The Sirex wasp and its biological control in plantations of radiata pine variably defoliated by Dothistroma septospora in north-eastern Victoria

Summary

The distribution and severity of pine needle blight disease (PNB), caused in spring 1989 by an outbreak of the needle cast pathogen Dothistroma septospora, was assessed within a 10 km radius of two study sites near Myrtleford in the Wangaratta Pinus radiata plantation complex of north-eastern Victoria. Areas of severe needle cast were located on colour aerial photographs in spring 1989, then checked by ground survey for the percentage of visible crowns infected. Population levels of the Sirex wasp (Sirex noctilio), rated in terms of the cumulative percentage tree mortality induced by the wasp since 1989, were estimated in autumn/winter 1991. Twenty-one unthinned plots, each of 400 trees aged 14–15 years, were examined for S. noctilio and associated symptoms of D. septospora. The effectiveness of parasitoids was evaluated in the laboratory by monitoring total insect emergents from 222 billets (0.8 m long), cut in winter 1991 from 100 herbicide-injected ‘Sirex’ trap trees. All emergents of S. noctilio were checked for presence of the parasitic nematode Deladenus siricidicola.

Sirex noctilio was most active in stands with severe D. septospora infection levels and large tree diameters at breast height over bark (DBHOB), as shown by positive correlations between (1) D. septospora infection levels and S. noctilio population ratings for stands of similar mean DBHOB, (2)mean DBHOB and D. septospora plus S. noctilio ratings, and (3) mean DBHOB and S. noctilio ratings for stands with low D. septospora infection levels. Unthinned plantations of intermediate age, stressed especially on the better sites from intertree competition and defoliation through severe D. septospora infection, are therefore highly attractive to S. noctilio. Prompt control of this pathogen with copper-based fungicide is therefore essential to reduce stress effects in such plantations unless thinning is imminent. Three species of parasitoid, released near Myrtleford between 1976 and 1984, collectively caused 47.9% mortality of S. noctilio (range 29.8–62.7%), with Ibalia leucospoides being predominant. Levels of D. siricidicola. introduced artificially from 1976 to 1983, were sufficient for effective sterilisation of the female S. noctilio population in only 20% of the study plots. Trap tree programs should therefore be intensified in all ‘Sirex’-susceptible plantations affected by PNB disease to facilitate the supplementary artificial inoculation of the nematode and its natural spread.