New aphid species creating problems across NZ

The branches of willows are blackened by sooty mould, caused by a new aphid species. Wanganui Chronicle Photograph by Stuart Munro.

A new aphid species that sucks sap from willows is making waves in the insect world and weakening the trees it feeds on.

Giant willow aphids arrived in New Zealand in 2013, and have spread to most parts of the country. They probably came from somewhere in Asia.

They suck sap from most willow species, and a few poplars, and their populations build to high levels in summer and autumn. As well as feeding on the sugary sap they exude some, which grows a black sooty mould across branches and leaves.

The sap they excrete attracts ants, wasps, bees, flies and some nectar-feeding native birds. As a result wasp populations are building to higher levels. People out walking in reserves are getting stung, and bees are also affected.

Region beekeeper Allan Richards has noticed a lot more wasps for the last few years. Early in the summer they search for protein to feed on. By autumn they need sugar to breed their drones and queens.

The extra sugar available from the aphids has extended their breeding season.

"When they run out of food they hammer the beehives," Mr Richards said.

The wasps also prey on other insects, including some introduced to control pest plants, Horizons Regional Council environmental co-ordinator Craig Davey said.

Bees also collect the aphids' sugars, but the honey they make from them is harder to work with and no use for feeding bees during winter.

"They can't use it. I have seen hives starve to death on it," Mr Richards said.

The aphids can also kill or weaken willow trees. Mr Davey said that was helpful, if the trees were pest grey willows, but bad if willows had been planted to stabilise stream banks or prevent erosion.

Beekeepers also value living willows, because they provide nectar and pollen in spring when food for bees is scarce.

There are other stories of wool being devalued by blackening when sheep rest under willows with the sooty mould. And orchardists fear the mould will get on to their fruit and make it less saleable.

Scion (formerly New Zealand's Forest Research Institute) is researching ways to control giant willow aphids. The most likely agent is a predatory wasp, Mr Davey said.