South Island farmers are being told to keep their eyes peeled for the first sign of a pest that cadges a free meal to wreak havoc on maize and sweetcorn crops.
So far there have been confirmed reports of fall armyworm in Northland, Westland and now Tasman this growing season, with an unconfirmed case in the Bay of Plenty and also near Gisborne.
On the West Coast, sightings of the highly invasive moth have increased to about 10 properties in maize crops as they appear to have survived over winter.
The pest was found in a sweetcorn crop in Canterbury last year.
However, larger numbers and the appearance of third generation populations in Northland have the potential to become a greater economic threat.
Foundation for Arable Research (Far) recommends farmers keep a close watch on maize crops regularly.
Far biosecurity officer Ash Mills said there had been no further Canterbury discoveries since small numbers of fall armyworm were found in sweetcorn at a property.
A windier summer, however, had increased the risk of more of them being found, he said.
"It will be the sweetcorn growers that will be the ones that will be the indicator because they are harvesting and hand-packing the cobs and checking some of them, and they will notice."
Growers of maize silage were less likely to see them unless they made regular checks, and this was key to identifying them. The pest was more likely to be economically damaging for sweetcorn growers than maize growers, except for maize grain, which could be more at risk, he said.
Higher death rates of larvae over winter are expected in the South because of its cooler climates, except for pockets with warmer micro-climates.
Mr Mills said fall armyworm was likely to be widespread in maize crops from Hari Hari to the north of the West Coast. They had also been found in Motueka and Golden Bay.
Maize crops appeared to have small populations of first generation fall armyworm on the West Coast, with their numbers likely kept down by cooler conditions during maize planting in October and November.
"It’s probably got to the point there’s not too much we can do because of the size of the plants as the larvae can easily get into the whirls and cobs and then hide away from any insect applications."
Mr Mills said researchers were trying to work out how they had managed to survive the West Coast winter.
"A big question for [us] is what exactly are they over-wintering on. Maize and sweetcorn is their known preferred food in New Zealand, so what are they feeding on successfully in these cooler climates when maize is completely removed?"
Among their food sources are kikuyu grass — usually found in the Far North — and other host plants, such as sorghum, sugar cane and rice, are not grown on the West Coast.
Fall armyworm is active through the night as adult moths are strong flyers and can travel long distances, especially when assisted by wind.
The best time for farmers to weekly scout for them is when crops are small and young larvae are easier to find in the whorl or cob. This is when they are more susceptible to wasp attacks or insecticide control.
The parasitic wasp Cotesia species is acting as a biocontrol agent against the pest and keeping small infestations in check.
An insecticide with softer chemistry called Sparta is being recommended for aerial and ground control, particularly in the evenings, as other insecticides could remove the wasp.
Mr Mills said they had seen a lot of parasitised activity by the wasp from the West Coast to Northland and this was promising.
