Weevils with appetite for waterway-smothering salvinia weed welcomed in Top End

Weevils with appetite for waterway-smothering salvinia weed welcomed in Top End

Weevils with appetite for waterway-smothering salvinia weed welcomed in Top End

Updated 8 October 2017, 9:40 AEDT

Weevils might be best known for attacking pantry shelves, but in Kakadu National Park a weevil with an insatiable appetite for waterway-smothering plant salvinia is proving a critical weapon in combatting the weed.

Weevils might be better known for attacking pantry shelves than combatting weeds.

But in Kakadu National Park these little bugs are proving a critical weapon in combatting a salvinia weed outbreak.

Kakadu is Australia's largest terrestrial national park and is home to more than one-third of Australia's bird species and one-quarter of its freshwater and estuarine fish species.

But Kakadu's waterways are under threat from a huge outbreak of salvinia weed.

Lou Elliott, a scientist with the NT Weed Management Branch, said it was regarded as one of the worst weeds in the country because it was highly invasive and had dire economic and environmental impacts.

"It's in waterways across the Top End and also down the east coast of Australia and it causes terrible impacts where it occurs," he said.

The aquatic weed chokes waterways, covering the surface with a thick mat of vegetation that can reach up to 400 tonnes. It blocks plant life and starves the water of oxygen.

Mr Elliott said the weed formed "floating mats" and wreaked havoc on healthy billabongs.

"[It] transforms billabongs, turns them from billabongs with fish and other aquatic life to basically being a clogged-up swamp," he said.

The weed doubles in size every two days.

Parks Australia's Anthony Mann, who runs Kakadu National Park, said it was a concern when the park was renowned for its pristine state.

"So what that means for a our lovely billabongs and waterholes — you go there one day [and] it's nice and clean, you come back a week or two [later] and the salvinia is spreading through that system," he said.

'Little champions' with huge appetite for salvinia weed

But scientists have found one bug with an insatiable appetite for the weed — the salvinia weevil.

"They pack quite a punch don't they, for such a little thing," Mr Mann said.

"They're little champions. I often think of them as Santa's little helpers."

The weevils munch through hundreds of hectares of salvinia by eating out the bud.

Mr Elliott, a weed scientist, said that means the "floating parts which keep it afloat will sink below the surface, and when it does that it dies".

The weevil was introduced to Australia from South America in 1980, near Mt Isa in Queensland, where within just 12 months it wiped out more 99 per cent of 400 hectares of salvinia.

Weevils can survive in the wild on their own, but their population fluctuates along with their food source, which rules out a cane toad-type scenario.

The population is in constant need of boosting to be effective however because of the extremity of the wet season in the Top End.

"Every year the salvinia gets flushed out and the weevil population drops 'cause it doesn't have enough food, so what we're trying to do is increase the weevil population particularly in the early part of the year to give that weevil a march on its food source," Mr Elliott said.

Salvinia-munching weevils welcomed by traditional owners

Demand has been so high the local Indigenous ranger group recently constructed their very own weevil-breeding facility.

In tanks salvinia weed is used to grow thousand of weevils.

Head ranger Matthew Rawlinson said every weevil in the pond lays an egg a day, which results in about four times the amount of weevils every month.

The weevils have come as a huge relief to many, including traditional owners who have only been able to watch on as their homeland has been destroyed.

Violet Lawson is a powerful traditional owner in this region and welcomes the devastating weed being eliminated.

She said a popular fishing spot in the region could not used anymore because there was too much salvinia choking up the waterways.

"[It was] a good place for mussel fishing, very good hunting," she said.

"[You could go] out on weekends and take your children and fish, get all the bush tucker you want.

"It's no more, and it's been like that for three to four years now. We haven't hunted."

The tourist industry largely keeps Kakadu afloat and it too is worried about the unsightly weed.

General manager of Cooinda Lodge, Brett Skinner, said eco credentials were becoming very important as a criteria for tourists.

"You know they not only want to come out and see and experience [the environment], but they also want to know it's being cared for and looked after for future generations," Mr Skinner said.

The Weeds Department estimates through the weevil program will be able to get rid of most of the salvinia weed within five years.

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