Farm Forestry New Zealand

PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Phylacteophaga froggatti, Eucalyptus leaf-blister sawfly

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Forest and Timber Insects in New Zealand No. 64: Eucalyptus leaf mining sawfly.

Revised 2009
Based on M.K. Kay (1986)

Insect: Phylacteophaga froggatti Riek (Hymenoptera: Pergidae)

 
Fig. 1 - Mines and emergence holes made by eucalyptus leaf mining sawfly in leaves of Eucalyptus botryoides.

Type of injury
Larvae of this small Australian wasp feed by mining inside the leaves of the host plant. The structure of the lower leaf surface remains intact, but the upper leaf cells are eaten, leaving just a thin, papery cuticle covering the mine. Individual mines may be some 5 cm² in area and several mines may coalesce, destroying much of the leaf. Affected leaves have a brown "blistered" appearance (Fig. 1) and may be shed prematurely. The tree is injured because of this loss of photosynthetic tissue. Damage is most common on foliage up to 3 m above ground level (Fig. 2), and small trees can be totally defoliated. In sheltered areas the damage to large trees can extend high into the canopy.


Fig. 2 - Damage caused by eucalyptus leaf mining sawfly to lower leaves of Eucalyptus cinerea.
 

Hosts
In both Australia and New Zealand a wide range of Eucalyptus species are susceptible to attack, but those of the subgenus Symphyomyrtus (e.g., E. botryoides, E. cinerea, E. nitens, E. perriniana, E. saligna) seem to be preferred. In both countries the sawfly has also been found on Lophostemon confertus (=Tristania conferta brush box). In New Zealand it has been discovered on Quercus (oak) where its mines could be confused with those made by the oak leaf-miner Phyllonorycter messaniella. It has occasionally been seen on Betula (birch) and Agonis flexuosa (weeping willow myrtle), while abortive mining has been found on Liquidambar styraciflua (liquidambar).

Distribution
This insect was first discovered in New Zealand in March 1985. At this time it was found only within a 7-km radius of Auckland International Airport. By September 1986 it was present over an area extending from Pukekohe to Waiwera, and isolated attacks had been confirmed at Huntly, Glen Massey and Waingaro near Ngaruawahia, and Whangamata. It is now widespread in the North Island, and is present in the South Island as far south as Dunedin. Distribution to new localities may also occur through such means as the transfer of infested plants from nurseries, and the inadvertent transport of infested leaves caught up on motor vehicles. The adults are also know to be transported on motor vehicles.

Economic importance
The current economic effect of this insect is unknown, but is likely to be minimal due to the success of the introduced biological control agent Bracon phylacteophagus Austin. Prior to this control, the growth rate of small trees which had been totally defoliated was significantly reduced having serious consequences for the establishment of eucalypt plantations, particularly for those in warm, dry areas which favour insect survival and where newly planted trees may be already under stress. The unsightly effects of attack can detract from the use of eucalypts as garden ornamentals.

Description
The adult (Fig. 4 and 5) is a 4-6 mm long, thick-waisted wasp with an orange head, prominent black compound eyes, and dark brown or black antennae. The legs are mostly pale yellow, and the wings are clear, with each forewing having a prominent black spot on its leading edge. The male is smaller than the female and has a black prothorax and abdomen, and a black spot enclosing the three simple eyes on the top of the head. In the female the top and sides of the prothorax and usually the last two segments of the abdomen are orange, and there is no black around the simple eyes. The saw-like egg-laying apparatus (ovipositor) under the tip of the female's abdomen gives this group of insects the common name of "sawflies".


Fig. 4 - Eucalyptus leaf mining sawfly adults: male on the left, female on the right.
The lines show natural length.

 
Fig. 5 - Eucalyptus leaf mining sawfly female with wings spread to show the thick waist typical of this group of insects.

The larva (Fig. 6) has an orange head and a soft, yellowish-white, translucent and somewhat flattened body with dark markings on the underside. A fully grown larva is about 10 mm long.

 
Fig. 6 - Eucalyptus leaf mining sawfly mine opened up to show larva.
The line shows natural length of the larva.
 

Life history and habits
The female may lay up to 70 eggs which are deposited singly near the midrib of leaves of any age. The egg is placed on the underside of a leaf in a slit cut by the ovipositor. On the upper side of the leaf a small raised lump, which blackens within a day or two, marks the position of the egg. In about seven days a narrow mine develops from the place where the egg was laid and rapidly broadens as the larva grows (Fig. 7).

 
Fig 7 - Developing mines made by larvae of eucalyptus leaf mining sawfly in leaves of Eucalyptus botryoides.

Larvae destined to be females grow to a larger size and make larger mines than those which will become males. Mines of adjacent larvae often join so that several larvae then inhabit one large mine. Larvae can successfully complete their development and subsequent pupation in infested leaves which have fallen or been removed from the tree. Pupation occurs within the mine about four weeks after mine initiation and takes approximately ten days. Before pupating, the larva spins a silken cocoon about itself which is attached to both leaf surfaces. The position of the pupa is indicated by the raised surface of the papery upper epidermis, and the newly formed adult cuts a hole through this to emerge. The adults live for about five days only and do not feed. Mating and egg-laying begin soon after emergence. The sawfly has many generations per year, and eggs, larvae, pupae, and adults can be found at any time. The life cycle slows down during the winter, but numbers rapidly increase in warm, dry, summer weather.

Control
Rain can be a significant cause of mortality - the thin covering of the mines is easily perforated and water enters, drowning the larvae. Competition must also play a part in limiting populations for observations show that, while up to 30 eggs may be laid in a single leaf, no more than 11 adults have ever emerged. General predators such as birds undoubtedly help to reduce sawfly numbers. A classical biological control programme began in 1988 using a larval parasitoid, Bracon phylacteophagus. Within three years, the parasitoid had spread through the sawfly-infested area, and collapse of the pest population occurred within a year at each site, and the Eucalyptus leafmining sawfly is no longer considered a serious forestry pest.

A systemic insecticide, such as an organophosphate, can be used to control the larvae on ornamental trees or nursery stock. Repeat applications would be needed to counter reinfestation.

Bibliography
Austin, A.D. and Faulds, W. 1989: Two new Australian species of Bracon F. (Hymenoptera: Braconidae) parasitic on Phylacteophaga spp. (Hymenoptera: Pergidae). Journal of the Australian Entomological Society 28: 207-213.
Farrell, G.S. and New, T.R. 1980: Some aspects of the biology of the eucalyptus-mining sawfly Phylacteophaga froggatti Riek (Hymenoptera: Pergidae). Australian Journal of Zoology 28: 83-90.
Faulds, W. 1990: Introduction into New Zealand of Bracon phylacteophagus a biocontrol agent of Phylacteophaga froggatti eucalyptus leaf-mining sawfly. New Zealand Journal of Forestry Science 20 (1): 54-64.
Kay, M.K. 1986: Phylacteophaga froggatti Riek (Hymenoptera: Pergidae). Eucalyptus leaf mining sawfly. New Zealand Forest Service, Forest and Timber Insects in New Zealand No. 64.
Nuttall, M.J. 1985: New insect pest attacks eucalypts. New Zealand Farmer 106 (19): 124-125.
Withers, T.M. 2001: Colonization of eucalypts in New Zealand by Australian insects. Austral Ecology 26 (5): 467-476.

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