Giant Willow Aphids
WINTER is upon us, yet nobody has told one of Gisborne’s newest invasive pests, the Giant Willow Aphid.
In the district since 2014, the aphid thrives in summer before usually disappearing over winter.
“This time last year they had long gone,” says Dr John McLean, a Gisborne entomologist and beekeeper.
Yet they are still here and scientists, farmers and beekeepers are concerned.
First identified in Auckland in December 2013, the aphids were found to be widespread around the country. Nobody knows exactly how they arrived in New Zealand, although one theory is they blew over from Australia.
They thrive on the sugar-rich sap in willows and are also showing up on apples, pears and kiwifruit.
During warm, dry weather the moisture deficit in the soil stresses out the willows. They go into “survival mode”, mobilising more sugars and proteins, which in turn attracts more aphids. They breed rapidly in the warm conditions with such a rich food supply.
The aphids probe into the willows and suck out the sap, excreting excess in the form of honeydew onto the branches and anything below.
Rapid spreadThe aphids are all female and multiply rapidly, with some producing up to 76 offspring.
The greatest number of Giant Willow Aphids ever seen in Gisborne appeared last summer, with some trees swarming with hundreds of thousands of aphids.
After a bit of rain and a few frosts their numbers have taken a hit but due to the mild start to winter they are still active.
“If they are like this in the middle of winter one can only imagine what they will be like come summer,” said Dr McLean.
Affected willows are easy to spot, marked with a black “sooty mould” produced by fungi attracted to the sugar-rich honeydew.
The honeydew can also drip on to fruit and when colonised by sooty molds it is blemished and has reduced value.
At around five to six millimetres “giant” might also refer to the effects they have on some of the region’s most important trees.
In hilly areas willows help keep land stable, near rivers they reduce sediment washing downstream and on farms they act as shade and occasional fodder for livestock.
Nick Seymour, president of the Gisborne branch of the New Zealand Farm Forestry Association, said they are increasingly worried about the potential impacts on willows.
“We rely on willows in this area. If the root systems in the willows go, we are knackered.”
Effects on farm profitsExcess sap the aphids cannot consume drips off the tree onto anything below including livestock seeking shade, which can ruin wool.
“It makes it hard for farmers. There is no positive side to it.”
In other areas of the country, wasps attracted to the honeydew have been causing havoc stinging cattle.
While it is not yet a major issue in Gisborne, Mr Seymour says they are keeping an eye on it.
Team leader in soil conservation at Gisborne District Council (GDC), Kerry Hudson, says they are watching the spread of the aphids and their impact closely.
“We are not sure of the long-term impact yet but they have spread quite quickly.”
At GDC’s poplar and willow nursery they have found the aphids affect growth in the younger trees.
“When they are under stress, given the dry conditions, something like this just accentuates the problem.
“A really bad drought year could add extra stress.”
The major growth time for willows is from September to February, the same time the aphids typically boom.
“Willows are a real niche in the district and we are watching with interest.”
He says for anybody with willows on their land to keep in touch.
“It's out there, we are concerned about it and would appreciate people getting in touch if see willows dying or under stress, especially if it happens in the next few months.”
Effect on honey productionDr McLean says the aphids have a great impact on apiculture, reducing yields and changing the flavour of the honey. Bees are attracted to the honeydew, and honey produced from honeydew is very sticky and includes “melezitose”, which crystallises in the hives and makes it difficult to extract the honey, reducing yields by around 30 percent.
The honey is much darker, and to some tastes “yuck” says Dr McLean.
“It tends to spoil the flavour of a lot of native honeys in our area.”
Another concern is the impact on the prized manuka sector. In the lead up to manuka flowering over summer, for 10 to 12 weeks, food in spring for bees is very important, and willow pollens and nectars are a vital food source.
“It is a pretty serious problem that raises a good question about what to do about them.”
Ministry of Primary Industries looked into it in 2013, but found it was so widespread they elected to “stand down” on the matter.
Scientists at Scion Research in Rotorua have applied for support to research this problem and have recently received a Sustainable Farming Fund grant to fully evaluate the Giant Willow Aphid.
One aspect they will be looking into is biological control with a parasitic wasp from northern Asia, where the aphid is thought to be native.
The Apiculture New Zealand Research Task Group Chair Barry Foster also supported this proposal.