Biological control of hydrilla
Hydrilla, Hydrilla verticillata (Hydrocharitaceae), was first introduced into the United States through the aquarium trade in the early 1950s and since that time it has greatly expanded its range from Florida to Delaware on the East Coast and westward to Texas and California. Current control measures are very expensive and economic losses are excessive. In south Texas during drought, hydrilla infestations clog the Rio Grande River impeding water flow and distribution to cities and farms. Hydrilla is present in the U.S. in both the monoecious and dioecious biotypes, probably as a result of two separate introductions.
Hydrilla in Texas, photo courtesy of USDA/ARS
The origin of hydrilla is unclear, but genetic evidence indicates that monoecious hydrilla closely matches material from Korea, and the more prevalent dioecious type is closely related to material from Bangalore, India, though literature records indicate Sri Lanka as the origin.
Management of hydrilla through chemical and mechanical control is ineffective in the long term, environmentally damaging, and costly. Biological control using aquatic invertebrates is considered to be the safest, most cost-effective and sustainable long-term solution to controlling hydrilla. However, biological control of hydrilla has not yet been realized with the existing agents that have been found during extensive worldwide surveys. Global surveys were undertaken to compile lists of the natural enemies of hydrilla throughout its native range. Foreign scientists were contracted to conduct most searches in conjunction with overseas trips by U.S. scientists. Surveys of northern and eastern Australia (1984-1988), China (1989 and early 1990s), eastern Africa (1976, 1981-1984), India (late 1960s, 1982) and Pakistan (1971-1976) were extensive, though trips to Panama (late 1970s, 1980), the Philippines (1982) and Southeast Asia - including Indonesia (1982), Malaysia (early 1970s and 1982), Thailand (1982 and 1996) and Vietnam (1996) - involved only brief surveys. Many phytophagous insects were found during these surveys, though few were selected as potential agents due to their specificity, availability and impact. Two Bagous weevils (Coleoptera: Curculionidae) and two Hydrellia flies (Diptera: Ephydridae) were released in the U.S. but have either not established or have had limited impact on the growth of hydrilla. New agents are needed.
Bagous weevil, photo courtesy of CSIRO
Hydrilla, photo courtesy of ABCL
ABCL renewed surveys in Southeast Asia and Australia in 1999 and more recently it has assisted with surveys in southern China in collaboration with USDA-ARS scientists in Florida and the Chinese Academy of Sciences in Wuhan, China. Several agents have been evaluated including four stem-boring Bagous weevils and several moth species including Paracymoriza vagalis, whose larvae defoliate hydrilla in North Sumatra, Indonesia.
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