Pest plant on the way out

Heather is dying off in parts of Tongariro National Park. PHOTO/ DANIAL VAN DER LUBBE

It's not pretty, but conservationists rejoice at seeing expanses of dead, grey plants in Tongariro National Park.

Tiny beetles let loose in there in 1996 are finally starting to make a difference by killing off another introduced species - heather.

Along parts of SH47 a 1km swathe of heather is dead and grey, and all but 10 per cent of the heather in a wetland near National Park has been killed, Conservation Department biodiversity ranger supervisor Danial van der Lubbe said.

The beetles' impact "suddenly exploded" last year. More is to come, because they are being collected and moved to other parts of the park, and a more robust species has yet to be added.

The native vegetation under the dead heather is starting to come through.

But it's been a long campaign, and there are many years of it yet, DOC Tongariro technical adviser Harry Keys said.

Heather was introduced into the park by a warden between 1912 and 1921. He wanted it to feed grouse, and wanted grouse shooting as a sport.

The introduction was stopped by government, on the advice of scientists. But it was too late, because the heather had taken hold. By the 1960s it was out of control.

It continued to spread until the 1980s. The Conservation Department (DOC) held a first workshop about it in 1988, Dr Keys said.

European beetles that eat heather were suggested as a solution. In the early 1990s the beetles were put through a "starvation test", to make sure they wouldn't eat any other native plants. It took four years to be certain of this.

They were finally released in January 1996, in two places in the park, but they only survived in one of them.

In 1999 their work became visible, and beetles were collected and moved to other parts of the park.

They haven't bred up quickly, Dr Keys said, because they are small insects in a cold environment, and feeding on foliage with low nutrients. A larger, more robust species is being bred in Rotorua.

It will be introduced in two years, and DOC staff hope it will be able to survive and do its work at altitudes as high as heather can reach.

Dr Keys is not surprised it has taken 20 years for much sign of heather dieback, and he is heartened to see native plants re-emerge.

"We always knew this was an ecological scale process, not an annual business cycle process," he said.