The Department of Conservation has been testing new aerially sprayed herbicides to kill wilding pines on Queenstown Hill.
The trial showed surprisingly positive results on one of the more troublesome species of pine, which has made Doc staff hopeful about potential savings.
Several experiments have been made in the fight against pines by spraying from the air, but the pines have been slow to absorb the poison, and the Pinus contorta variety especially appeared hard to eradicate this way.
For a 32ha area on Queenstown Hill, Doc biodiversity threats ranger Jamie Cowan tried mixing the usual herbicide with vegetable oil in an attempt to improve the trees' absorption of the herbicide.
The spraying was done back in February, and the result is now showing.
The addition of vegetable oil meant the targeted Douglas fir had died 30% quicker than in previous trials.
The new mixture also proved to have a significant effect on the sturdier and more invasive contorta pine.
"It was a surprise to see that in less than a year the troublesome contorta variety in the area is also dying.
This kind of pine is spreading drastically at many locations around the country, so finding a way to battle it is an important breakthrough.
"We will now be doing further trials with dense stands of Pinus contorta on the flanks of Coronet Peak near Queenstown and Mid Dome in Northern Southland," Mr Cowan said.
Cost-saving is one of the driving factors behind Doc's research in aerial spraying of pines.
Spraying the 32ha at Queenstown Hill cost about $18,000 in total, or less than $570 per hectare, which was split between Doc, the Queenstown Lakes District Council and private landowners.
Mr Cowan said it cost $2000-$3000 per hectare to clear the pines by conventional methods of tree felling and manual weeding.
Doc's weed technical support officer, Peter Raal, said much research was still to be done.
However, a conservative estimate indicated savings of 40% by implementing aerial spraying in pine control.
Given Doc's annual cost of $250,000 for pine control in Otago, and an estimated $2.5 million spent nationwide, it could mean savings of $1.1 million a year for Doc, and equivalent savings for local and regional councils, and private landowners, involved in pine control.
Mt Aspiring Station owner John Aspinall, a trustee of the Mid Dome Wilding Trees Charitable Trust, is optimistic about the prospect of cheaper pine control.
"In the past, we have spent up to $2000 per hectare clearing pine growth with chainsaws, and as much as $4000 per hectare to completely clear and mulch a pine area.
We have an area of almost 250ha of dense contorta pine growth at Mid Dome, so if the aerial method proves effective, we could be looking at substantial savings."
A risk of aerial herbicide spraying is that the wind can cause it to drift to surrounding areas.
The Queenstown Hill area was close to the residential area of the hill, and called for special care.
Mr Raal said: "We do not like spraying near residential areas, but when the severity of the pine problem calls for it, we take great care and make sure the conditions for spraying are 100% right. We had a comprehensive risk-management plan for the Queenstown Hill project."
Mr Cowan had also considered the effect on other vegetation in the sprayed areas.
"There will inevitably be a bit of damage to the plant life in the fringe areas. But the land we are trying to save from the pines is mainly covered with tussock, which only takes a light scorching from this herbicide, and then bounces back.
"If you look at an area of dense pine, you will see it kills all undergrowth. So it is better to kill a bit of grass and scrub along with the pines to give room for the native vegetation to grow back," Mr Cowan said.
