Plants of the aquatic weed, water lettuce, on water.

The aquatic weed, water lettuce (Pistia stratiotes).

Biological control of water lettuce, Pistia stratiotes

Water lettuce is one of a suite of aquatic weeds that have been controlled in Australia using biological control agents.

  • 22 March 2007 | Updated 14 October 2011

The weed

The aquatic weed, water lettuce, Pistia stratiotes, occurs throughout the tropics and subtropics.

It rapidly forms dense floating mats on rivers, dams and irrigation canals. These can disrupt navigation, fishing and paddy crops as well as restrict water flow. It can also serve as a breeding ground for mosquitoes, which are the principal vectors of many diseases.

Water lettuce is a free-floating perennial with ribbed, spongy, velvety leaves and dense feathery roots. Its flowers are small and partly concealed at the base of the plant. It spreads by both vegetative reproduction and seeds. The common name comes from its lettuce-like appearance.

Water lettuce thrives in ecologically-disturbed aquatic habitats, particularly those that are man-made.

It is a serious weed in Africa, India and South-East Asia.

The CSIRO research on water lettuce was the first time biological control had been attempted for this weed.

It was first collected in Australia in the Northern Territory in 1946 and eventually spread to New South Wales, Queensland and Western Australia.

In Australia, it is not as big a problem as water hyacinth and salvinia but infestations in Queensland caused problems in ponds, channels and streams.

By 1976, it had noxious weed status in most states.

The origins of water lettuce are uncertain but it is considered to be native to Asia, Africa, equatorial America and, probably, the Northern Territory, Australia.

Biological control

CSIRO scientists investigated the possibility of using biological control to manage water lettuce.

The insect they chose was a small weevil, Neohydronomus affinis, from Brazil. After testing in quarantine in Brisbane, Queensland, it was approved for release in Australia in 1982.

The first releases were made in Queensland where it readily established. Within 12 to 18 months, it reduced the weed to insignificant levels at release sites. The weevil spread and has effectively controlled water lettuce throughout most of its range in eastern Australia.

Populations of the weevils can build up quickly as they have a very short life cycle (approximately 30 days). Adult weevils, which are about 3mm long, feed on and burrow into the leaf, while the larvae mine the inside of the leaf. This eventually kills the plants.

The CSIRO research was the first time biological control of water lettuce had been attempted and the impressive results in Australia paved the way for international success in tropical countries throughout southern Africa, Cote D’Ivoire in West Africa and in the United States.

Plants of the aquatic weed, water lettuce, on water.

The aquatic weed, water lettuce (Pistia stratiotes).

Dam surface clear of aquatic weeds.

The Ayame Dam II in Cote D'Ivoire in late 1998 after a biocontrol agent had been released on water lettuce.

The CSIRO team supplied starter colonies of N. affinis to:

  • Papua New Guinea
  • United States of America
  • South Africa
  • Botswana
  • Zimbabwe.

In Zimbabwe, for example, weevils from Australia were released in the main water source for Harare, in April 1988. By July of the same year, the weevil was well established and water lettuce coverage was declining.

Different biological control agents have been used in other countries.

Learn more about the Ecology and management of Australian weeds.