Their endurance has defied initial assessments they would struggle to get through the winter months.
First detected in March 2022, the fall armyworm has persisted in warmer regions and was expected to overwinter in Northland, but not this far south.
Lincoln AgResearch scientist Craig Phillips said the tropical insect species was surprisingly hardy in cooler climates.
"It is easy to forget that continental places like Florida and the southern United States can get much colder in winter than many parts of New Zealand. It is that intense cold in mid-winter, even in tropical areas, that kills fall armyworm."
New Zealand’s temperate, oceanic climate did mean it could survive "fairly easily", he said.
Dr Phillips told 50 researchers and industry people attending FAW research workshop last month a model indicated it could overwinter as long as the mean annual temperature did not drop below 10°C.
Modelling showed the armyworm was less likely to survive further south, and further inland. As New Zealand gets warmer, its year-round distribution was expected to move southwards, inland, and to higher altitude areas.
Experts wonder if climate change or unmeasured microclimates are also at work.
Lincoln AgResearch scientist Scott Hardwick said consistent populations had continued in Westland throughout the growing season over the past three years.
"There is definitely overwintering there; either that or there is a constant flow into that region and I don’t think that is the case. However, as it is not as warm as more northern regions, larvae are not growing as quickly, only getting to two generations over the growing season, with potentially a third generation lasting over winter."
Tasman was an interesting case with moderate infestations throughout the growing season.
Infestation starts early in the season and is ongoing. The region grows a limited amount of sweetcorn, along with maize for dairy and grain.
The pest probably reached four generations in Northland in the 2024-25 season, the most since it was first detected. Young sweetcorn and maize crops required replanting in some cases after early-season damage and late-finishing grain crops were also impacted.
Reports of crop damage were lodged in Auckland, Waikato, Bay of Plenty and Taranaki. Gisborne also had significant damage in late finishing sweetcorn.
Mr Hardwick said more research was needed on FAW’s dispersal and its overwintering hosts. — APL
