Northland biosecurity: Wasps recruited to kill Northland's willow aphids

Tiny parasitic wasps are released into willows by Northland Regional Council nursery coordinator Matthew Mabbitt at the NRC's Mata poplar and willow nursery near Whangārei.

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One of the released Pauesia nigrovaria wasps with a ballpoint pen nib for scale.

SUP270421NADwasps4.JPG The target pest; giant willow aphids which feed on willows in large numbers producing a sticky honeydew. The insects can damage – or even kill – infested trees.

Killer wasps that leave the mummified corpses of their victims hanging in trees have been recruited by the Northland Regional Council as a biocontrol agent to deal with a sticky pest problem.

The tiny wasps leave the mummified corpses of their victims hanging in trees as telltale evidence of their presence and they have been released by the NRC.

The species the wasps have been recruited to help control is the giant willow aphid, which were first sighted and reported in Auckland in late 2013 and spread quickly throughout the North Island.

Willows are the aphid's only recognised host plant and although just 5mm-6mm long, they're a problem because they form dense clusters which tap into the sugar flow in a willow's stem.

The pressure of sap through the aphid produces honeydew; a sticky nuisance for farmers and orchardists and which also attracts large numbers of German and other wasps. The willow trees also suffer from the aphid infestation, experiencing branch dieback and occasionally death.

The target pest; giant willow aphids which feed on willows in large numbers producing a sticky honeydew. The insects can damage - or even kill - infested trees.

However, help for the trees is coming from an unlikely source, 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.

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Funding from the Ministry for Primary Industries Sustainable Food and Fibre Futures programme - www.mpi.govt.nz/funding-rural-support/sustainable-food-fibre-futures - had also made the Northland release possible.

Councillor Jack Craw, chairman of the NRC's Biosecurity and Biodiversity Working Party, said the wasp lays eggs in the aphids, which hatch and consume their host, before emerging as an adult, leaving their now mummified nursery behind.

Craw said the parasitoid's presence can be confirmed by the mummified remains of large numbers of aphids which remain fixed to willow stems for weeks or even months after the wasps have 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.

One of the released Pauesia nigrovaria wasps with a ballpoint pen nib for scale.

Craw said the wasps join a variety of biocontrols already in Northland, all of which have been rigorously tested for host-specificity to ensure they won't attack other, non-target species.

People interested to learn more about the regional council's biocontrol programme and its other work in pest control should visit www.nrc.govt.nz/nasties