A release site for biocontrol agents of the aquatic weed, hydrilla, on the Rio Grande River in the USA.

Biological control of the aquatic weed hydrilla in the USA

The USDA’s Australian Biological Control Laboratory is conducting surveys across South-East Asia for potential biocontrol agents of the aquatic weed hydrilla.

• 16 May 2007 | Updated 14 October 2011

• The weed
• The search for biocontrol agents
• Future research
• Partners

CSIRO and the United States Department of Agriculture are working together to control the spread of the aquatic weed, hydrilla, in the United States of America.

The weed

In Thailand, part of hydrilla's native range, it grows in balance with other aquatic plants and doesn't form invasive monocultures.

Hydrilla, Hydrilla verticillata, was first introduced into the United States through the aquarium trade in the early 1950s. Since then 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. Recently, hydrilla has become resistant to aquatic herbicides.

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 US in both the monoecious (having male and female flowers on the same plant) and dioecious (having male and female plants) forms, probably as a result of two separate introductions. The dioecious biotype has been found in 13 states.

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. However, literature records indicate Sri Lanka as the origin.

The search for biocontrol agents

Management of hydrilla through chemical and mechanical control is ineffective in the long term, environmentally damaging and expensive. Biocontrol using aquatic invertebrates is considered to be the safest, most cost-effective and sustainable long-term solution.

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 US scientists.

Although these extensive surveys found several potential agents none have yet been used in a successful biological control program.

Extensive surveys were conducted in:

• northern and eastern Australia, 1984-88
  New surveys are underway for potential biocontrol agents for hydrilla in the USA.
• China, 1989 and early 1990s
• East Africa, 1976, 1981-84
• India, late 1960s, 1982
• Pakistan, 1971-76.

Brief surveys have been conducted in:

• Panama, late 1970s, 1980
• South-East Asia
  • the Philippines, 1982
  • Indonesia, 1982
  • Malaysia, early 1970s and 1982
  • Thailand, 1982 and 1996
  • Vietnam, 1996.

Although many plant eating insects were found during these surveys, only a few were selected as potential agents. The rest were not specific enough, were not readily available or had little impact on hydrilla.

As a result of this research, two Bagous weevils and two Hydrellia flies were released in the USA. None of them have been successful in controlling hydrilla as they either not established or have had limited impact on the weed’s growth. New agents are needed.

Future research

Further exploration in hydrilla’s home range is needed to find new agents that are adapted to the range of environments where hydrilla is invasive.

Since 1999, scientists at the United States Department of Agriculture's (USDA’s) Australian Biological Control Laboratory (ABCL) in Brisbane, Queensland have been conducting new surveys in:

• Thailand
• Indonesia
• Singapore
• Malaysia.

As a result of these surveys, research has focused on the leaf-defoliating moth, Paracymoriza vagalis from Indonesia and a suite of stem-boring Bagous weevils from Thailand.

The ABCL is planning further exploration in:

• China
• Vietnam
• Cambodia.

Partners

The ABCL is hosted by the CSIRO laboratories at Long Pocket in Brisbane, Queensland, Australia. It is operated by the USDA's Agricultural Research Service.

Find out more about The US Department of Agriculture’s Australian Biological Control Laboratory.