Giant aphids have been linked to an increase in wasps.
The Ministry for Primary Industries (MPI) first identified the giant willow aphid species in January, after a report from an Auckland insect specialist.
Its presence has since been confirmed in Northland, Auckland, Waikato, Bay of Plenty, Taranaki, Wellington, Nelson, and North and Mid Canterbury. It has also been reported to be in Fiordland, Westland, Gisborne, Wairarapa, Hawke's Bay, Marlborough and South Canterbury.
Department of Conservation technical advisor for threats Chris Green said the insects produced honeydew, which attracted wasps. Female aphids could reproduce without mating, potentially leading to more food being produced for foraging wasps and increased cases of wasp stings.
In February, nine pupils from Nelson's Victory Primary School were taken to hospital after being stung when a wasp nest was disturbed at Tahunanui Beach.
Green said foraging wasps were not usually a problem as they were not defending their nests.
Entomologist Richard Toft said the aphids spread quickly because they were carried by winds.
"There's a direct correlation between numbers of wasps and people being stung," he said. "If you disturb a willow tree they will drop out of a tree on to a person potentially."
MPI principal adviser George Gill said the insects did not pose a significant threat to New Zealand but could be a nuisance in large numbers as they fed on willows and poplars, which could be used as shelter belts or flood mitigation measures along waterways.
There were general pesticides that could be used to manage aphids, he said.
Green said he was unsure whether the aphids would continue to live throughout winter, or die off.
If they did not die off, then it was likely wasps, which thrived in spring, summer and autumn, would have a much bigger food source later this year. Fairfax NZ
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