Driving through an explosion of white butterflies

What was left of Dave Craig's vegetable patch, after the white butterfly caterpillars had their fill.

What was left of Dave Craig's vegetable patch, after the white butterfly caterpillars had their fill.

An explosion in the population of white butterflies in parts of New Zealand can make even short drives a messy business.

On the outskirts of Palmerston North Dave Craig has given up trying to grow cauliflower and broccoli and pulled the plants out.

The white butterflies were everywhere outside. "(Natural insecticide) Derris Dust is like candy to these guys this year," he said.

Hundreds of white butterflies spotted above a paddock in Temuka.

Hundreds of white butterflies spotted above a paddock in Temuka.

"It's pretty evident when you're driving, they're flying across the roads."

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During a drive home he "must have smashed 50 or 60 of them".

Roger Harper who commutes to Ashburton each day said numbers were higher last week when there were "clouds of them". "The front of my car is testament to that."

Lisa Rogers who works in Timaru said, "there are millions of them - flying up at the side of the road, must be eating all the kale crops".

Commenting on a report that white butterfly numbers were three times higher than usual in South Canterbury, one reader said the pests were all over the car after a trip.

"... while in Amberley I pointed it out to my partner there's so many moths around . My new car grill took a large number out," the reader said.

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Hayden Lewis said Rotorua had also seen a big rise in white butterfly numbers. "They seem to mostly like my kale plant," he said.

"I've resorted to every couple of days having to squishing all the bloody eggs and the caterpillars otherwise I'd have nothing left."

Cathy Mountier said she had been astounded by the number of white butterflies she saw a few weeks ago in sub-alpine vegetation in the Mt Holdsworth, Wairarapa area of the Tararua Ranges.

"I have never seen a throng of them up high like that before. There were also hundreds and zillions of them in the valley floor by the river," she said.

Reports of exploding white butterfly populations have also come from Central Hawke's Bay, Central Otago, Oamaru, Tokoroa, Taupo, West Coast and Upper Hutt. 

Plant & Food Research entomologist Graham Walker said a range of factors probably contributed to the high numbers of white butterflies but the warm summer was probably the main cause,.

"It's not just white butterflies, we're getting lots of reports of pests in a lot more crops," he said.

The unusual warmth meant that in a six to seven week period, there could be one more generation of the insects than normal, with each generation producing far more offspring than the  one before.

Another factor could be the amount of crops - of the kind eaten by white butterfly caterpillars - grown for the dairy industry.

Around 300,000 to 400,000 hectares of fodder brassicas such as kale, turnip and rapeseed were grown each year. "It's the biggest crop in the country."

Possibly the introduced small wasp that normally killed most white butterfly caterpillars hadn't done so well this summer, Walker said. One reason for that could be heavy rain, which could kill up to 80 per cent of the insects living in or on plants.

A further factor might be timing of pesticide spraying, with growers encouraged to only spray when pests became too bad. "So there's been a window of opportunity in this warm weather, and the pest's gone crazy."

Dr Robert Hoare of Landcare Research said one of the problems with butterflies and moths was that little monitoring was done so most indications of numbers was anecdotal.

Efforts were under way to try to get a better idea of native moth numbers, with many species thought to be declining. While the warm summer probably meant some native species had a good year a range of factors, such as habitat loss and predation by introduced predators, might counteract any breeding increase.

Of around 2000 species thought to live in New Zealand, about 85 per cent lived nowhere else.

Few were thought to be extinct and their breeding capacity meant they should be able to bounce back if predators were controlled and natural habitat was reinstated, Hoare said.

Among the more spectacular and better known of the native moths, the large, bright green puriri moth, was probably doing okay because it had few predators.

 - Stuff

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