PEST ALERT

Blue Gum Chalcid

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LATEST PEST ALERT: Possible Leptocybe invasa in Zululand

Click here for a PDF to identify the wasp and for symptoms observed in the field.

This pest alert was put together for the November 2005 TPCP newsletter.

Introduction

The Blue Gum Chalcid, Leptocybe invasa Fisher & LaSalle, (Hymenoptera: Eulophidae) is a  new genus and species that was first recorded in the Middle East in 2000 and has spread to most Mediterranean countries, as well as to many of the Eucalyptus areas in northern and eastern Africa. It was detected in Uganda (2002), Kenya (2002) and Tanzania in (2005) and is reported to spread fast and cause serious damage to young plantations and nursery seedlings. It was first reported in South Africa in June 2007 and poses a serious threat to Eucalyptus forestry. Monitoring is of critical importance to now determine the distribution of the pest and identify tolerant species/clones.

Damage

The insect forms galls on the mid-ribs, petioles and stems of new growth of young trees, young coppice as well as nursery seedlings, thereby stunting growth. Galls induced by this wasp can cause substantial injury to young trees and may eventually seriously weaken the tree. In an outbreak situation wasp pressure is intensive and all new growth may be damaged. The impact of the wasp on the development of an adult tree is not yet clear, although galls can be found on most leaves if the wasp occurs in large numbers (Figs A & B).

Distribution and host range

Although this wasp is probably native to Australia, its Australian distribution is still unknown. Presently, the wasp is reported from Algeria, Iran, Israel, Italy, Jordan, Kenya, Morocco, Spain, South Africa, Syria, Tanzania, Turkey and Uganda. Suitable hosts for this insect include: E. camaldulensis, E. globulus, E. gunii, E. grandis, some E. grandisxcamaldulensis clones, E. botryoides, E. saligna, E. robusta, E. bridgesiana, E. viminalis and E. tereticornis.

Description of the insect

The female is a small wasp with average length of 1.2 mm. The head and body are brown in colour with a slight to distinct blue to green metallic shine (Figs C & D).  Only females have emerged from rearing procedures so far and not a single male has been recoded.

Symptoms and gall development

When exposed to new growth, wasps insert their eggs in the epidermis of the upper sides of newly developed leaves, 0.5-5 cm in length, on both sides of the midrib, in the petiole of such leaves and in the parenchyma tissue of twigs. The attack takes place within 1-2 weeks of bud break. There are 5 stages of gall development reported on E. camaldulensis in Israel.

Click here for a PDF of the Symptoms observed in the field.

Stage 1 begins 1-2 weeks after oviposition, with the first symptoms of cork tissue appearing at the egg insertion spot. This stage is characterized by a small change in the morphology of the attacked tissue, the cork scar becomes bigger and the section of the midrib that carries the eggs often changes its colour from green to pink.

Stage 2 is characterized by development of the typical bump shape and the galls reach their maximum size of about 2.7 mm wide.

Stage 3 is characterized by the fading of the green colour on the surface that tends to change to pink while retaining its typical gloss.

Stage 4 is characterized by the loss of glossiness of the gall surface, with colour changes to light or dark red according to whether the galls are present on the leaves or on the stem.

Stage 5 is characterized by emergence holes of the wasps where the colour changes to light brown on the leaf and red on the stem.

Close-up of female Leptocybe invasa wasps (1.2 mm ave. length) (Photos Zvi Mendel)
Young galls and older galls showing exit holes of the wasp Leptocybe invasa on the midrib of Eucalyptus leaves.
Galls showing exit holes of the wasp Leptocybe invasa on the young Eucalyptus branches and leave petioles.
Extensive galling on leaves and twigs, and stunting of Eucalyptus caused by Leptocybe invasa attack in Tanzania. The tree in the foreground on the right is the same age as the resistant clones in the background.