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