Giant Willow Aphid
Concern from farmers and beekeepers about the threat of this tree parasite
Giant willow aphids, Tuberolachnus salignus, were first reported in Auckland in December 2013 and are now widespread throughout New Zealand. These 5-6mm long pests tap into the sugar flow in willow stems and the wounds they cause leak a honeydew which attracts wasps.
Under threat are willows especially selected by the New Zealand Poplar and Willow Research Trust and regional councils to suit New Zealand farms. Willows are used to control erosion and flooding and are also a vital source of pollen and nectar for bees in spring.
New Zealand Poplar and Willow Research Trust chairman and Hawke’s Bay farmer, Bruce Wills, is alarmed at the invasion of a giant willow aphid which targets willows.
The species was first seen on crack willow in Auckland, in December 2013 and also in southeast Auckland. Within six months it had been spotted in Ashburton and now covers the country having spread extraordinarily quickly. The North Island East Coast and other drought-prone areas including Marlborough are likely to be the worst affected.
“These aphids are widespread around the world but at this stage seem more aggressive in New Zealand,” Bruce says. “In what’s only its third season, we are seeing widespread infestations and big numbers peaking in late summer/autumn.”
Willows as well as poplars are valuable for holding fragile soils on steep farmland and stream banks, he says. They provide shade and shelter for stock while beautifying the landscape and are an alternative feed source during drought.
The giant aphids are a serious threat, sucking vital willow sap in late summer and autumn causing the damaged trees to secrete a honeydew which attracts wasps. Tree growth stalls under attack, as the photosynthesis rate and water uptake increases. This impedes storage of nutrients for winter and growth in spring.
The impact on roots could reduce willows’ effectiveness for flood and erosion control.
Bruce first saw giant willow aphids at his Hawke’s Bay farm Trelinnoe, in autumn 2014, only months after they were first spotted in Auckland.
“I was heading out on my bike to chainsaw down a branch broken by the wind. I went to cut the branch and was chased back by a swarm of wasps. I went to investigate and saw the tree was covered with these black insects I had never seen before.”
Bruce spent the next few hours riding around his property from top to bottom, finding 400-500 willow trees all swarming with wasps and large aphids – brown to dark brown with rows of black patches and wart-like lumps.
He rang the poplar and willow research team at Massey University and found they already had two reports of giant aphids on willows and councils confirmed they were being spotted around the country.
Mysteriously, the pests disappeared from Trelinnoe as quickly as they arrived. Bruce dug around in tree bark with his knife but found no aphids and decided winter cold must have chased then away. Sure enough they were back, when the weather warmed.
Adult aphids, all female, give birth to about 10 nymphs per day meaning populations can go from ten to hundreds of thousands in weeks. Nymphs have four moults before maturing and only a few grow wings. “One day you’ll see 12 on a tree. A couple of weeks later the trunk will be black.” Pass your hand over a colony and aphids wave their long hind legs, a behaviour which may deter predators.
Bruce says the jury’s still out on how giant willow aphids entered New Zealand.
Insecticides will kill these pests but also beneficial insects. Ladybirds have been seen predating them.
Climate change is likely to increase the reproductive rate and shorten the life cycle of this pest, due to warmer summers and numbers are likely to increase, Bruce says.
Beekeepers are dismayed at the arrival of these new aphids, predicted to cause a 30% drop in honey yields.
In late summer bees collect large amounts of a honeydew secreted by the aphids and take it back to their hives where it forms crystals which clog filters during extraction. Any honey which does come through has a distinctive green apple taste.
Especially in the North Island, beekeepers are discovering cells may be capped but on closer inspection contain crystallised honeydew sugars not totally digestible by bees, which eventually try to remove it from the comb.
One beekeeper who processes 150 boxes a day cleaning filters just once, goes through a filter every seven boxes when extracting willow honeydew.
Beekeeper John Berry says willows are a first class source of nutritious pollen and energy-rich nectar for bees to collect and take back to the hive to feed to their young in spring, especially in drought. The trees are most vulnerable to attack in autumn, when they drop their leaves after moving nutrients to their stems. That’s when aphids grow really fast, intercepting sugars and proteins. A heavy infestation weakens trees as they go into winter, compromising pollen and nectar production the following spring – a major disaster for beekeepers.
Willow honeydew also attracts wasps which attack beehives. “What we’re finding is wasps causing damage in areas where they have not done so before,” John says. Last season he lost 30 hives to wasps in one area.
Some beekeepers saw willow honeydew as a useful source of winter feed, collected in autumn. However, evidence was starting to mount that bees did not overwinter well on this feed, which is only partially digestible and sets like concrete, so was impossible to extract, John said. Samples would be taken this autumn, towards discovering whether this willow and also perhaps also poplar honeydew was damaging bees. He’s tasted willow honeydew and says it has a strong flavour and is sought after in some export markets.
John observed good willow flowering last spring – good for bees perhaps but also a worrying sign of trees stressed by aphids.
The farming and apiculture industries have joined forces with Crown Research Institutes Scion and Plant & Food Research, the Poplar and Willow Trust and regional councils’ River Managers Group to apply for Ministry of Primary Industries funding of giant willow aphid research. In late 2015 this collective was shortlisted for Sustainable Farming Fund support for research into the species’ abundant breeding and lifecycle.
Studies have already started into whether some willow varieties are more tolerant than others to this pest and looking for possibilities for biological control.
So far there is $33,500 of co-funding, plus in-kind research support has been pledged by branches of the National Beekeepers’ Association combined with people, businesses and organisations in the wider apicultural industry including Federated Farmers Bee Industry Group.
Scion will do a small-scale trial at Whakatane this summer with support from a beekeeper there.
The National Beekeepers’ Association wants people to email accounts of experiences with giant willow aphids to email@example.com