[Sunraysia Daily, 18 June 2010, p. 14 by Graeme O'neill] AS THE submerged waterweed dense elodea (Egeria densa) continues to spread unchecked along the length of the Murray River from Lake Mulwala to South Australia it is still being sold legally as an aquarium plant in Victoria and Queensland. The weed has formed dense infestations in slow-flowing reaches of the river, including popular swimming spots off sandbars, where it poses a drowning hazard to children and even older swimmers. It has also choked shallow water at local fising spots like Fort Courage, on the Murray downstream from Wentworth.
"I personally think dense egeria is worse than cabomba, yet it hasn’t been declared weed of national significance like cabomba," Dr Shon Schooler, a water weeds expert with CSIRO’s Division of Entomology in Indooroopilly, in Brisbane, said.
Cabomba, or fanwort, (Cabomba caroliniana) has become a major menance in Lake Nagambie, in Victoria, and was found in Mildura’s Ornamental Lakes. The lakes were drained two years ago to prevent it getting to the Murray. Three other South American water weeds, ailigator weed (Alternanthera philoxeroides), salvinia (Salvinia molesta), and cabomba, were declared Weeds of National Significance in the early 2000s.
Dense elodea has so far been overlooked for a similar classification.
NSW has declared it a noxious weed, but it has not even been declared in Victoria or Queensland, and is apparently still on sale in Victorian aquarium supplies shops.
Dr Schooler said a referral to the National Aquatic Weeds Management Group was the first step towards declaring dense elodea a weed of national significance, a status it probablymerited, because itwas now a serious problem in most coastal rivers in NSW and south-east Queensland.
"The weed management group is trying to persuade the aquarium trade not to sell dense elodea, but it’s still legal in most states, and they complain there’s nothing else available it’s still Australia’s top seller.
"Despite such claims, there are native substitutes that would not become a problem, like curly pondweed (Potamogeton crispus), blunt pondweed, (Potamogeton ochreatus), and water thyme (Hydrilla verticillata).
Dense elodea cannot be controlled with aquatic herbicides, because herbicides generally cannot be used in drinking water.
Nor is mechanical harvesting a solution, because it is so extensively distributed in the Murray system, and many infestations are inaccessible to floating harvesters.
Dr Schooler said that until a biological control agent is found, swimmers, anglers, boaters and other river users on the Murray, and now in the Goulburn-Broken river system, will just have to put up with the weed.
DrSchoolersaid state environmental authorities in California are searching for a potential biological control agent for dense elodea, which has become a serious problem in many rivers in the southern US.
"People tend not to see it as a problem in Australia, because they don’t frequent rivers, and they usually think it’s a native plant," Dr Schooler said.
He says CSIRO is in contact with Californian state researchers who have been searching the aquatic weed’s native range in Argentina, hoping to identify a biological control agent that will only attack dense elodea.
They have identified a fly that lays its eggs on the floating flowers. On hatching, the larvae tunnel into the submerged stems and eventually kill the weed.
Dr Schooler said aquatic weeds tend to thrive in areas where there are high levels of nutrients in rivers, like the Mildura weir pool.
Low stream flows concentrate nutrients, which is why infestations are now flourishing.
Floods or periods of high flow tended to wash dense elodea infestations downstream, reducing their density for a time, but they invariably grow back.