Scientists hope a thistle-munching beetle already reducing prickly weed infestations in the south will also lay waste to hard-to-control Californian thistles on North Canterbury hill country farms.
Landcare released European-sourced green thistle beetles - also known as cassida or tortoise beetles - on about 50 farms, mostly in lowland Southland, five years ago.
AgResearch scientists want to find out more about the beetle and see if it can go to work in the hill country of North Canterbury and the North Island.
"We're starting to see the effects of the beetle, and Southland farmers are thinking that it's having quite a good effect on the thistle populations there," said AgResearch scientist Michael Cripps. "Basically, they're seeing observational evidence of a decline in the thistle population."
Cripps worked with Landcare scientists to bring the beetle to New Zealand. Its primary host is Californian thistle - which is not, as the name suggests, from California but from Europe. It also feeds on other thistles, but to what extent is unknown.
"We don't even know if the beetle can develop and complete a whole life cycle on some of these other thistles, so that's what we're trying to determine," Cripps said. "Can it survive and complete its life cycle, or does it just occasionally nibble on some of these other plants?"
A variety of thistles is being grown by AgResearch in Lincoln, and beetles are being released on to the plants.
"We'll look at some of the leaf traits of the other plants and try to understand its specificity - is it something physical about the thistle, or chemical, or somehow related to the primary host in terms of its genetics?"
The beetle was known to only eat thistles, which made it a good candidate for release in New Zealand, said AgResearch weed scientist Graeme Bourdot.
"There's probably no threat at all to any New Zealand native species, which is a really good thing - and fortunately, New Zealand doesn't have any native thistles."
With the beetle apparently making an impact in Southland, Bourdot and Cripps, together with a group of North Canterbury and North Island west coast farmers, have submitted a proposal to the Sustainable Farming Fund to investigate its release in hill country.
"There are huge tracts of hill country that has Californian thistle where you can't get conventional control methods. You can't get a tractor [to spray or mow]; you can get a helicopter but it's too expensive," said Bourdot.
"This beetle could really come into its own hill country where it can fly and move around on its own. It just has to be released and an initial population started, and then they can disperse and multiply and move through the hill country.
"We're interested in what other thistles it attacks, but we're also interested in what impact it has out in the field on Californian thistle in particular, and in particular in the hill country, where we think its benefits are really much greater than on the flats, where you can mow and spray."
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