Tiny parasitoid wasp packs a punch

The Californian wasp was released in New Zealand as a biological control agent, beginning in 2020.

After just one year it was detected up to 100km from release locations.

Steve Penno, Ministry for Primary Industries’ (MPI’s) director of investment programmes says the presence of giant willow aphids has decreased each year of the trial, and the proportion of aphid-free trees has increased.

“This is an extraordinary result in such a short space of time,” Penno says.

He says the research has directly benefited beekeepers, river managers, soil conservationists, and farmers.

"Indirectly, the public also benefits as riverbanks become less prone to erosion and fewer pest wasps are drawn to the honeydew secreted by the aphids.

"This research has demonstrated that Pauesia nigrovaria are a highly cost effective, sustainable, and environmentally sound control method for controlling the giant willow aphid population."

MPI’s Sustainable Food and Fibre Futures fund contributed $260,000 to the project in partnership with Scion, Plant & Food Research, and Apiculture NZ to deal to the exotic pest, which was first detected in New Zealand in 2013.

Project lead and forest entomologist Stephanie Sopow from Scion says she’s "blown away" by the results so far.

"Normally a biocontrol agent tends to disappear a few years while it settles in before signs of establishment are seen, followed by slow spread, so the project has gone even better than we expected,” Sopow says.

She says the parasitoid wasp lays an egg in the aphid. When the larva hatches, it eats and eventually kills the aphid host as it develops. In 2 to 3 weeks a new adult parasitoid emerges from the dead aphid.

"Pesticides aren’t an option because they would transfer into the honeydew that the aphids secrete, putting nectar feeders such as honeybees, tūī, and bellbirds at risk."

Sopow says giant willow aphid feed on willow tree sap and have a particularly devastating impact on bee colonies.

"The total impact of giant willow aphids has been estimated to be over $300 million each year.

"It’s vital that we protect our willow trees as they’re an essential part of New Zealand farming for slope stabilisation, flood protection, crop and livestock shelter, fodder, and as pollen and nectar sources for honeybees in early spring."

Monitoring and reporting on the trial will be ongoing, with assistance from scientists across New Zealand.