One summer morning near Ruatoria on the East Coast, Doug Lambert wandered across a paddock to little Waiomatatini School, where he was principal. It was the Christmas holidays, but the diesel generator—which also powered his home—had been on the blink. All around, maize rustled on tall stalks, the cobs plump and sweet, ready to be harvested for cattle fodder. The schoolyard was quiet, but as Lambert rounded the buildings, he spotted something he’d never seen before and hasn’t since: an army of caterpillars.

“They were everywhere, hundreds and thousands of them. It was sort of like a biblical crisis.”

The caterpillars, dark green with pale markings and about two centimetres long, were heading northwards. “They looked like an army on the move, but moving at a snail’s pace.” Lambert watched as they crossed the playing field then onto his property, where some attempted to climb his weatherboard house and others headed straight into the chicken run, meeting hens lined up behind the wire netting, ready to gobble them up. The majority, though, headed into the maize paddock next door, “like a plague”.

By the next day, the caterpillars had all passed. Lambert could only imagine they had marched, and munched, right through the night. Amazed by what he’d seen, he called authorities in Gisborne, who knew instantly what he was describing: armyworms. It’s likely these were cosmopolitan armyworm (Mythimna separata), which once regularly wiped out maize and sweetcorn crops. That same year, 1972, a tiny wasp was introduced from Pakistan to kill the armyworms, and for a while, it felt like we’d mostly won the war against them.

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This March, Rhonda and Sam Mojel started nightly patrols of their Auckland vege garden. Tropical armyworms have descended on the plot the Mojels have tended for more than 30 years. Their beds of spinach and kale, which usually produce more than they can eat, were stripped almost bare. For weeks, the couple hunched over in the dark, picking caterpillars off their leafy greens, until Rhonda decided it was enough. Their foe was too numerous, its ranks seemingly endless. “I love insects,” she says, “but this feels like war.”

Liz Heape is a stomper, and she stomps hard. She grows vegetable seedlings in Dargaville, north of Auckland, which she sells to home gardeners. Her pest-control philosophy? “Be brutal. Be a murderer of insects.” Earlier this year, armyworms ate a great many of her seedlings; the only things they didn’t eat were spring onions and leeks. Though Heape stomped on all the caterpillars she found, more kept appearing—and eating.

Further north, Bells Produce grows kūmara, citrus, maize and other crops on 351 hectares of prime land in Pukepoto, Kaitaia. Farm manager John Reed has been in horticulture for 40 years, but last January he was caught off guard. “We went out to the maize crop and it was just literally crawling—it was just like a plague.” Armyworms, which had not been a problem for decades, had been multiplying under the frost cloth.

By the time Reed and his team found the hordes, “it was too late. The damage was done.” They tried spraying, but two days later, more larvae would turn up. The team gave up on that block, then relaxed a little as the winter chill seemed to see off the invaders. In October, they peeled back the frost cloth covering their early sweetcorn plants and found caterpillars there. “We were like, ‘Oh shit.’”

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In Maungatapere, just outside of Whangārei, the Salisbury family runs Old Wheelbarrow Farms, growing all manner of vegetables in long rows protected from the wind by lines of banana trees. On Saturday mornings, they sell carrots, kale, taro leaves and other seasonal produce at the Whangārei Growers Market. They’ve found some armyworms on the farm, but not too much destruction. Dawn Salisbury says to account for bugs eating some of their produce, they grow extra. She sees bugs as part of the ecosystem of the garden, and prefers not to spray. The leaves on the table at the market are pristine, being the pick of the bunch.

Murray and Nicci Burns enlisted frogs in their war on caterpillars. In Kauri, north of Whangārei, surrounded by what was once pastoral land and is now lifestyle blocks, they grow all manner of leafy greens in a spray-free, outdoor hydroponic system. In 2022, the couple noticed bright green frogs living in their greenhouse amongst the basil, hunting caterpillars, so when they saw armyworms on their outdoor crops, they popped one adult frog in each net-covered bay. It worked—until Cyclone Gabrielle blew off the nets, the frogs moved on, and a legion of caterpillars proceeded to strip four weeks’ worth of newly planted seedlings. As a result, HydroHealthy has been missing from the Whangārei Growers Market. They’ve had nothing to sell.

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This summer, armyworms were once again spotted marching—along footpaths, driveways and even the road. News of their presence has spread through gardening groups and communities, with gardeners at first shocked to find their plots devastated, then frustrated that these caterpillars are such formidable foes. “They’re the worst thing that could have happened,” says Nena Rogers, a gardener in the small community of Ōakura Bay, in Northland. “The worm has just demolished everything.”

Across Northland, armyworms have stripped vege gardens bare, emptied out greenhouses, and made their way indoors, making light work of houseplants. In Whangārei Heads, they’ve laid eggs on the sides of boats, and in Glinks Gully, fluffy egg masses have appeared on houses, washing lines and cars.

The life of an armyworm is tied to heat—the warmer it is, the more they eat and the quicker they reproduce, build up numbers and spread. A warm, wet summer has resurrected an old threat.

There are three types of armyworm in Aotearoa. Cosmopolitan armyworms, which Doug Lambert saw and which are eaten by the introduced wasp; tropical armyworms, which the Northland gardeners and farmers have been battling over the summer; and fall armyworms, which only just got here. Despite the name, they’re actually all caterpillars—and hungry ones at that. They’re notorious crop pests around the world, especially the new arrival.

The fall armyworm’s favourite foods are corn and maize, but it has a taste for a multitude of other plants, depending on what’s available. We don’t yet know what it might eat here, and DOC is worried native plants could be on the menu. Hundreds have been earmarked as potential hosts for the fall armyworm, including kawakawa, tarata, kiekie and kūmara, as well as the endangered ngutukākā. So far, no infestations have been spotted on native plants, but it seems no one has been specifically tasked with looking out for them, either.

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One night last March, a tired brown moth flitted around Tauranga, looking for places to lay thousands of eggs. She landed on a slippery green surface, laid a mass of about 200, covered them in furry beige webbing and took off.

On a clear day soon after, a field officer was routinely checking a spongy moth trap. The trap is designed for moths to fly into, but this time, stuck to its outside was a suspicious fluffy splotch—some insect’s audacious “I was here”. The egg mass was sent to a lab to be identified. On March 24, it was confirmed as the calling card of an insect classified as an unwanted organism—one that could “harm New Zealand if [it] arrived”: the fall armyworm.

The fall armyworm has long been waging war in other parts of the world. Its home range is from Argentina to Mexico, where it eats 20 per cent of all maize grown. In the past seven years, it’s invaded almost 50 countries. Not one has been able to eradicate it. In 2020, the caterpillar reached Australia and pinged Biosecurity New Zealand’s radar. Soon, our local farmers were receiving flyers on how to look out for it on their crops.

The soft brown moth that laid the alarming egg mass probably flew here from the east coast of Australia, and could have laid 2000 eggs that night, spread over 10 or so masses. Although Biosecurity New Zealand was braced for its arrival, it thought the armyworm would struggle to thrive here, “as areas with preferred hosts do not necessarily have the correct climate to suit fall armyworm”. That was strangely optimistic: Northland, in fact, has a combination of the perfect weather and favourite food.

Two weeks after the initial egg mass was discovered, a Waikato grower found big, juicy caterpillars on his corn plants. Nine days after they were confirmed to be fall armyworm, Biosecurity New Zealand deployed staff to inspect the surrounding area. They found fall armyworm in another field two kilometres away. The troops had landed and were claiming territory.

By the end of April, the caterpillars had also been found in South Auckland, Waitara and Gisborne. Farmers were sent monitoring traps and instructed to use insecticides or destroy plants on which fall armyworm was found. A public campaign encouraged people to report sightings, and that autumn, fall armyworm was found in 25 more locations. Meanwhile, a collection of government agencies started studying the impacts of this pest in Aotearoa.

Entomologist Jenny Dymock set out to lure male moths with the scent of female moths, in an attempt to track the population in the Far North. Her traps were positioned on the edges of six corn and maize fields, the “absolute favourites” of fall armyworm. From November to April, she checked them every Wednesday—rain, shine or cyclone. No week could be missed, because then she wouldn’t have the data to make “a nice beautiful graph” of their numbers.

The traps look like children’s sand buckets tied onto metal stakes about a metre and a half from the ground. Inside is a ton of fly spray and an old, soft piece of egg carton—it’s a measure of comfort, in the hopes the moths will stay calm and rest on it, rather than thrash around and destroy their wings, which makes them difficult to identify. It’s common practice among entomologists, says Dymock. She remembers some particularly tatty moths that she sent away to be identified. “They dissected the genitalia, dare I say it. They even sent me photos!” It was more than she wanted to know.

Over the summer, Dymock’s graph zigzagged. At first she found nothing. In mid-January, she hit a peak of 50 moths, bottoming out again three weeks later. Then came a second sharp spike: 90 moths caught in mid-March. These booms and busts follow the generational cycle of the bugs. The quiet periods are the enemy gathering strength—eggs hatching, caterpillars pupating, adult moths emerging. With each generation, the population multiplies exponentially, growing into an army.

By May, Dymock’s traps were empty. The armyworms went somewhere, and no one knows where. “It’s like trying to find a needle in a haystack,” says Dymock. She thinks the pupae could be hiding in the fresh shoots of corn seedlings, down the twirled leaves inside their straw-like centre. She checked a few maize clusters, but there are no caterpillars, no pupae, no moths and no eggs—only dew. There are questions for Dymock that only time and observation will answer. “We scientists like to have lots and lots of data, years and years over heaps of sites, and we just don’t have that here.”

She hopes that next year, after more research and monitoring, scientists will be able to predict when the pests will be out and about. Then, farmers can prepare. They must, because by April 2023, it became clear that fall armyworm was widespread, particularly in the North Island, and Biosecurity New Zealand conceded that eradication was probably impossible. Sightings are no longer to be reported, because there would be too many to deal with. Fall armyworms are here to stay.

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A tiny moat, the width of a shoe, surrounds the single-storey building that is Plant & Food Research’s “bug shop” in Mt Albert, Auckland. Here, Anne Barrington has been rearing insects for 36 years. In room five, plastic containers you could keep your lunch in line the shelves. It’s always 23°C and the lights are on from 6am to 10pm. For 20 years, this room has been home to a colony of tropical armyworms.

The colony is separated out by life stage, in containers dated with felt tip. The tiniest caterpillars, whose dark heads look like poppy seeds, are clumped in their hundreds on top of what look like thin slices of tofu; it’s an artificial diet infused with antibiotics that’s made down the hallway. Barrington cares for this colony on Mondays and Thursdays, feeding, sterilising, and moving the creatures from stage to stage. The final stage is the freezer, or the room across the hall. There, tropical armyworm caterpillars are fed to native stink bugs.

The bugs have a mouthpiece like a sucking straw. They pierce the caterpillar and suck out its guts, leaving an empty sack.

These armyworms help us understand what we’re fighting and how our weapons will work. They’re used to test new insecticides and as controls, to figure out changes in wild populations. Entomologist Frances MacDonald is hoping to start a similar colony of fall armyworm. She wants to do feeding experiments, to see which native plants they find tasty.

MacDonald’s specialty, biocontrol, provides an avenue of real hope. Already, parasitic wasps, the same species that keep the cosmopolitan armyworm in check, have been attacking the fall armyworm. These wasps, Cotesia ruficrus, lay dozens of eggs inside each caterpillar. “As those eggs hatch, the [larvae] munch through the stomach of the caterpillar, then make an exit hole out of the cocoon. New wasps emerge, and then they go off and do the same thing.” Growers here have reported seeing cocoons torn open from the inside by hatching wasps.

In Australia, a legion of entomologists collected fall armyworm eggs, larvae and pupae, reared them, and watched. Over 20 species of parasitoids burst their way out, including the very same Cotesia ruficrus. Researchers hope that in time, natural predators will keep armyworm populations down, but it’s not about standing back and hoping the wasps turn up. “We’re not saying growers shouldn’t spray,” says MacDonald. We don’t yet know how effective the wasps will be—especially as the climate warms to benefit armyworms.

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In Devonport, Rhonda and Sam Mojel are making plans for next season. As soon as they plant out seedlings, they’ll cover the beds with a nylon netting so fine not even just-hatched larvae could pass through it. “It’s going to be a fortress,” says Rhonda.

Liz Heape found a bacterium that kills the caterpillars, Bacillus thuringiensis. Spraying with this once a week has kept their numbers right down. She’s been passing on pest-control knowledge to home gardeners who come to the Whangārei Growers Market for her seedlings. Many of them have had to replant their gardens “again and again and again”. Others have given up.

Murray and Nicci Burns in Kauri are steering clear of sprays, because frogs are sensitive to them. They’re focused on learning what makes a happy frog—shelters, wriggling offerings from the worm farm—so that the frogs will stay. And they’re enjoying the nightly chorus of croaks. “It’s beautiful, I love it,” says Nicci.

Some of the team at Bells Produce questioned whether they’d be able to continue growing maize and sweetcorn, or if armyworm would always destroy it, but in October, they sprayed before the fall armyworm had a chance to multiply and infest their crops. “It was nowhere near the drama that we thought,” says John Reed. All that was needed was knowledge, and a tweak. “Once you spray ahead of them and you keep [that] cycle, you keep on top of it, no problem.” They continue to monitor and spray only when necessary. During summer, they grew 800,000 cobs of sweetcorn. As is their habit over winter, the armyworms seem to have vanished.

The Salisburys’ stall at the Whangārei Growers Market has been busy recently; sometimes, almost all of their veges are sold by 8am. The sweet orange carrots, sold in bunches with bright green leaves, have been a particular hit. In May, Dawn was in the greenhouse planting trays of winter veges—spinach, kale, lettuce, broccoli. She lined up some seedling trays, and tipped out a bucket of soil and softly decomposing leaves that had been sitting in the warm greenhouse. Then she looked closer. Strewn all through the soil were dark, reddish-brown pupae. When she poked them with her finger, they wriggled.

Winter is coming

Cold can stop the fall armyworm in its tracks—but many areas are already warm enough for it to survive the winter. These hotspots will act as reservoirs, fuelling the pest’s spread. Where are those hotspots, exactly? Short answer: we’re not sure. And the boundary lines will shift as the climate does.

“It’s going to be more of a pest some years than others,” says Craig Phillips, a senior scientist at AgResearch. “It’s going to be a little bit variable and sporadic, but it is going to be a headache.”

It’s clear that Northland is warm enough for fall armyworm to establish year-round and spread seasonally. In the US, Florida stays warm enough for the pest to overwinter, and from there, reach Canada in summer. Its ability to spread so far “is quite spectacular biology really”, says Phillips. He’s impressed by a statistical model used overseas that works on the basis that if the air stays warmer than 10°C, the fall armyworm will usually truck on through winter. This model predicts that the pest could make it through winters in Auckland northward. There’s also a band around the North Island and the top of the South Island where they will be able to scrape through in warmer winters. This is an area that will expand as the climate warms.

Monitoring work happening in Northland and Hokitika this winter will be partly guided by these predictions. Phillips hopes there will also be money for monitoring in places the fall armyworm is not expected to survive; that would let him tweak the temperature threshold on the model, and give growers and gardeners an early heads-up of invasions to come.

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