Weedy waterways

Huge rivers flow through Papua New Guinea which, with the country’s dense vegetation and limited road networks, are vital in providing links between villages and routes to gardens, markets, schools and hospitals. Like roads, waterways need to be kept free-flowing and clear for traffic – in this case, boats. During the 1980s and early 1990s, many of Papua New Guinea’s rivers and wetlands were being taken over by an outside invader, the water hyacinth.

Water hyacinth

A native of South America, water hyacinth is a freshwater plant with pretty blue flowers. Away from its natural enemies, it became a vigorous growing aquatic weed that damaged water quality by blocking sunlight and oxygen and slowing riverflows. 

The Sepik River in Papua New Guinea was especially affected by the water hyacinths and, in the ten years after it was first introduced as a decorative plant, it had spread so much that in places:

• waterways were too clogged for boats to get through
• water plants and animals died because sunlight could not reach them
• water quality was reduced for drinking, cooking and washing
• people lost income as they could not get to their gardens and markets and tourists could not visit villages.

In some villages people even died because of the water hyacinth:

• Snake bite victims could not get to hospital in time.
• People starved because they could not get to their gardens or to markets.
• Contaminated water caused diseases and provided breeding grounds for malaria-carrying mosquitoes.

The problem became too big for manual clearing (mechanical control); and herbicides (chemical control) would have been ecologically disastrous, uneconomical and would only have had a short-term effect – so a different solution was needed.

Control by weevils

In nature, every plant and animal has natural enemies, which help to keep the environment in balance. Using these natural enemies to attack weeds and pests is called ‘biological control’. Scientists found that a particular weevil was the natural enemy of water hyacinths in South America and that it would eat only water hyacinth and not harm any other plants. The weevils feed on the leaves, while the larvae tunnel into the leaf stalk and crown, destroying the growing points. This causes the plants to rot and die and eventually sink. Once the weevils become established on water hyacinth, the impact is rapid, visible and long-lasting.

The Papua New Guinea Government, with the assistance of the Australian Government, worked on a project to locate, breed and release the weevils into the worst-affected areas. Field workers kept records of the numbers of insects released and their impact on the water hyacinth. Community members were taught to identify the water hyacinth as a weed and report any that they saw growing. The partnership of government, scientists and community was successful in clearing the waterways, bringing back fish stocks and allowing people to use the river again.

Applying the learning

The project, using the weevil for control of the water hyacinth and involving the community in the program, has also been applied to clearing huge infestations of water hyacinth in Lake Victoria (Kenya, Uganda and Tanzania), Benin (West Africa), South Africa and Thailand.