Experimental studies on the responses of European siricid woodwasps to host trees.
Studies were carried out in England in 1968-69 on the oviposition responses of the siricids Sirex noctilio F., S. juvencus (L.), S. cyaneus F. and Urocerus gigas (L.) to experimentally debilitated host-trees of Scots pine (Pinus sylvestris) and spruce (Picea sitchensis). Treatments included felled, fire-burned, logged and girdled trees that were treated at intervals before bioassay with caged siricid females. With comparable treatments, S. noctilio exhibited a distinct preference for pine, although it was also quite successful on spruce, while the other siricids showed a preference for spruce. The Sirex species successfully attacked more recently treated material, although U. gigas exhibited a preference for timber in a more advanced state of debility. All species showed preferences for girdled trees, although other treatments were also favoured by the different siricid species. Overall, the siricids preferred the area above the girdle irrespective of tree species and there was a higher rate of oviposition above the girdle in treated spruce trees. A wide range of moisture content in the timber appeared to have no effect on drilling activity or the successful development of eggs. The response of a wild population of S. juvencus to the treated spruce is also described. The results are discussed in relation to the ecology of Siricidae in Europe and the establishment of S. noctilio in Australia. It is suggested that, of the species studied, only S. noctilio poses a serious threat to the living pine stands of Australia.