The fall armyworm has flown over from Australia, and Biosecurity New Zealand is hoping it is only a tourist.
More than 26 cases have been confirmed in New Zealand by Biosecurity New Zealand since it was first found early this year, including on three properties in Kaitaia and one in Hikurangi.
Anecdotal sightings have also been reported in the Waipu and Ruawai areas, according to crop managers Northland Seed and Supplies.
John Walsh, director for readiness and response for Biosecurity New Zealand, said cases had been confirmed in six regions in the upper North Island, including Northland.
Walsh said all confirmed infestations had been found on maize or corn crops so far, with most on self-sown plants that pop up after harvest.
Farmers and growers are being asked to report any sightings on crops or crop remnants.
"They are quite easy to identify as they have distinct markings that are different to the tropical armyworm. We're encouraging people to go to our website for more information or to ring 0800 809966 to get instructions on submitting photographs for help with identification of this unwanted organism.''
The notifiable pest had most likely been blown across from Australia on storm fronts.
"It's been in Australia for a couple of years and had spread down from Queensland to Victoria and New South Wales, so our risk assessment was that it was likely to be blown across to New Zealand at some stage floating in strong wind,'' he said.
"Travel has been very limited with border closures due to Covid-19 so we are confident the fall armyworm has been blown here rather than being brought in by a traveller.''
While officially first detected in May, Walsh said there was some evidence it may have been in New Zealand since February.
The fall armyworm is more invasive than its tropical armyworm cousin because its rapid life cycles give it the ability to quickly create a population boom.
Scientists believe the fall armyworm would struggle to become established in New Zealand because it was more suited to a tropical climate and it would likely struggle to survive in a chilly winter.
However, Northland's moist, subtropical climate could allow it to overwinter and gain a footprint, especially if temperatures were to increase in the future.
Walsh said in the tropics, the fall armyworm could sustain between 10 and 12 life cycles in a year, whereas it was believed in New Zealand it would be more likely to be three or four, "which is still enough to be a problem".
While they prefer corn and maize crops, they were known to have "quite a broad appetite" and could attack crops in the brassica family, cereal crops and grasses. They are known to feed on more than 350 plant species.
Walsh said farmers would be unlikely to spot the moth stage, as they are nocturnal and most active during late summer and early autumn.
"They will certainly notice damage from the caterpillars.''
Larvae would feed on stems and leaves – reducing them to a skeleton – as well as getting into corn and maize ears, silks, cobs and kernels.
Egg masses might be spotted underneath leaves, usually in clustered masses of 100 to 200 and covered with a felt-like layer.
Fall armyworm caterpillars grow to about 4 centimetres, and have a pale inverted Y shape between the eyes and four distinctive dark spots arranged in a square on the second-to-last abdominal segment.
"It's not all happening on the plant. During the pupae stage they burrow down into the soil.''
Pheromone traps could be used to trap moths to help farmers and home gardeners gain an understanding if they have a problem.
The same range of methods used to control the tropical armyworm could be used for the fall armyworm, including insecticides.
However, their preference to hide inside plant whorls and cobs could make them difficult to control.
"People should be vigilant, especially if it turns up in crops next year,'' Walsh said.
Since the current findings of fall armyworm have been on pop-up "volunteer" plants which have appeared after the main crop has been harvested, suggested management options concentrate on removing host plants and plant material using herbicide, insecticide or hand pulling followed by tilling to a minimum of 10cm soil depth.
Mowing or intensive grazing could also help to starve the caterpillar.
Using rotation techniques to plant a different crop for next season could also help.
However, evidence to date indicates that most of New Zealand's environment would be unsuitable for these pests, so they would have limited distribution. A related species, the tropical armyworm, has been recorded to have some negative impacts when conditions are unusually warm and humid.
More information about the fall armyworm is available at Biosecurity New Zealand's website under the category "Plants, spiders, or insects". An online report form could be used to notify Biosecurity New Zealand of any sightings at https://report.mpi.govt.nz/pest/
Instructions on how to submit photos for formal identification could be obtained by phoning 0800 809966.