KEY POINTS:
An invasive moth from Australia has "got away" in Auckland with hundreds of thousands of its caterpillars munching their way through the foliage of eucalyptus trees which line the city's streets.
The Auckland City Council says the population of gum leaf skeletoniser caterpillars has exploded in Auckland this month.
Council arborist Simon Cook said they had been around for a few years but numbers had increased hugely this spring.
The caterpillars were fond of the Queensland box which were popular street trees.
They skeletonise leaves by eating the green parts and avoiding the veins, resulting in mass browning off and destruction the tree foliage.
Mr Cook said some trees in Epsom had been left totally bare.
"Fortunately though they don't like new leaves ... however if the trees lose even one third of their canopy they will suffer."
Mr Cook said the caterpillars were particularly rampant around Glenn Innes and Pt England, but had spread throughout Auckland even in apple moth spray areas.
The caterpillars also liked silver dollar and red flowering gums which had been hit quite badly.
The first population of the native Australian moth was discovered in New Zealand at the Mt Maunganui golf course in 1992, but was successfully eradicated.
"They probably came over in someone's golf gear."
Another population was found at the Mangere Lawn Cemetery, near the international airport, in 2001, possibly arriving with travellers from Australia.
Mr Cook said the Ministry of Agriculture and Forestry had now decided it was impossible to eradicate them and had gone into a pest management phase.
The main thrust was to find a biological control but in the meantime the council was considering stem injections where a hole is drilled into tree trunks and chemicals inserted to poison the caterpillars.
Mr Cook was concerned at their impact on amenity values given they were affecting many eucalyptus in streets, parks, schools and private properties.
Each tree could host thousands of the caterpillars, and each female moth could lay hundreds of eggs.
"We are never going to get rid of them."
Control methods could however reduce their numbers and minimise their spread.
The caterpillars could also sting and cause health problems such as itchy skin rashes. Their hollow stiff spines contained a venom which could be injected into human skin upon contact and could cause severe pain and welts.
Mr Cook was advising council tree contractors to wear long sleeve shirts when working on affected eucalyptus trees.
The New Zealand Medical Journal this year reported how three girls were stung by the caterpillars after climbing a eucalyptus tree in Avondale.
Lisa Berndt, a research entomologist at Ensis Forest Biosecurity and Protection, was investigating a suitable biological control for the pest.
Dr Berndt said she had been working on the problem for four years, with the assistance of the Sustainable Farming Fund.
The most likely control would be the introduction of a parasitoid insect from Australia that looks like a tiny wasp, to lay eggs in the caterpillars.
"But we need to ensure they do not attack anything else."
Dr Berndt said with approval from the Environmental Risk Management Authority the wasps could be released by 2010.
She said the moths had been trapped in the Auckland region as far north as Warkworth and south as Meremere. Auckland and Northland were most at risk from the pest because the warmer weather allowed them to go through two lifecycles in a year.
It could spread wherever eucalyptus trees grew, even down to Southland, but at least in cooler climates it would likely only produce one generation a year.
Dr Berndt said commercial eucalyptus growers were very concerned.
