Controlling gum leaf skeletoniser

6 July 2010

Scion’s application to the Environmental Risk Management Authority (ERMA) for permission to release a parasitic wasp into New Zealand to control the gum leaf skeletoniser has been approved.

The gum leaf skeletoniser Uraba lugens is an Australian moth that has the potential to defoliate thousands of hectares of Eucalyptus plantations as well as ornamental Eucalyptus grown for shade and shelter throughout New Zealand.

Scion entomologist Dr Lisa Berndt says the young caterpillars skeletonise gum leaves by eating the green parts of the leaves, avoiding the veins, which results in a skeleton appearance to the leaf.

“It’s a very serious threat to our eucalypt forests,” she says. “Repeated defoliation by this pest can slow the tree’s growth, or even kill individual trees. Outbreaks occur regularly in Australian eucalypt forests, and this could happen here too.”

Although initially found in Mt Maunganui in the 1990s the gum leaf skeletoniser was eradicated from that location by the Ministry of Agriculture and Forestry (MAF). However, in 2001 it was discovered in Auckland and was too far spread for eradication to be feasible. It is now widespread in the Auckland region, and also present in Waikato, Coromandel and Bay of Plenty and is likely to continue spreading.

“As devastating as it can be for eucalypt trees, gum leaf skeletoniser is also an urban problem because it is hazardous to people,” says Lisa. “The long hairs of the caterpillar inject venom into human skin on contact, which can cause a painful skin irritation.”

Scion has been involved in research into the gum leaf skeletoniser since it was first detected in New Zealand. Lisa and her team have been developing biological control to help manage this pest since 2007, as part of a MAF Sustainable Farming Fund project. The most suitable biological control agent was found to be the Australian parasitic wasp Cotesia urabae. This wasp was chosen because it will specifically target the gum leaf skeletoniser caterpillar. It will be sourced from Tasmania, with the help of Dr Geoff Allen (University of Tasmania/Tasmanian Institute of Agricultural Research), so will be adapted to a climate similar to New Zealand.

After three years of research into the safety of the wasp’s release into New Zealand, and consultation with a wide range of industry and community groups, Scion made its application to ERMA on behalf of the Gum Leaf Skeletoniser Stakeholder Group.

In arriving at its decision to grant approval ERMA considered all the potential effects of the parasitic wasp to the environment, human health and safety, the economy, society and local communities, and Maori culture and values. Overall the benefits of release of the wasp outweighed any possible adverse effects.

“Our research has shown that the wasp poses no significant risk to native insects in New Zealand, and unlike the gum leaf skeletoniser the wasp has no known effects on humans,” says Lisa. “Biological control has been a very effective and safe means of control for other eucalyptus pests in New Zealand. We hope this new agent will be as successful on gum leaf skeletoniser because it is the only sustainable alternative to applying chemical insecticides to our forests.”

The Sustainable Farming Fund project will run for another 12 months during which time the wasp will be brought into New Zealand and released, with the help of the Gum Leaf Skeletoniser Stakeholder Group and any landowners or community groups associated with the release location. The release will probably be made in the Auckland region where gum leaf skeletoniser is most abundant. The success of the release and ongoing impacts will be closely monitored over the next few years.