This paper examines the essentials of the biology, impact and control of the European tree-killing wood wasp Sirex noctilio, which was found established in exotic Pinus radiata plantations in Australia about 1950-51. The wasp, with its pathogenic symbiont Amylostereum areolatum, has severely damaged some unthinned pine stands of intermediate-age, and valuable shelterbelts on farmland in Tasmania and Victoria. One-year life cycles are predominant, but three-month cycles and two or three-year cycles are also known. Emergence of adults, followed by attack on pine, occurs mostly between mid and late summer, when soil moisture levels, growth rates and tolerance of pine to pests and diseases are low. Various volatile substances produced by phloem/cambium tissues of stems and large branches, are important in the attraction of the pest. Susceptible trees are normally physiologically stressed, and attack on them is associated with a reinforcement of stress due to mucus injection, and a subsequent insect-pathogen development phase in the wood.In Tasmanian plantations, the introduced ichneumonid, ibaliid and stephanid parasitoids, and the parasitic nematode Deladenus siricidicola, have been effective control agents, though some outbreaks have collapsed even without natural enemies. In some parts of Victoria, where droughts are frequent and summer temperatures are high, an ecological balance between host and parasite populations may occur only after an unacceptably high level of tree mortality in stands that have remained unthinned due to unfavourable markets and unavailability of funds for sanitation felling. For such stands, a control strategy is being devised that will combine the effects of established biological control agents and minimum, selective noncommercial thinning, with the strategic placement of groups of trap trees, which have been predisposed to S. noctilio attack by injection with herbicide during spring.

Basal stem injection of Dicamba herbicide (20% 3,6-dichloro-2 methoxybenzoic acid) into the outer sapwood was tested as a technique for predisposing conveniently placed groups of Pinus radio/a trees to attack by the European tree-killing wood wasp Sirex noctilio. The trials included injection of Dicamba during spring and summer, injection of de-ionised water during spring and untreated controls, and were undertaken in unthinned plantations of two ages, 8 and 15 years.Sirex noctilio was attracted almost exclusively to the Dicamba-treated trees, which provided suitable breeding habitats for the pest and its introduced natural enemies, both wasp parasitoids and nematode parasites. Spring injection gave more satisfactory results than the summer injection. The trees in the older stand were more susceptible to attack, as were the trees in the smaller diameter classes in both stands. Infested treated trees contained an average of 275 wasps irrespective of tree age. Dicamba injections did not induce major attacks on any nearby untreated trees.The practical importance of these findings is that S. noctilio can now be readily detected, its populations periodically monitored, and control improved in plantations, by attracting the pest, and its lethal wasp parastoids, to strategically placed and widely distributed groups of Dicamba-treated 'trap' trees. Inoculation of the trees with the nematode parasite, De/aden us siricidicola, which infests S. noctilio larvae, will cause the infested females which subsequently emerge to be sterile and, as a result, the population is expected to fall.

In June 1981 heavy falls of wet snow with air temperatures near 0°C and strong winds caused extraordinary damage in both pine plantations and native eucalypt forests in much of southeastern Australia, especially at altitudes between 700 m and 1200 m. Using data from a thinning trial, inventory plots, forestry records and extensive inspections of plantations and arboreta, a study was made of this damage to identify types, incidence and mechanisms of damage in Pinus radiata plantations, and their silvicultural implications.The most widespread but least injurious types of snow damage were the bending and breakage of branches. Uprooting was important in only one plantation. The main damage was the bending or breakage of stems. Some 500 ha of pine plantations have had to be salvaged by clearfelling and a further 1700 ha by heavy thinning, most of them about 15-30 m tall.Snow damage toP. radiata stems was found to be influenced by: (a) bending moments (due to imbalance of the loaded crowns and lean of the tree), (b) the height/diameter (HID) ratio of the stems, (c) the shelter enjoyed by the trees below the general canopy, (d) the height/spacing ratio (and hence the chance of one tree falling on another), (e) selective removal of the weaker, less stable individuals of the stand, (f) effects of growing space on diameter growth and hence HID ratios, and (g) effects of age on HID ratios and the intrinsic strength of the wood. Comparisons are made with other species.It is concluded that the HID ratio provides a valuable indication of the risk of snow damage, and that this risk can be largely controlled by spacing-thinning strategies. In plantations of P. radiata, trees should be allowed as much growing space from an early age as is compatible with other constraints; they should be established at no more than I 000 stems/ha and thinned by removal of the more slender trees before the dominants exceed 20 m in height.

In the Pinus radiata stands of north-eastern Victoria the symptom "dead top" is common and consistently associated with the fungus Diplodia pinea. In order to resolve the pathogenic status of D. pinea, its association with disease symptoms has been studied by histological examination and tissue isolations following greenhouse and field inoculations. Malformations were associated with infection by D. pinea and although the amount of tissue killed was small, the defect induced was considerable. Damage to the leader caused the greatest loss in wood quality. The fungus attacked the pith very readily after gaining access to this tissue through injuries and via needle or cone traces. In the pith, the fungus spread slowly downwards and was protected from changes in the external environment. Injection of D. pinea spores into the pith resulted in a very high percentage of infection, and caused the malformations and other symptoms associated with attack by this fungus in the field. Histological studies showed that the fungus spread rapidly in the pith tissues, causing alterations in cambial function which produced traumatic resin canals, parenchymatous cells, and compression wood. Field trials showed that fast-growing trees were more severely affected than slower-growing ones, and the latter recovered more quickly. The fungus persisted in the infected tissues for up to 400 days.

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