The munching invader

UNWANTED: The male great white butterfly.
UNWANTED: The male great white butterfly.

Wow, it's 2013. How did that happen? The last time I looked, it was 2000 and something. That's how life goes, each year going "faster" as a smaller percentage of the journey.

And, of course, we should remind ourselves that it's the journey, not the destination, which is the essence of it all. Unless you're Buddhist, of course, and hope to come back as a higher being. I've always fancied coming back as my cat, because she has such a lovely, leisurely life, dozing on the sofa or sleeping in the cool of the garden shade in summer.

She's been watching me this week from under the hydrangeas as I get ready to plant winter brassicas. You should be planting yours this month so they romp away in the warmth of summer, ready for autumn and winter harvests.

But, without wanting to alarm you, life in the brassica patch may never be the same if the ravages of yet another alien in our midst aren't stopped right now.

While not as headline-grabbing as, say, the fiscal cliff debate in the United States, it is nonetheless much more insidious, with potentially lasting effects.

It's a silent, creeping, crawling, floating, munching invader, possibly already in a garden near you. It's called Pieris brassicae, the great white butterfly.

Its caterpillar is a super-sized eating machine that mob chomps its way through anything in the cabbage and other related plant families, and makes the regular-sized white cabbage butterfly look like a pussycat in comparison. And you should be afraid.

It's a gardeners' and growers' nightmare and, if it escapes beyond central Nelson, could also be an ecologist's nightmare, wiping out native brassicas. All would be vulnerable to the ravages of this great white invader.

And why should you care about our native brassicas, most of which you've probably never seen or even heard of? Well, as Department of Conservation botanist Shannel Courtney says, native brassicas and cresses are a significant part of our indigenous flora, comprising our eighth-largest plant family, Brassicaceae, with several species yet to be named.

The family also includes the weird and wonderful alpine penwiper plant and Cook's scurvy grass, Lepidium oleraceum, made famous by Captain Cook, whose crew ate it to prevent scurvy.

Although it was once widespread around the coastline and outlying islands, Cook's scurvy grass is now limited and classified in botanical terms as "nationally vulnerable".

Closer to home, Lepidium banksii, native to only a few discreet locations in Tasman Bay, could well succumb to attack and become extinct, as two other species did earlier last century, if the great white butterfly finds it.

Of our 79 species of native cresses, 57 are on the threatened plant list, with 18 classified as "critical", meaning there are fewer than 250 individuals left in the wild or their total habitat is less than a hectare.

What's also alarming about the great white butterfly is that it's more cold-tolerant than the regular one, which has only rarely been recorded in alpine environments. The frightening implication is that for the first time, our endemic penwiper genus, Notothlaspi, could meet its match in its alpine habitat where, until now, it's been safe from attack.

With its tolerance for cool temperatures, longer, more prolific breeding season, and stronger flight than the white cabbage butterfly, it's conceivable that the great white invader could get as far as the Sub-antarctic and Chatham islands to devastate their flora.

Losing more native species to extinction might not mean much to some, but would be more links missing in our country's story. "If it was the kiwi, everyone would say, ‘Crikey'," Courtney says.

But, even if the bleak botanical outlook doesn't push your buttons, the great white has huge implications for home gardeners and vegetable growers.

Unlike the white cabbage butterfly, which mostly lays solitary eggs, the great white lays eggs in clusters which then hatch and mob feed, skeletonising every brassica in their path.

Found in many parts of the world, including Britain and Europe, the great white butterfly is attracted by the smell of brassicas, the magic mustard oil we're told we should be eating as part of the health benefits of vegetables such as broccoli, cabbage, cauliflower, brussels sprouts, rocket, cavolo nero and watercress.

Great whites also go for alyssum and honesty, two valuable attractants for beneficial bugs in the garden, and nasturtiums, another crop useful for repelling some unwanted insects in the garden.

The great whites will also eat plants in the Capparaceae family, including cleome, which can be grown in the vegetable garden as a sacrificial plant for stink bugs. But if you're trying to garden organically using beneficial plants such as cleome, alyssum and nasturtiums, or if you grow honesty, Lunaria annua, to attract beneficial hover flies to the early spring garden, you might instead attract great white butterflies.

And imagine if you're a commercial grower. The brassica vegetable crop alone is worth about $80 million a year to the country. Great white butterfly caterpillars also eat forage brassica crops grown to feed stock.

Although it was discovered in Nelson two years ago, it's still within a limited radius of Port Nelson, so eradication is still "quite achievable", Courtney says.

Despite this, the Ministry for Primary Industries abandoned moves to eradicate it, saying the cost-benefit analysis wasn't accurate enough. Instead, DOC has stepped up to lead the multi-agency campaign.

The department is taking its cue from the white cabbage butterfly, which spread more than 200km within two years of being discovered here in 1929.

The fear is that if the great white gets out, there will be little chance of stopping it - which is why gardeners are asked to come together to find it and, hopefully, get rid of it for good.


Help with the hunt for the great white butterfly and caterpillars which may be lurking on brassica plants and nasturtiums in your garden. If you suspect you have the great white butterfly caterpillar in your garden, phone the Ministry for Primary Industries hotline on 0800 80 99 66 immediately.

The adult great white butterfly looks similar to the small white butterfly, though it is about twice the size. A distinctive difference is that the great white's caterpillars are mostly found in tight groups, whereas the small butterfly's caterpillar is more often found on its own.

The great white butterfly lays its yellow eggs on host plants in batches of 30 to 100 eggs. In contrast, the small white butterfly lays its more cream-coloured eggs singly or in pairs. Research is being carried out to help with eradication and management of the great white butterfly.

The Ministry for Primary industries is conducting research on detection and surveillance tools (eg lure and kill trapping) to help with the eradication of this pest. Information supplied by the Department of Conservation.


Plant brassicas and leeks in the vegetable garden now for autumn and winter harvests. Dig and divide and replant bearded irises any time this month.

Lift garlic, main crop onions and shallots on a dry, sunny day and leave hanging in a dry place for the tops to completely die back and the bulbs to cure.

Summer prune stonefruit trees to remove unwanted "water sprouts". Cut back herbs that are going to flower, such as basil and oregano, to promote renewed leafy growth.