Biological control of Salvinia

• Kelaniya University, Sri Lanka
• Department of Agriculture, Sri Lanka
• University of Sri Jayewardenepura, Sri Lanka

Salvinia (Salvinia molesta) comes originally from south-eastern Brazil, where specially adapted insects keep its growth in check. However, man has spread this floating fern to many tropical and subtropical countries, including Australia, and there the plant grows extremely quickly until it completely covers many water surfaces. Thick mats of salvinia halt the movement of boats, block irrigation channels, stop rice-growing and fishing and kill submerged plants and animals by cutting off light and oxygen. The plant has no know economic uses. And, because it grows so fast, doubling in less than 3 days under ideal conditions, control with herbicides or by physical removal requires indefinite, frequent and expensive repetition, leaving biological control as the only economically and environmentally viable method.
In 1978, scientists from CSIRO discovered salvinia's native range and collected host-adapted insects. After completing host-specificity tests, they released two species in Australia. The weevil, Cyrtobagous sp., proved spectacularly successful in controlling salvinia near Mt Isa in only 1 year and is performing well at other Australian sites. The moth, Semea multiplicalis, has been less effective, but has severely damaged salvinia in coastal Queensland. In a collaborative project with the United Nations Development Program (UNDP) and the government of Papua New Guinea, the weevil has been released in the Sepik district, PNG, where it is starting to control a huge salvinia infestation.
Sri Lanka has experienced problems with salvinia since the mid 1940s, and the weed now threatens large irrigation and development schemes.
This ACIAR project aims at increased understanding of interactions between salvinia, the environment and biological control agents, so as to maximise the likelihood of achieving biological control of the weed in Australia, Sri Lanka and, ultimately, other infested countries. Having confirmed that the weevil Cyrtobagous sp. will not attack any plants of economic importance in Sri Lanka, the scientists will organise a ground survey covering all parts of the country that salvinia infests, to identify the range of growing conditions and to collect sampled of the weed for NPK analysis. On the basis of the ground survey, they will select up to ten contrasting sites for future monitoring and obtain meterological records from the nearest weather station to each one. Only the will they establish and distribute and distribute a population of the weevil in Sri Lanka. As well as reducing current salvinia problems there, the project will reduce the threat of new salvinia infestations to the Mahaweli and other Sri Lankan irrigation schemes.
In Australia, the team will continue to measure interactions between salvinia, biological control agents and the environment, to develop strategies for biological control in a range of environmental and climatic regimes. This approach identified the cause of early difficulties in the Papua New Guinea project and led directly to a successful solution. The research will involve construction of a computer model, incorporating results obtained in Sri Lanka, Australia and Papua New Guinea. Initially, the model will link the effects of temperature and nitrogen concentrations in salvinia to population interactions between the weed and its predators. It will be used to predict how different climatic conditions, or manipulating the system by applying fertilisers or herbicides, will affect biological control. The aim is to predict conditions under which the insects can and cannot control the weed, and to suggest management options that may convert potential failures into successful control.