Entomologist Dr Jenny Dymock checks a fall army worm trap alongside a maize crop near Kaitāia. Photo / Mike Dinsdale
The top-of-New Zealand’s biggest maize contractor says news of a potential increase in fall army worm populations is “bloody horrific”.
Whangārei-based Greg Sowry’s comments come as the Foundation of Arable Research warns of the country’s first likely occurrence of four generations of the pest in a season.
Sowry said this was not good news because fall army worm infestations could result in maize crop production loss of as much as 50%.
“It is a definite threat and concern,” he said.
The maize crop pest came from Australia and was first officially confirmed in New Zealand in 2022.
The Christchurch-based Foundation of Arable Research is warning New Zealand will probably for the first time see four fall army worm generations this season that will most likely play out in the Far North.
Its numbers expand as the number of generations increases.
Modelling predicts New Zealand will get up to five generations of the pest in a season.
Globally, the pest likes hot tropical conditions where up to six generations breed in a season.
The Far North’s warmer temperatures make it the country’s fall army worm epicentre.
However, the pest is found throughout Northland and has spread to Auckland, Waikato and potentially Bay of Plenty and Gisborne – plus the South Island’s West Coast and now Tasman.
Doubtless Bay-based entomologist Dr Jenny Dymock identified this season’s first fall army worm moth at Sweetwater near Kaitāia on November 12.
This formally indicated the start of the New Zealand season three weeks earlier than its typical beginning, which Dymock said was due to the pre-Christmas warm weather.
The second generation began on December 27, while the third generation was yet to appear.
Dymock has been monitoring Far North fall army worm numbers for the federation for three years. She monitors Kaitāia district maize crops weekly, reporting the pest’s presence nationwide.
She is working with organisations nationwide to develop models that will predict fall army worm population growth to assist with management and biological control.
Northland maize seed retailer agronomist Mark Dunham said up to 40% of some Far North farms’ maize crops were damaged by the pest in mid-January.
Fall army worm infestation levels varied within paddocks, farms, districts and the region, he said.
Dunham said populations increased as the number of generations climbed.
“You get the situation where there are different life stages from successive generations all happening together. This brings greater densities of the pest,” he said.
The pest is featuring predominantly in the Far North at present and to a lesser extent in Hikurangi Swamp.
Northland Regional Council (NRC) chairman and Hikurangi Swamp farmer Geoff Crawford said farmers should be vigilant about checking maize crops for the pest until the end of harvesting in June.
The fall army worm’s presence has stopped Kaitāia’s Bells Produce from growing local-market sweetcorn after Christmas.
The company’s on-farm manager, Daniel Masters, said after that time, the pest’s infestation levels meant it damaged cobs being sold to the public.
Masters described fall army worm as a “pain in the arse”.
“It’s an added cost to production.”
It started appearing about November-December and could be seen until May, he said.
The pest had also been a factor in the operation shifting away from contract maize production on its Pukepoto property.
Meanwhile, in the northern Wairoa stronghold of maize production for grain rather than silage, the pest has not been as much of a problem, according to Northland’s biggest maize grain producer.
Grainco director David Wordsworth said the parasitic wasp Cortesia ruficrus was starting to turn its attention to eating fall army worm with positive results.
The parasitic wasp was brought into New Zealand in the 70s to control other sorts of army worms, but was starting to include fall army worm in its diet.
NRC biosecurity incursions and response manager Nicky Fitzgibbon said it was encouraging to see early signs of beneficial insects preying on the fall army worm.
Dymock is also researching biological control options for the pest at her Far North base.
She said warmer Far North temperatures meant the pest was overwintering there rather than dying off.
She was looking for where they were doing this.
Sowry said the moths flew over maize paddocks spraying clusters of eggs, which in turn became caterpillars that burrowed into the centre of leaf-growing whorls.
The caterpillars ate the outside of leaves and moved into the growing maize cobs and central leaf whorls from where vegetation grew.
Their faeces caused fungus and bacteria in the maize cob. This led to an overall crop quality drop. It could also mean the silage-making process failed, at worst turning the maize into compost rather than silage.
The fall army worm takes its name from the time of year the pest attacks in the United States, where it originated.
■ LDR is local body journalism co-funded by RNZ and NZ On Air.