KEY POINTS:
Waiheke Island will be the first home in New Zealand to voracious South African caterpillars that are expected to chomp through an invasive shrub that has gained a stronghold on the island's cliffs.
In a joint project, the Auckland Regional Council and Landcare Research plan to release today the first batch of 50 leafroller caterpillars to battle boneseed.
Introduced from South Africa as an ornamental plant, boneseed has spread rapidly in the past 20 years along coastlines throughout the North Island and parts of the South Island.
Fast-growing and a prolific seeder, boneseed can rapidly replace native plants and its dense thickets also restrict access to beaches.
Landcare research technician Chris Winks has mass-reared the caterpillars since their release from quarantine in January.
The caterpillars are rapacious feeders of the shrub's foliage and can kill entire plants.
Mr Winks said that in Cape Town they went through three lifecycles a year but how they would thrive on Waiheke would depend on the temperature.
He said the caterpillars webbed leaves together to make homes for themselves, hence the name leafroller.
They would serve an important role as many areas where boneseed grew were inaccessible and environmentally delicate.
"Often the cost of control is prohibitive, or the use of herbicide is environmentally undesirable. The caterpillars will be of most help in those cases."
The caterpillars would not eradicate the boneseed shrub but they could slow it down enough to prevent a monoculture and give native plants a better chance of competing.
He said extensive tests in South Africa by Landcare Research and by Australian researchers had shown the risk of leafrollers eating anything but boneseed was negligible.
ARC biosecurity manager Jack Craw said boneseed was now common on Auckland's coastline including many of the Hauraki Gulf islands.
Scientists were working with other councils to release boneseed leafrollers in other parts of the country.
The leafroller moth had been released in Australia and scientists were testing a promising rust fungus which, if found suitable, might be brought here to attack boneseed.
Mr Craw said boneseed had invaded coastal cliffs, sand dunes, grassland and the side of roads.
"It's even crowded out gorse on coastal sites, and its thick growth cuts off access to beaches, especially when it takes hold in sand dunes."
The Environmental Risk Management Authority had approved the release of the caterpillars as a natural biocontrol agent.
Boneseed is on the national pest plant accord list, meaning it is banned from sale, propagation, distribution and commercial display within New Zealand.
Smaller plants can be pulled out by hand and larger ones cut off close at ground level and the stump treated with herbicide.
Spotting boneseed:
* Bushy shrub grows up to three metres high.
* Thick, leathery leaves with slightly toothed edges.
* Masses of bright yellow daisy-like flowers bloom in spring.
* Clusters of fruit turn black when ripe.
* Single seed inside is hard and white.
