Tiny wasps with a big job to do

A Pauesia nigrovaria wasp, with a ballpoint pen nib for scale. Photo / NRC

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.

Crown research institute Scion is leading a three-year research programme focusing on long-term control of the aphid, including introducing the wasp. Funding from the Ministry for Primary Industries Sustainable Food and Futures programme has also made the Northland release possible.

Cr Jack Craw, who chairs the regional council's Biosecurity and Biodiversity Working Party, said the wasp laid eggs in the aphids, which hatched and consume their host before emerging as an adult, leaving their now mummified nursery behind.

The parasitoid's presence could be confirmed by the mummified remains of large numbers of aphids that remained fixed to willow stems for weeks or even months after the wasps had emerged.

Last week the council released 30 mated females of the little wasps at its Flyger Rd, Mata, poplar and willow nursery, where it's hoped they will establish and then eventually spread from.

Craw said the wasps were joining a variety of biocontrols already in Northland, all of which had been rigorously tested for host-specificity to ensure they would not attack other, non-target species.

Those biocontrol agents included beetles that fed on tradescantia, fungi and rusts that collectively attack a variety of pest plants and insects including tropical grass webworm, mistflower, gorse, ragwort and woolly nightshade.

Anyone who wants to know more about the regional council's biocontrol programme and its other work in pest control can go to www.nrc.govt.nz/nasties