The Giant Willow Aphid - Tuberolachnus salignus
An insect pest has arrived in New Zealand which is affecting willows throughout Northland and the rest of the country.
The Giant Willow Aphid was first detected in New Zealand in December 2013 and large dense colonies form on all known willow varieties over summer.
The aphids suck the sap of willow trees and populations have spread fast. As the aphid feeds it secretes honeydew, which attracts a number of insects, including wasps, which congregate in vast numbers.
The aphids are of concern as willows are widespread and used extensively by regional councils throughout the country to protect hill country from erosion and as a part of river protection schemes, to buffer the effects of flooding. Horticulturalists also use willows as shelter for their orchards.
The Ministry for Primary Industries says the insect has not been assessed as a significant threat to New Zealand. It doesn't know how or when the species got here, but says, given how widespread the problem is, containment and eradication tools are not feasible.
It was initially thought that poplars were also affected by the aphid but observations suggest that this is due to how close the poplars are to willow trees generally, both in nurseries and in the field.
Willow trees that have been infested by the aphid look noticeably stressed and lacking in vigour. Other tell-tale signs include blackening of the stems and ground under the willow trees, as a sooty mould grows on the energy-rich honeydew that the aphids exude.
The long-term effects of the aphid are still unknown however research is underway on both the insect and potential biological control options. Ladybirds and a few native birds have been making the most of the new food resource.
At this stage the arrival of the Giant Willow Aphid is not influencing council's use of willows for erosion control and flood protection as the issue seems to be seasonal. Past experience has shown that after a few seasons of aphid attack a balance returns and trees become less susceptible with predators bringing pest numbers back to manageable levels. However, it's a good idea to keep an eye on any future developments.
Find more information on the Giant Willow Aphid with the following resources:
Download the 'Tuberolachnus salignus - the giant willow aphid' article (PDF 526KB) - Poplar & Willow Research Trust website