrilla will hydrilla (Morrow et al., 1997; Kirk et al., 1996, Kirk spread throughout the state and cost millions of dol- et al., 2000). Grass carp populations have reduced lars annually to manage (CDFA, 2000b). the infestation levels of hydrilla; however, additional Ecological damage. Native plants act as the pri- stocking may be needed to maintain the current level mary producers in most ecosystems (Drake et al., of control (Kirk et al., 2000), which will also add to 1989; Pimm, 1991). In the United States, hydrilla fre- the management costs of this program. quently forms large monocultures that displace na- Hydrilla was first reported in California in 1976, tive vegetation (Haller, 1978), reducing biodiversity and at that time the state established an eradication and altering native ecosystems. These alterations also management plan. This program has eradicated affect the primary and secondary consumers in af- hydrilla from various sites in ten counties. At some fected communities (Westman, 1990; Frankel et al., sites, treatment of hydrilla continued for six to eight 1995; Schmitz and Simberloff, 1997). Massive years before eradication was achieved. Funding for amounts of hydrilla can alter dissolved oxygen, pH, this program has gradually increased over time, and and other water chemistry parameters (Smart and during the last three years, California has spent more Barko, 1988). The portion of the water column oc- than $5.39 million (nearly $1.8 million annually) to cupied by aquatic plants also influences the presence eradicate hydrilla infestations in that state (CDFA, and size distribution of fish (Killgore et al., 1993; 2000a). Harrel et al., 2001). In dense hydrilla mats, feeding Geographical Distribution by certain predatory fish is hampered, and small in- sectivores predominate, reducing community diver- Hydrilla is now almost cosmopolitan in its distribu- sity. (Dibble et al.,1996). tion. Antarctica and South America are the only con- Extent of losses. Hydrilla is a major aquatic weed tinents from which it has not been recorded. It is very problem throughout the southeastern United States common on the Indian subcontinent, many of the (Center et al., 1997). It was introduced to North Middle East countries, Southeast Asia, and northern America in 1951 or 1952 by an aquarium plant dealer and eastern Australia. Based on C. D. K. Cook’s (pers. who discarded six bundles of hydrilla into a canal comm.) list of herbarium specimens, hydrilla is found near his business in Tampa, Florida (Schmitz et al., in the Southern Hemisphere as far south as the North 1991). Since then, it has spread explosively because it Island of New Zealand (approximately 40° S). In the 92 Hydrilla Northern Hemisphere hydrilla is found as far north leaves. The leaves are usually strongly serrated with as Ireland, England, Poland, Lithuania and Siberia. the teeth visible to the naked eye, and each leaf ter- The Lithuanian sites, at about 55° N latitude, are the minates in a small spine. The midvein is sometimes furthest from the equator that hydrilla is known to reddish in color, and is usually armed with an irregu- occur. Since virtually the entire continental United lar row of spines. The squamulae intravaginales (nodal States, except Alaska, lies below a latitude of 48°, scales) are small (ca. 0.5 mm long), paired structures hydrilla is climactically suited for growth in any of at the base of the leaves and are lanceolate, hyaline, the contiguous states as well as Hawaii. Even Alaska and densely fringed with orange-brown, finger-like cannot be considered entirely safe from invasion by structures called fimbrae. Flowers are imperfect (uni- hydrilla since places such as Juneau are at approxi- sexual), solitary, and enclosed in spathes. The female mately the same latitude as the hydrilla infestations flower is white, translucent, with three broadly ovate in Lithuania and Siberia (Balciunas and Chen, 1993). petals, about 1.2 to 3.0 mm long; the three petals al- The female form of dioecious hydrilla arrived ternate with the sepals that are much narrower and in Florida in the early 1950s (Schmitz et al., 1991) slightly shorter; the three stigmas are minute; the and quickly spread throughout the southeastern ovary is at the base of a long (1.5 to 10+ cm) hy- United States. Although the monecious biotype of panthium. The male flower is solitary in leaf axils. hydrilla was not detected in the United States until Mature flowers abscise and rise to the surface. Sepals the late 1970s (Haller, 1982; Steward et al., 1984), it and petals are similar in size and shape to those of too is now spreading rapidly, especially into north- female flowers. Each of three stamens bears a four- ern states. Monecious hydrilla has now been detected celled anther that produces copious, minute, spheri- as far north as the Columbia River in Washington cal pollen. Hydrilla plants occur as two biotypes. state in the western United States, and in Pennsylva- They can be either dioecious, with flowers of only nia and Connecticut in the eastern United States (Ma- one sex being produced on a particular plant, or deira et al., 2000).