Australasian Science: Australia's authority on science since 1938

Parasitic Wasp to NZ’s Rescue

The gum leaf skeletoniser larvae leave just the skeleton of eucalyptus leaves.

The gum leaf skeletoniser larvae leave just the skeleton of eucalyptus leaves.

By Stephen Luntz

A Tasmanian parasitic wasp (Cotesia urabae) has been approved as a biological control for a less appreciated import to New Zealand: the gum-leaf skeletoniser (Uraba lugens).

As the name suggests, the skeletoniser, a sort of hairy caterpillar, strips gum leaves to their veins and oil glands. Once older, it eats the entire leaf. Although there have been occasional large outbreaks of the skeletoniser in Australia, it is generally kept under control by various parasitic wasps, predatory insects and spiders.

In 1992 a population of the skeletoniser was detected in eucalyptus plantations in New Zealand. Chemical methods were deployed, and this was believed to have succeeded. However, a 2001 invasion in Auckland has grown too large for eradication to be successful, so scientists at the New Zealand forestry research institute Scion consulted University of Tasmania entomologist Dr Geoff Allen about a suitable biological control. They settled on C. urabae as the biological agent.

“People get a bit nervous when you start talking about wasps because they usually think of the European wasp,” Allen says. “But Cotesia urabae is only about 3 mm long and is harmless to humans. It’s an effective control against the gum-leaf skeletoniser because the wasp lays its egg inside the caterpillar, and when ready to pupate the larva eats its way out and kills the caterpillar.”

Eucalypt plantations are a substantial part of New Zealand’s forestry industry, but the skeletoniser also poses a threat to the use of eucalypts as windbreaks and shade trees, as well as amenity trees in urban areas. “The caterpillar has poisonous spines on its body and can cause rashes,” Allen says. The potential damage of the gum leaf skeletoniser has been estimated at $100–142 million.

Biological controls have evolved a long way since the cane toad became a pest that was much worse than the one it was intended to stop. Allen says: “We’re quite fortunate in that the skeletoniser has only one remotely closely related species in New Zealand, although we also tested to see it wouldn’t attack much more distantly related species.”