A new insect pest has been breeding up the wasps and threatening the survival of bees, beekeeper Rob Butcher says.

Giant willow aphids have been in New Zealand for about two years. They were first noticed in Auckland in 2013, but were already widespread.

The spotted grey insects can be 6mm long, larger than most aphids. They suck sap out of the stems and young branches of willow trees, and some poplars, which can kill the trees.

They also exude honeydew. Bees eating it produce honey that is impossible to extract. The honeydew also attracts wasps and builds up numbers. When it is gone, the wasps attack bees, and can wipe out hives.

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"At the moment most beekeepers are only just becoming aware of how bad the problem is," Wanganui Beekeepers' Club secretary Neil Farrer said.

It was a hot topic at a National Beekeepers' Association meeting he attended on Monday.

The problem was especially hard on people with five to 50 hives wintering near willow trees. Mr Farrer was told wasps had reduced a Hawke's Bay man's 100 hives to 40.

Rob Butcher has been keeping bees for 50 years and fears the aphids will harm conservation as well as the bee industry. He had always had wasp problems, but never the "ruthless destruction" of this season.

The honey made by bees eating the honeydew was in "a comb, set like plastic glass, with sticky runny fluid that resists extraction," Mr Butcher said.

Mr Farrer said the honey had a different sugar, and he called it "concrete honey". It fed bees over winter, but clogged filters and had no other use.

The other reason the aphids were bad news for beekeepers was their effect on willow trees.

"We rely on the willows in spring, because they're a great producer of pollen and nectar for bees," Mr Farrer said.

Beekeepers are looking for scientific research that will help, and will discuss the aphids at their national conference in two weeks.