Mild winter feels good but scientists see its downsides

The giant willow aphid arrived just over two years ago but has rapidly spread around the country.

The giant willow aphid arrived just over two years ago but has rapidly spread around the country.

Warmer weather is proving a mixed blessing down on the farm.

While it may make working more pleasant, the continuing fine spells mean poorer quality feed, the spread of destructive bugs, and the failure of fruit to achieve high sugar levels.

Niwa forecaster Dr Chris Brandolino said the six months to June were likely to be the warmest for more than a century, although the agency is predicting cold southerlies to arrive at the end of July.

Where the willow aphids make honey dew, the wasps follow.
KATIE JONES

Where the willow aphids make honey dew, the wasps follow.

AgResearch scientist Dr Warren King said while pasture may look good, it is just a cosmetic effect.

READ MORE: Warm winter may finally be on the way out

"To the naked eye, grass is grass. If our winters are a bit warmer we might get extra pasture growth but we simply don't have the day length or sunshine hours to drive good quality pasture growth. Typically what winter growth you do get can be short on carbohydrates.

Northland farmers last month discussing pasture management. Grass growth may appear to be good in a mild winter, but it ...

Northland farmers last month discussing pasture management. Grass growth may appear to be good in a mild winter, but it lacks the high energy of spring pasture.

"Farming in New Zealand is a business of turning solar energy into protein and if you don't get that solar energy then everything's compromised. You won't be able to detect a visible colour change and the grass is fully hydrated, but you cannot detect changes in energy content visually," King said.

An expert in the damaging South African black beetle, King has noted a southward movement of the pasture pest in recent years.

Its distribution has been up until now north of Taupo, but along coastal areas it has spread as far south as Foxton.

Warren King from AgResearch with a giant size model of a black beetle, a pasture pest which is moving south as ...

Warren King from AgResearch with a giant size model of a black beetle, a pasture pest which is moving south as temperatures rise.

"Most insects would shut down at 10 degrees but black beetles are closer to 15. We need cold wet weather to drive the population down, and if we're not seeing those prolonged cold spells, then we're setting ourselves up for a really difficult summer," King said.

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Hawke's Bay farmer Bruce Wills has noticed the impact of the destructive giant willow aphid as it has continued to spread around the country, spurred on by warmer than usual weather.

As well as threatening to kill the trees, the aphids produce a honeydew which attracts wasps, rains down on sheep sheltering below the trees and spoils wool.

As a result wasps are more widespread, whereas up until now they have plagued native beech forests in particular.

First discovered in Auckland in December 2013, the aphid has swiftly spread.

"It was one of the faster ever incursions, within six months it had gone from one end to the other, quite extraordinary. Nowhere in the world are they experiencing anything like the numbers we are - there are hundreds of thousands on an individual tree," Wills said.

"I've got a spot here with willows along the edge of a steep gorge track and down below is a lovely stream, with normally attractive native bush. But now all the bush is black, the ferns are black, the stones in the water are black."

Scion scientist Dr Stephanie Sopow is working on an application to import a parasite wasp from the northern Asian region, where the aphid is thought to be native.

She said a Northland farmer had recently been shown a patch of willows that had been infested, and a year later in the small gully he found 28 wasp nests. Some of his cows had been stung.

"It's worse in later summer-autumn when the populations build up; it feels like it's raining under the heavily infested willows," Sopow said.

Beekeeper Ricky Leahy from Murchison said he worried about the impact of the aphids on willows because the trees were vital as a pollen and nectar source for bees in spring.

And while the aphids' honeydew might appear to be a consolation for bees, it in fact contains a sugar called melezitose, which crystallises hard and does not spin out of the combs.

Waikato University agribusiness professor Dr Jacqueline Rowarth said warm weather could play havoc with wines like sauvignon blanc which needed cold weather to bring out its sharp flavours.

Fruit all benefited from cold snaps by elevating sugar or brix levels.

She echoed King's warnings about the impact of mild winter weather on pasture.

"Even if people are mowing their lawns, there's not much energy in the food. Black beetles and crickets are moving south -  Waikato used to be an easy place to grow grass but now pasture management is challenging," Rowarth said.

Lincoln University scientist Steve Wratten said plants and animals were finding it difficult to adapt to the climate.

He said grass grub, which had once been a pasture pest, was now attacking vineyards.

 - Stuff

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