The corn-consuming moth was found for the first time in Hokitika maize crops and, late last season, in a sweetcorn crop in Canterbury.
Maize-growing in the eastern province remains free of the pest, but growers are bracing themselves for the first outbreak.
While trap monitoring and scouting in Northland and the West Coast have shown no moths or obvious signs of larvae overwintering, that is expected to change soon as the weather warm up.
The Foundation for Arable Research is making use of this early window to alert growers of the need to inspect crops for early damage before numbers build up.
Business and operations general manager Ivan Lawrie said the life-cycle of fall armyworm slowed down over winter and no activity was found, as expected, in southern regions.
A visit to Northland had also revealed no sign yet after moth capturing traps were placed surrounding the earliest sweetcorn crops , he said.
"The last capture of a moth was in early winter and as expected there’s been very little activity since then. On inspection of early crops we haven’t found any larvae damage yet or early presence of larvae, but it is expected as spring progresses we shall start to see some."
A colder and wetter winter was likely to suppress their re-emergence for a later start. To cause economic crop damage their numbers had to build up to at least three population cycles in maize or sweetcorn, he said.
Mr Lawrie said a warmer and drier spring would accelerate their build up.
"We are particularly concerned for crops that go in later, as they are the ones that tend to be more vulnerable. [In] the earlier sown crops usually we might see only some damage in the early stages from a few individual caterpillars.
"The recommendation is not to rely on the traps, but to actively monitor all your crops. We are asking sweetcorn and maize growers to go and scout their crops in the early stages. If they find fall armyworm damage it’s really important to think of early control."
As temperatures rise, the life cycle will begin to speed up, with any winter larvae that have survived pupating, and early moths laying egg masses.
"We already know from last year’s experience that there are pockets — especially on the West Coast of the South Island — that are frost-free and that’s where we spotted fall armyworm last year in maize crops in and around the Hokitika regions."
Mr Lawrie said there was no point in being alarmist about it coming to Canterbury maize crops, as there were other maize pests.
"We need to be very clear about the messaging here. Fall armyworm is one of the pests that can attack these crops and there are others. There’s the cosmopolitan armyworm and corn earworm and so these pests are also present in these crops and can be easily confused even by experts. It’s important to take good photographs and report them in, but more than anything, detect any sign of damage in small windows in the early crops stages in the leaves — that’s the best way to grab them and address the problem early."
The FAR would be keeping growers updated in weekly emails.
A parasitic wasp from the Cotesia species was "quite promising" against fall armyworm and the vegetable pest tropical armyworm last year, he said.
Growers were reminded to only use the insecticide Sparta for aerial and ground applications to control them. Fall armyworm had resistance against synthetic pyrethroids and organophosphates and spraying crops would likely knock out the beneficial wasp, he said.