The discovery of myrtle rust in a Kerikeri nursery earlier this year sparked a dramatic but probably futile bid by the Ministry of Primary Industries to contain and destroy it, but not all rusts are equal.
Two rust fungi that attack Lantana camara have not only been welcomed - they were deliberately introduced, and according to Cable Bay-based entomologist Jenny Dymock, one is beginning to have a noticeable impact on the pest plant in Northland.
The Environmental Protection Authority approved the release of Puccinia lantanae and Prospodium tuberculatum to control lantana in 2012. Both were released in Northland (the Whangaroa, Doubtless Bay, Awanui and Kohukohu areas) in autumn.
Dr Dymock, who works with the Northland Regional Council, said they worked by restricting the growth rate and fruit and leaf production of lantana, one of the world's most invasive weeds.
While Puccinia lantanae (a blister rust) did not appear to have established itself in Northland yet, the leaf rust Prospodium tuberculatum was beginning to have a big impact on lantana populations, as evidenced by the number of plants with dead or dying branches.
Lantana was a serious problem in Northland, where it formed dense thickets that invaded a wide variety of areas, from native and exotic forests to domestic gardens, roadsides, sand dunes, quarries and wasteland, Dr Dymock said.
Typically a low, scrambling shrub with small, colourful flowers, lantana could be poisonous to people and grazing stock. It had thorny stems, strong-smelling leaves, especially when crushed, and produced fruit that was attractive to birds, which then spread its seeds to uninfested sites.
The NRC was behind the original application to the EPA to import the rusts, part of a growing number of host-specific weapons in the council's biocontrol arsenal.
She was delighted to see the rust, native to Brazil, making a noticeable dent in the local lantana population. Surprisingly, it appeared to be most effective in late winter, which she attributed to wetter conditions.
"Many people will be unaware that we have a range of more than two dozen biological control agents already in use in Northland, with even more likely to be available in the future," Dr Dymock added.
"Biological control is the use of naturally-occurring enemies and diseases to control pests and weeds. A cost-effective and environmentally-friendly method of pest control, it's not designed to eradicate a species; instead it aims to keep populations at low levels."
As well as fungi and bacteria, biological control tools included an "army" of insects, some of which targeted other insects, and even tiny internal parasites.
The process from lowly insect or fungus to biological control agent was a painstaking, lengthy and initially costly one, however.
"The NRC typically spends at least $80,000 a year on biological control work, and is part of a national group that collectively spends more than $600,000 on this annually," she said.
The first step in finding a biocontrol agent was a survey of the target's natural enemies, in New Zealand and/or overseas. If no natural enemies were found in New Zealand, potentially suitable overseas candidates from areas with a climate matching Northland's had to be extensively tested in a secure quarantine facility to determine whether they would attack any native New Zealand species, or any species that was of economic value.
"Only when researchers are confident the potential biocontrol agent attacks just the target species alone is an application for release from quarantine made to the EPA, followed by consultation with the public and stakeholders," she said.
It could take years to progress from beginning an initial hunt for a biological control agent to approval of its release.
Anyone who was interested in learning more about biocontrol in Northland should visit www.nrc.govt.nz/biologicalcontrol.