Weevils Help Control Salvinia

Land for Wildlife member, Leanne Field, releasing Salvinia Weevils into a Salvinia infestation on her property.

Regarded as one of the worst weeds in Australia, Salvinia (Salvinia molesta) is a floating aquatic fern  inhabiting still or slow moving water. Salvinia thrives in nutrient rich waters. This allows it to colonise most water bodies such as agriculture run-off, wastewater, wetlands and dams. Salvinia is a Class 2 Declared Pest under Queensland legislation and is also a Weed of National Significance (WONS). State Government legislation requires landholders to manage infestations on their properties. Salvinia was brought to Australia from Brazil in 1952 as an ornamental plant. Infestations soon exploded and almost all water bodies nearby were colonised by Salvinia; the introduction of this weed has been regretted ever since.

Salvinia has unusual features allowing this species to dominate and succeed. The leaves have tiny egg-beater shaped hairs that repel water and enable it to float. Each node has false leaves that are submerged and modified to act as a root. Morphological variation (variable size and shape) within the species is considerable and greatly depends on age, nutrient availability and the size of infestations.

Infestations in our waterways are a major hurdle for biodiversity, water quality and the aesthetic values of riparian areas. Salvinia dominates the surface of exposed water bodies, forming dense mats and altering ecosystem function by excluding light penetration, preventing the transfer of oxygen from the air and effectively decreasing dissolved oxygen levels.

Competing with native vegetation and preventing aquatic wildlife from finding refuge, food or nests, Salvinia plays an influential role in prohibiting the distribution of Australia’s native flora and fauna. In some regions, Salvinia also adversely affects pastoralism, tourism, recreation and traditional hunting practices. Hydrogeological regimes have also been altered by Salvinia infestations impeding the access of wildlife and stock to floodplains and water bodies.

Like many weeds, Salvinia has traits that make it highly invasive. It can be dispersed through various methods such as floodwaters, birds and other wildlife. It grows rapidly, has limited natural predators and out-competes native aquatic plants. Salvinia reproduces by forming new branches that break off and re-establish quickly. Salvinia still has fern-like traits where spores are contained in sporocarps attached to the roots, however they are either empty or contain sterile spores. This is an unusual feature for a fern!

Salvinia dies in salt water; therefore, floodwaters flushing Salvinia out to sea can play a key role in managing infestations. Preventing further spread outside of core infestations should be a main management objective. It is easier and more cost effective to prevent new infestations than to control an established infestation.

One effective method of Salvinia management is biocontrol using the Salvinia Weevil (Cyrtobagous salviniae). In 1980, the Salvinia Weevil was released by CSIRO into Lake Moondarra near Mount Isa. This lake had a Salvinia infestation that covered an estimated 800 hectares and weighed more than 50,000 tonnes. This was significantly reduced to just one tonne within one year. It is important to note that eradication of Salvinia will not be achieved with the sole use of biocontrol as a healthy amount of weed is always needed to sustain an ongoing population of weevils.

Weevils can live for about six months with a completed life cycle only taking six to eight weeks. Weevil larvae feed inside the stems and the adults feed on the leaf buds, therefore they both help to manage the weed. Over a period of one to three years after weevil introduction, the matted Salvinia turns brown and sinks to the bottom of the water body. Preferring open water with little shade, all stages of the weevil’s lifecycle are temperature dependent; therefore, weevils should be released during spring and summer months. Salvinia Weevils are commonly used by landholders in South East Queensland and are available from your Council – just ask your Land for Wildlife Officer.

Equipped with knowledge, the right information and a positive commitment, weed management can help restore and protect the great diversity our riparian areas have to offer.

References & Further Reading
Julian M, McFadyen R and Cullen J (2012) Biological Control of Weeds in Australia. CSIRO Publishing.
Sullivan P & Postle L (2012) Salvinia Biological Control Field Guide. NSW Department of Primary Industries.
Walden D, Boyden J, Bayliss P & Ferdinands K (2012) A preliminary ecological risk assessment of the major weeds on the Magela Creek floodplain, Kakadu National Park. Commonwealth of Australia.
Department of Environment and Heritage (2003) Weeds of National Significance: Weed Management Guide Salvina (Salvinia molesta). CRC Weed Management.
Walton C (2005) Reclaiming Lost Provinces a Century of Weed Biological Control in Queensland. Department of Natural Resources and Mines.

Article by Stephani Grove Land for Wildlife Officer Ipswich City Council

View Full Newsletter