Leptocybe invasa

Introduction

The blue gum chalcid (Leptocybe invasa; Hymenoptera: Eulophidae: Tetrastichinae) is one of the most severe invasive eucalypt pests affecting plantation forestry worldwide. Since its initial establishment overseas in the Middle East and the Mediterranean region in 2000, L. invasa has spread within a period of 10 years to all continents where eucalypts are grown.

Description

The female lays her eggs in young shoot tips and midribs of juvenile leaves, which induces extra growths (galls) to develop around the eggs. When the eggs hatch, the maggot-like larvae feed and develop inside the gall. Fully developed larvae pupate inside the galls and emergent adults chew their way out of the gall. The adult blue gum chalcid is mainly black and about 1.4 mm long.

Distribution

Native – Australia;  Invasive – Iran, Israel, Turkey, Mediterranean Basin, sub-Sahara and South Africa, India, Thailand, Laos, Vietnam, Argentina, Brazil, Chile, Uruguay and Florida (United States).

Hosts

Many Eucalyptus spp. including Eucalyptus tereticornis, E. camaldulensis, E. urophylla, E. exerta, E. botryoides, E. bridgesiana, E. globulus, E. gunnii, E. grandis, E. robusta, E. saligna, E. viminalis and Eucalyptus hybrids

Damage/symptoms

  • Galls form mainly on young shoots and along both sides of the midrib on juvenile leaves, but in heavy infestations egg-laying may cause galls to develop in the leaf tissue and twigs. Gall size varies.
  • Gall formation on leaves of eucalypts causes deformation in terminal shoots and leaves, and causes more rapid leaf abscission and shoots to dry out.
  • Sap oozes from the oviposition holes and galls begin to develop. The infested tissue swells, becoming pinkish and developing into a series of lumps

Severe infestation of L. invasa galls on Eucalyptus nursery seedlings showing galls on leaf midribs, petioles and stems. (image © Department of Agriculture Fisheries and Forestry, Queensland).

Severe L. invasa galling on Eucalyptus grandis seedling in field.  Note extensive galling on stems.  (image © Department of Agriculture Fisheries and Forestry, Queensland).

Monitoring

Galling is easily seen. If damage is severe within a plantation, monitoring plots should be established to determine the amount of crown damage and which section of the crown has been affected (entire, lower, middle or upper sections of the crown). Repeat regularly to determine the rate of spread of Leptocybe invasa.

Impact

  • Severe galling causes the loss of apical dominance, growth retardation and stunting in trees.
  • Most plantations are even-age monocultures, and sometimes genetically identical. All the trees may be susceptible at the same time (e.g. when they produce growth flushes, resulting in a massive pest or disease outbreak).
  • Outbreaks may occur in nursery seedlings and plantations, particularly up to two years of age.

Management

Chemical control

Application of broad-spectrum systemic insecticides may help protect nursery seedlings and at plantation establishment, but are not effective in managing the pest in plantations over the longer term.

Resistance

Where possible, eucalypt species or clones that have demonstrated resistance or tolerance should be used for plantation establishment.  Active screening programs are being carried out in a number of countries for L. invasa, but resistant germplasm is still not widely available

Biological control

Four species of parasitoids have been introduced from Australia into Israel to control this insect. More recently another Australian parasitoid (Selitrichodes neseri) has been identified by the biological control program for L .invasa in South Africa run by FABI at the University of Pretoria.

Known L. invasa parasitoids are:

  • Selitrichodes kryceri, Selitrichodes neseri and Quadrastichus mendeli (Eulophidae)
  • Megastigmus zvimendeli and Megastigmus lawsoni (Torymidae)

These may become available for release in other countries where the blue gum chalcid occurs. This offers the best chance of long-term control.