8 April 2009
|Aquatic weed outbreaks in some of south east Queensland’s dams are proving no match for the humble weevil as a result of a biological control program co-ordinated by Seqwater.
Salvinia weed - Salvinia molesta - is regarded as one of the most invasive aquatic weeds in Australia and has considerable environmental and economic impact.
Salvinia floats on still or slow-moving water, growing rapidly to cover the surface like a thick mat which blocks out submerged plant life and limits the transfer of oxygen, making water unsuitable for fish and other animals. Weevils attack by eating the leaves and stems, destroying the roots and causing the weed to sink.
Seqwater spokesperson, Dr Adrian Volders, said weevils were successfully eliminating salvinia in many of the region’s dams as part of integrated control program.
“Keeping weeds under control without damaging our catchments is a delicate battle. Weed outbreaks can cause water quality problems which increase the cost of water treatment,” Dr Volders said.
Recognising the success and cost savings of biological control, Seqwater has established a weevil breeding facility at the Capalaba Water Treatment Plant. Weevils are currently reared in large above-ground pools and plans are being made to upgrade the facility over the course of this year. Seqwater also supports a similar breeding facility managed by the Brisbane City Council.
“Having an expanded program will mean Seqwater and our team of ‘tiny assistants’ can respond to salvinia outbreaks quickly and efficiently across the region. This will have significant benefits for the water quality of our dams and waterways,” Dr Volders said.
“Our plan is to make sure we always have access to a healthy population of weevils, which ironically means we also have to keep a ready supply of salvinia weed in controlled conditions.
“An outbreak of salvinia weed on Leslie Harrison Dam is the next battleground for our tiny assailants. When an outbreak occurs, we simply fill large plastic containers with weevil infested weed from our breeding pools, and place it at the outbreak site.”
In addition to biological controls, Seqwater employs other methods to control salvinia including floating containment barriers, mechanical harvesting and spraying if required.
Salvinia molesta is declared as a class 2 pest plant under Queensland legislation the Land Protection Act 2002 and also classified as a Weed of National Significance.
The dark sub-aquatic salvinia weevil (Cyrtobagous salviniae) measures 2 to 3 mm long.
To be fully effective the complete life cycle – from eggs to adults - is required for the weevil to reduce salvinia levels. This normally takes around 7 weeks in good conditions.
Both the adults and larvae contribute damage to the salvinia weed.
Adults feed on new buds, preventing growth which in turn stimulates the plant to produce more buds which are subsequently eaten. The continual bud production eventually depletes the plant’s reserves. Larvae also tunnel through the stems and destroy the structure of the plant.
Plants cannot tolerate this combined level of damage and eventually deteriorate, become waterlogged and sink.
Weevil damage is visibly apparent in the form of a gradually expanding brown area. The damaged salvinia eventually sinks, creating a widening area of open water where the weed was originally located.
A number of factors play an important role in maintaining the weevil population and their effectiveness as a control measure such as the host plant, warm water temperature and sufficient bioavailable nutrients. Under these optimal growth conditions an infestation can double in size in less than 3 days.
Early detection of Salvinia is important. If plant material is found early in its primary growth phase there is a better chance of preventing the progression of an infestation from occurring when using the biological control agent.