Tiny wasps that leave the mummified corpses of their victims hanging in trees as tell-tale evidence of their presence have been released by the Northland Regional Council as a biocontrol agent to deal with a sticky pest problem.
The species the wasps have been recruited to help control, the giant willow aphid (GWA), was first sighted and reported in Auckland in late 2013, and spread quickly throughout the North Island.
Willows are the aphids' only recognised host plant, and although they are just 5mm-6mm long, they're a problem because they form dense clusters and tap into the sugar flow in a willow's stem. The aphids use the sap to produces honey dew, a sticky nuisance for farmers and orchardists, and which also attracts large numbers of German and other wasps.
The willow trees themselves also suffer from the aphid infestation, suffering branch dieback and occasionally dying.
Help for the trees is now coming from an unlikely source, however, in the form of a tiny host-specific wasp, Pauesia nigrovaria, introduced from the United States in 2017.