Parasitic wasp tackling poisonous caterpillar

A parasitic wasp released in the Whangarei area to target a newly-discovered pest caterpillar bristling with poisonous spines is the latest addition to more than two dozen biocontrol agents now at work in Northland.

Leaf damaged by gum leaf skeletoniserKane McElrea, a biosecurity officer with the Northland Regional Council, examines a leaf badly damaged by a gum leaf skeletoniser.

The gum leaf skeletoniser (GLS) - whose caterpillar stage has stiff, hollow spines that can deliver painful stings – was discovered several months ago in Whangarei, prompting warnings to be vigilant for it from local authority staff.

Last month staff from the Northland Regional Council and Scion Forest Protection released dozens of GLS larvae infected with the eggs of a small parasitic wasp (Cotesia urabae) as a biological control agent in a Morningside gum plantation.GLS infected with parasitic waspGLS infected with parasitic wasp

Don McKenzie, the council’s Biosecurity Senior Programme Manager, says
biocontrol is the use of naturally-occurring enemies and diseases to control pests and weeds.

“It’s not designed to eradicate a species; instead it aims to keep populations at low levels and we’re hoping that eventually numbers of these wasps – which only target GLS – will build in numbers and eventually spread throughout the Whangarei area.”

GLS eggs with house key for scaleGum leaf skeletoniser eggs shown beside the tip of a standard house key.

Mr McKenzie says the wasps lay their eggs inside GLS larvae, with the wasp larvae eventually hatching and killing their host as they grow. The wasps, which were specially bred by researchers in Auckland, join more than two dozen biological control agents already in use in Northland for a variety of pests.

Gum Leaf Skeletoniser was found late last year ago at a port in the Whangarei area through MAF's high risk site surveillance programme.

Mr McKenzie says GLS – which damages gum trees by eating their leaves – has been in the greater Auckland area for about a decade, but is not thought to have been in Northland until now. (Previously, it had only been recorded as far north as the Warkworth area.)

Although it favours eucalypts, Gum Leaf Skeletoniser also thrives on silver birch and can damage some oak, copper beech and plum.

Mr McKenzie says fortunately, GLS is not generally attracted to our native plants, although it can feed on them when they’re growing very close to eucalypts.

The pest which goes through four very different looking life stages.

Its eggs are about 1 mm in diameter and are laid in groups 100-200 in parallel rows on young leaves. The eggs are yellow-green at first, turning brown as they develop.

Caterpillars are hairy and pale yellow with black and grey markings. Older caterpillars have a distinctive 'hat' on their heads.

Its cocoons are usually formed under bark or in leaf litter and are rarely seen while moths are dull grey with silver-grey forewings and a wingspan of 20-30mm.

Mr McKenzie says two life cycles are completed annually, one in summer and one winter.

He says a booklet, factsheet and series of frequently asked questions about Gum Leaf Skeletoniser’s health effects and management is available online at: http://www.biosecurity.govt.nz/pests/gum-leaf-skeletoniser

Further information is also available from Northland Regional Council offices.