Bulletin No. 29

The sirex wasp in Victoria.  F.G. Neumann, J.L. Morey and R.J. McKimm (edited by D. Meagher).  1987.  Lands and Forests Division, Department of Conservation and Lands.  41 pp + plates.


In Victoria the sirex wasp, Sirex noctilio Fabricius (Hymenoptera:Siricidae), from southern Europe, is a serious pest of exotic Pinus radiata D. Don (radiata pine), which has been widely planted as the principal source of softwood. In the 1960s many valuable pine shelter belts on farmland were severely damaged or destroyed, and substantial tree mortality also occurred in privately owned P. radiata plantations, especially in central Gippsland, east of Melbourne. The worst outbreak in government-owned plantations occurred between 1972 and 1979 in a 12 to 15-year-old plantation of 1906 ha near Delatite in east-central Victoria. In this plantation, where biocontrol agents were present, the wasp killed an average 77% of trees on 25 ha, 63 % on 79 ha, 35 % on 379 ha and 5 % on 701 ha, whereas in 16 to 18-year-old thinned stands, tree mortality was below 27%. Only 20% (388 ha) of the plantation area remained undamaged. In the worst affected stands, the total and merchantable wood volumes were reduced by 50.4% and 48% respectively. Stocking density declined from about 1700 stems ha-1 to less than 420 stems ha-1, which corresponds to a level approximately 62 % below the prescribed stocking after a first thinning.

The sirex wasp's life cycle usually extends over a one-year period, though some individuals may pass through a three month or a two year cycle. Emergence of adults, followed by attacks on trees, occurs mostly between mid-summer and early autumn, when soil moisture levels, growth rates and resistance of pine to pests and diseases are low. Various volatiles produced by the phloem/cambium tissues of stems and large branches of living trees are important in the attraction of the wasp. Susceptible trees are usually above 12 years old, and physiologically stressed. Sirex wasp attack causes a reinforcement of stress due to the injection of phytotoxic mucus and spores of the symbiotic pathogenic basidiomycete Amylostereum areolatum (Fries) Boidin (commonly known as the sirex fungus) into outer sapwood during oviposition. This pathogen desiccates the wood, causes white rot, and is also a source of nutrients for the wasp's larvae. Additional degrade in the wood occurs through the tunnelling activity of the larvae, and through secondary decay fungi which enter via flight holes and render the wood unmerchantable within a year.

Since the discovery of the sirex wasp in locally produced logs near Melbourne in December 1961, it has spread to all major pine-growing regions of Victoria and beyond into south-eastern South Australia, southern New South Wales and the Australian Capital Territory. This has occurred despite a rigorous application during the 1960s of extensive search and destroy operations, the prohibition of log movements from quarantined sirex-infested properties, and periodic mill-inspections by specially trained staff, followed in the 1970s by extensive biocontrol measures. The total cost of these operations amounts to about $3 million (about $10.5 million in terms of 1984 monetary values). All State Governments (and since 1983 also the Commonwealth-administered ACT Forests) have met these costs by annual contributions approximately in proportion to the areas of Pinus species under cultivation. Until 1974 the Commonwealth Government also made annual allocations on a dollar for dollar basis with the states. Many private forestry organisations and local government authorities have contributed, especially during the 1960s. Funding has been progressively reduced from a peak of $315 720 in 1964-65 to $26 175 in 1984-85.

The sirex wasp is essentially a secondary opportunistic wood-boring pest. The prevention of economically important outbreaks in plantations is therefore largely a management problem that can be alleviated by routine surveillance of plantations, the application of silvicultural measures including timely selective thinning for sustained vigour, and the early removal of multi-stemmed, unhealthy or damaged trees. Biocontrol is not required in some well-managed plantations. In fact small numbers of sirex wasp in these may be useful in killing unwanted trees between scheduled thinning treatments. However, the wasp needs to be controlled in plantations that have been damaged by protracted drought, as well as those in poor silvicultural condition because thinning has been delayed by unfavourable markets, unavailability of funds for early non-commercial thinning or other reasons. An effective multi-pronged control strategy that enables the early detection and rapid suppression of even small sirex wasp populations is now available. It combines the use of aerial reconnaissance, routine ground observations and the deployment of herbicide-injected (and hence highly susceptible) trap tree systems with the effects of biocontrol agents. Most useful among these biocontrol agents are the parasitic nematode Deladenus siricidicola Bedding and two parasitoids of genus Ibalia (Hymenoptera:Ibaliidae). The nematode is artificially inoculated into the outer sapwood of sirex-infested trap trees and significantly suppresses the fecundity of females without impairing their general vigour and sexual competitiveness. The parasitoids kill early larvae of the wasp.

Until 1983, the Forests Commission Victoria was responsible for the culturing and supply of large numbers of biocontrol agents for use in Victoria and interstate. After a restructure of government departments in 1983, this became the responsibility of the State Forests and Lands Service of the Department of Conservation Forests and Lands. In late 1985 it was transferred to the Department's Land Protection Service. In recent years the population levels of the sirex wasp have been economically unimportant, largely as a result of the effectiveness of current pest management measures. However, the insect remains a potential threat to Victoria's pine resource. Vigilance needs to continue, and preventative control measures applied, in areas where endemic populations are showing signs of build-up.