Salvinia

Salvinia (Salvinia molesta) is a fern from South America. It is classified as a category 3 pest plant under Queensland's Biosecurity Act 2014, and it has a Weeds of National Significance status. This means landholders, including the City of Gold Coast, must take steps to keep their land and water free of this plant.

The species is spread over substantial areas of the state. Its impact is so serious that it requires active control to avoid further spread to properties still free of the pest. Salvinia can rapidly double in size within a week under optimal growth conditions. It forms mats that completely cover lake systems, affecting water quality, flow, wildlife and recreational activities, such as fishing.

It is a free-floating aquatic fern with small, spongy, green leaves positioned in pairs along a common stem. The species can be confused with the native azolla species. Salvinia though has pairs of hairy floating leaves along the stems.

Our integrated management program for this pest plant involves mechanical removal, application of registered herbicides and the introduction of biological control agents.

The salvinia weevil (Cyrtobagous salviniae) was discovered by the CSIRO in 1979. It originates from the same native range in Brazil as salvinia. The adult weevils feed on the salvinia plant, but most damage is done by the larvae, which tunnels into the plant's stem (rhizome). This causes the plant to turn brown, lose buoyancy and sink.

Weevils can control mats of salvinia within a month, but the plant's regrowth can continue for up to 3 years. Weevils need time to increase their population to a size where they are eating the salvinia quicker than it can grow. As the plant is eaten, their numbers decrease allowing salvinia to regrow. This is then followed by an increase in the weevil numbers. This cycle continues until the salvinia and weevils reach an equilibrium.

Extensive testing was completed to ensure the weevil was not going to affect other species before being released. As part of our integrated approach, weevil infestation levels are monitored and we mechanically remove salvinia if it is growing too fast. Removing damaged salvinia is done before it decomposes and has a negative impact on water quality.

Reference:

Waterplants in Australia, G.R. Sainty and S.W.L. Jacobs, Sainty and Associates Pty Ltd, 2003

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