It’s morning tea time at Martin Heine’s house beneath Botanical Hill in Nelson, but there’s not much relaxation going on. The cuppas sit neglected as Heine and two friends wield butterfly nets between the vegetables in his garden.

“Got some,” shouts Graeme King, whose bespectacled head appears suddenly from behind the nasturtiums, a look of glee on his face. He offers up his find—not a butterfly but a pair of inch-long caterpillars.

There’s great excitement as the three men inspect the distinctive green, yellow and black crawlers before handing them over to Candy Eason, one of the field operations staff in the Department of Conservation’s newest biosecurity battle.

The specimens are caterpillars of New Zealand’s latest public enemy, the great white butterfly, Pieris brassicae, first detected in Nelson in 2010.

Eason slides the two latest finds and a sample of their food into a small, clear jar with a screw top and puts it into a brown paper bag, labelled with the name of the plant on which it was found, together with the address, time, date, longitude and latitude.

“It’s worrying there are only two, because they usually feed in mobs,” she says. “It looks like the others may have wandered off but we can’t find them here.”

As Heine shows me a tub containing some of the (dead) great white butterflies he and his friends have caught, there’s more excitement: Carl White, sporting an Iron Maiden T-shirt, discovers another caterpillar. It joins the first two in Eason’s paper bag in its own clear jar.

“These butterflies are easy to spot because they’re not just bigger than the small whites, they have more black on the tips of their wings as well as bigger black spots,” Heine tells me as he replaces the lid on his tub of lifeless but­terflies and takes up his net again to rejoin King and White in the garden.

The invaders chose the wrong property this morning­ Heine formerly managed DOC’s biosecurity staff for the Nelson and Marlborough regions.

Typically a northern hemisphere butter­fly, the great white has been reported else­where in New Zealand only once. In 2011 live pupae were found in a caravan that had been imported into Christchurch from the United Kingdom. The Ministry for Primary Indus­tries (MPI) intercepted and destroyed them.

The specimens in Heine’s vege patch will be formally identified and monitored for parasites at the labs of Entecol, the Nelson-based ento­mological and ecological agency contracted by DOC to lead the eradication programme.

So far, this new butterfly pest has not spread far from Nelson city, but the implica­tions of it expanding its range could be devas­tating for New Zealand. The butterfly—which feeds on a range of host plants in the families Brassicaceae, Capparaceae and Tropaeol­aceae—could threaten New Zealand’s culti­vated Brassica such as broccoli, cauliflower, cabbage, brussels sprouts, bok choi, swede, kale, rape, turnip and rocket. in home gardens, market gardens and forage crops of the wider agricultural industry.

However, there is another concern for DOC: a number of already endangered native cresses, herb-like plants, are also part of the Brassica family and likely to be subject to attack.

Throughout New Zealand there are 79 native cresses, nearly 60 of which are already at risk of extinction, and some which haven’t yet been formally described, partly because they are so infrequently encountered. Over 90 per cent of native cresses occur only in this country and 18 are classed as nationally criti­cal—which means there are fewer than 250 specimens remaining, or that their existing habitat is less than one hectare in total extent.

“These could be decimated by this new butterfly,” says Kerry Brown, DOC’s Nelson-based threats adviser, who championed the eradication attempt and is now involved in its planning. “We have a lot to lose if we leave this pest to establish itself.”

The great white butterfly has a diminu­tive cousin in New Zealand, the small white butterfly, itself an introduction in 1929, becoming well established throughout the country within just five years.

“The great white has a similar range of host plants to the small white, which would include these cresses, but the impact would be far more destructive,” explains Brown. “Small white butterflies lay a single egg at a time, which poses less of a threat to the host plant. In contrast, the great white lays its eggs in clusters of anything from 30 to 100.”

When the eggs hatch, the caterpillars stay together and devour the plant, completely defoliating it before moving on. They can roam up to 350 metres without a food source.

“This butterfly is such a major pest in so many other parts of the world, it’s one of the most researched animals on the planet,” says entomologist Richard Toft, founder of Entecol.

European research has found that when populations of this insect reach high densi­ties, they tend to mass-migrate on the wing over hundreds of kilometres. They also have a greater tolerance for cold conditions than the small white, extending their theoretical range to include New Zealand’s rare alpine cresses—which are currently above the alti­tude limits of the small white—even as far as the Chathams and subantarctic islands, each of which host vulnerable endemic species.

In November 2012, having gathered the support of the MPI, Vegetables New Zealand, Tasman District Council, the Foundation for Arable Research and Nelson City Council, DOC was given approval to launch a great white butterfly eradication programme.

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The day I met DOC biodiversity ranger Roger Gaskell, he’d rowed out from Mapua against the strong incoming tide to tiny No Man’s Island, just 16 kilometres from Nelson (as the butterfly flies), where he’s helped establish a habitat for a fragile plant that is arguably most at risk from the great white butterfly incursion. On his arrival, he’d faced the wrath of hundreds of anxious nesting gulls to reach a clump of small coastal peppercress in need of treatment for an aphid infestation.

The nationally critical coastal peppercress, Lepidium banksii, is found only in Tasman Bay, a short flit for great white butterflies.

“Sometimes we’re down to less than 100 specimens of peppercress, and the few sur­viving wild populations are mostly confined to islands managed by us,” says Gaskell. “It has many other threats, too, including the small white butterfly, fungus, aphids and the diamondback moth.”

Since the species prefers low-lying coastal areas, king tides threaten it too.

“The chips are down for this plant, because it’s already lost most of its natural habitat. In the right conditions it grows quickly, but it needs guano-rich soil and the constant disturbance of sea birds so that other plants can’t become established and compete for sunlight. The last thing we need is another threat like this one.”

Also at risk, according to DOC botanist Shannel Courtney, is iconic semi-succulent Lepidium oleraceum, better known as Cook’s scurvy grass, so called, the story goes, because Captain Cook had it served up as a nutritious food to help ward off the disease. Its main stronghold now is in the outer Marlborough Sounds, where predator-free islands generate a habitat rich in bird-life, perfect for these native cresses to thrive.

“This voracious new great white could wipe out whole populations of species,” says Courtney. “The thing that makes the Bras­sica family a target for these butterflies is the chemicals they contain called glucosinolates, which give them their mustardy smell and which the butterflies use to help find them,” he explains.

These chemicals are actually part of the cress’s natural defence against insect attack, but unlike native insects, both small and great white butterflies have evolved to tolerate the bitter glucosinolates and gained access to a nutritious food source.

Two species of native cress have already become extinct, the small white butterfly being implicated in the demise of at least one of these.

Little wonder that DOC is treating the incursion of the larger variety as a matter of priority, even ‘rebranding’ the insect from the ‘large white’, as it is known elsewhere, to ‘great white’ to emphasise the threat by asso­ciation with a man-eating shark.

The MPI, which had been charged with the pest’s management for the first two and a half years following the incursion, had favoured a ‘slow the spread’ approach. In con­trast, DOC, with its wider conservation responsibilities, is seizing the opportunity to pursue eradication through an intensive ground-based programme. This includes widespread active surveillance and a height­ened public awareness programme. On Nel­son’s Port Hills, where the core population of this insect has been identified, every single property will be searched.

An explosion of numbers at the start of summer prompted hundreds of calls from the public and a demanding schedule for the team.

Until then, two factors had restricted the pest’s spread. One was a parasitic wasp intro­duced to the country in the 1930s as a bio­control agent for the small white butterfly. Ironically, it is better suited to parasitising the caterpillars of the great white. The wasp lays its eggs inside the caterpillars. These eggs hatch and then chew their way out before their host has a chance to pupate. More than 50 per cent of great white caterpillars tested have been found to contain parasites, a rate that is expected to be higher in autumn.

The other factor slowing the spread, some believe, is that the great white came from the northern hemisphere and had its biological clock back to front. Most butterflies emerge from chrysalises in spring, later producing eggs and caterpillars, but the great white, in New Zealand, had been most proliferate during autumn, with no sightings at all over mid summer.

“By the end of winter 2012, unfortunately, it seemed to have adapted to the change of seasons,” says Brown, who notes that more butterflies hatched in spring and then formed second-generation chrysalises that also hatched in spring rather than in autumn— something not detected before in New Zealand.

Just two days before Christmas, new pos­itive sightings were confirmed beyond the six-kilometre radius within which the butterfly had previously been found. Two locations were confirmed eight kilometres from the port, swiftly followed by another four sightings 12 kilometres away in Richmond, a short distance from the commercial Brassica fields.

More DOC staff were drafted in to help with active surveillance. Bearing in mind that in some parts of the world (such as the Mediter­ranean region) these butterflies can reach up to five generations annually, the team—who were working to a budget of only $300,000 a year—were facing a grave situation.

Their efforts, however, were welcomed by commercial Brassica growers.

“We are always concerned about any incursion of pests,” says Mark O’Connor, a director of Vegetables New Zealand whose 130-hectare market garden lies just 13 kilo­metres from Nelson in Appleby.

O’Connor’s firm, Appleby Fresh, had dis­patched more than 2000 crates of produce, including cauliflowers and cabbages, as far as Auckland and Invercargill the day I accompa­nied him to inspect some of his cabbages. They were growing at one of his blocks just over two kilometres from the furthest positive sight­ing. Small whites flitted about in abundance but fortunately for O’Connor, a fourth-gen­eration market gardener, the only large white he’d seen was a dead specimen used as part of a demonstration during a meeting.

“We’re already dealing with the small white and other things like aphids and having to spray for those. There’s a good reason, though, to be concerned, because if this new butterfly gets out of control, what do we do then?”

Around 4300 hectares of vegetable Bras­sicas are cultivated throughout the country, worth approximately $90 million per year. There has also been a rapid expansion in pro­duction of forage Brassicas, mainly for dairy production, accounting for a further 230,000 hectares annually. All these crops would be targeted by the great white butterfly.

Although it’s expected spraying cam­paigns for the small white will protect against this new invader also, extra spray cycles would be needed, since the great whites have been emerging earlier in the year and con­tinuing later than the small white.

However, just as the eradication programme appeared to rest on a knife’s edge between control and runaway proliferation towards commercial Brassica crops, the insect’s relent­less march slowed almost to a halt.

The great white butterflies had gone into a sort of hibernation state, called aestivation, in response to high temperatures, which on Christmas Day reached 30ºC in Nelson.

“Aestivation occurs in summer rather than winter and involves a slowing or pause in activity at the chrysalis stage,” explains Richard Toft. “This happens in response to excessive heat or drought. The metabolism slows right down and the organism enters a state of dormancy rather than continuing to be active in periods where environmental conditions would put them at risk.”

This may have begun early in December for some, while for others it may have been towards the end of December or even later. For a few, it might not occur at all—those in damp, shady gullies may still reach maturity.

By late January, there were very few reports of eggs, caterpillars or butterflies, leading the team to conclude there were lots of pupae waiting for the onset of shorter days and cooler temperatures.

“We wouldn’t expect a whole population of organisms to adopt the same strategy unless conditions were really extreme,” says Toft. “It’s like financial planning. When the share market is in serious decline, money moves into gold, which is like financial aes­tivation—a safe way to rest up until the situ­ation improves.”

As a result of the rapid reduction in posi­tive sightings, the team were given a chance to log and assess their data, gleaning clues to the butterfly’s behaviour, and re-arm them­selves ready for an anticipated mass re-emer­gence of butterflies in autumn.

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It’s 7.30AM on a grey, drizzly summer morning at the headquarters of Nelmac, a contractor which maintains gardens, parks, reserves and public spaces on behalf of Nelson City Council. The 60 gardeners and conservation rangers are the newest draftees in Toft’s army. As well as their constant vig­ilance, the programme’s success depends heavily on the public to report findings via a hotline managed by the MPI.

Toft’s team respond to each positive finding in Nelson by removing the eggs or caterpillars, then treating the host plant with Spinosad, an organic insecticide. They re­visit 7–10 days later to check again.

Active surveillance is initiated 150 metres around that location, with neighbouring properties inspected by the team under the powers of the Biosecurity Act 1993, which gives them rights of access to locate unwanted organisms. Each new find sparks a fresh sur­veillance area with its own 150-metre radius, except on the outer boundaries, where a new sighting initiates a search up to 200 metres from the location.

“We want to stop that outward spread because if it gets into the Waimea Plains beyond Richmond, and the commercial Bras­sica fields, we’re basically stuck,” says Toft.

Part of the eradication programme includes host-plant management, particularly around nasturtium, which grows abundantly in Nelson and which, in sheltered, overgrown gardens, has proved to make ideal breeding sites. (Although nasturtium isn’t a member of the Brassica family, it also contains glucosi­nolates which attract the butterflies.)

During early summer, ‘sentinel sites’ that were easily checked were kept in place to help prevent the butterfly moving outwards in search of new food sources. Once the insect moved into aestivation, emphasis was shifted to removal of as much of this plant as possible in preparation for autumn.

With the anticipated mass re-emergence DOC updated its message, encouraging people not just to report sightings but also to kill great white butterflies.

“The eradication window is still definitely open but I really believe we couldn’t have left this any longer,” says Martin Rodd, area manager at DOC, who along with incident controller Bruce Vander Lee is heading the programme. “Even if people aren’t sure whether it’s a small or great white, our message now is to kill the butterfly on sight. It might sound harsh but it’s what’s essential if we are going to be able to do what’s neces­sary. We can’t afford to take any chances.

“Ultimately it’s not about DOC or the part­ners of the operational team, it’s about the res­idents of Nelson and Tasman deciding that they don’t want this thing and working together to eradicate it.”