Biological control of giant willow aphid in the form of parasitoids wasps is identified and tested by Scion entomologists, approved and released in New Zealand; the work directly benefits beekeepers, river managers, soil conservationists and farmers

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ROTORUA, New Zealand , January 14, 2022 (press release) –

A biological control agent for giant willow aphid has been identified and tested by Scion entomologists, approved and released in New Zealand.

Giant willow aphid (Tuberolachnus salignus), or GWA, was first reported in New Zealand in 2013. The aphids feed on willow sap, damaging and occasionally killing the trees. Willows are widely used in New Zealand for slope stabilisation, flood protection, crop and livestock shelter, fodder and as pollen and nectar sources for honeybees in early spring.

The aphids also secrete copious amounts of honeydew, which attracts insects such as honeybees and pest wasps. Honey made from the honeydew is granular and cannot be extracted from the comb, and bees are at risk of being killed by the wasps. Excess honey dew also causes sooty mould to grow on and beneath infested trees.

Biological control, where natural enemies control a pest, is a cost effective, sustainable and environmentally sound control method. While GWA has very few natural enemies in New Zealand, Scion scientists found evidence of parasitism in natural populations of GWA in California by the parasitoid wasp Pauesia nigrovaria. The parasitoid wasp lays an egg in the aphid, a larva hatches, and this then eats and eventually kills the aphid host as it develops. In two to three weeks a new adult parasitoid emerges from the dead aphid.

Stringent testing is needed before a biological control agent is released. The first parasitoids were imported into containment at Scion in 2017, followed by host specificity testing using non-target aphid species to ensure the parasitoid only attacked GWA. Following favourable results, an application to the Environmental Protection Authority was successful, with permission to release granted in December 2019.

Parasitoids wasps have been released by Scion and partners via a New Zealand-wide community effort. Scion shipped mated female parasitoids to beekeepers, regional council staff and others for release on infested trees.

Those involved in the releases, and citizen scientists, are now monitoring the successful survival and spread of the parasitoid. Not only did the tiny wasps overwinter, they have multiplied exponentially and spread up to 100 km from the first release sites. This is phenomenal - normally a biocontrol agent tends to disappear a few years while it settles in before signs of establishment are seen, followed by slow spread.

This work directly benefits beekeepers, river managers, soil conservationists and farmers. Indirectly, the general public will also benefit, for example, from river banks remaining less prone to erosion, fewer pest wasps and less sticky, sooty mould in public spaces.


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