The effect of Sirex noctilio infestation and fire damage on the chemical composition of South African-grown Pinus patula pulpwood

The South African pulp industry is reliant on Pinus patula as a main constituent of the softwood raw material. However, trees in plantations can periodically be physiologically stressed due to a variety of biotic and abiotic factors. Damaged or dead trees can constitute a loss of pulpwood production to the mill, with the economic consequence of having to source the timber elsewhere on the open market. Sirex noctilio (Hymenoptera: Siricidae) is recognised as a plantation pest in Pinus stands with significant economic consequences to the industry in the short and medium term. In 1987, 1.8 million P. radiata trees died in South Australia and the south-western parts of Victoria as a result of a S. noctilio outbreak (Haugen and Underdown, 1990). In response, an extensive inoculation program was started whereby 147 000 trees were inoculated with the parasitic nematode Deladenus siricidicola. Although biocontrol has been introduced in southern Africa in the form of the nematode D. siricidicola, introduction success, unlike in Australia, was reported to be poor (Hurley et al., 2007). In its natural range, which spans northern Africa and Eurasia (Wingfield et al., 2001), S. noctilio is not considered a serious economic problem (Slippers et al., 2001). The first sighting in South Africa of S. noctilio in 1994 was at Tokai near Cape Town (Baxter et al., 1995; Tribe, 1995), and a new era of integrated pest management was entered after its subsequent spread to the Eastern Cape and KwaZulu-Natal. In the large monoculture stands of P. patula in South Africa, favourable bioclimatic conditions and general absence of natural enemies have elevated this wasp to primary pest status. A recent survey established that as much as 46% of the P. patula plantations in the KwaZulu-Natal Midlands area are already infested at an intensity of 6%, which can escalate to 25% unless the problem is effectively addressed (Edwards, 2006). A mutual symbiosis exists between siricid wood wasps and Amylostereum spp. (Talbot, 1977; Martin, 1992). The relationship between these organisms is specialised and obligatorily species specific as far as the insect is concerned. The principal advantage for the fungus is that it is spread and inoculated into suitable wood substrates during wasp oviposition. In turn, the fungus rots and dries the wood, providing a suitable environment, nutrients and enzymes for the developing insect larvae (Slippers et al., 2001). Fires usually cause variable damage to plantations and, when extensive, compartments usually are clear felled as The infection and association between the wood wasp Sirex noctilio and the fungus Amylostereum areolatum is responsible for large-scale tree mortality in the Midlands of the KwaZulu-Natal province in South Africa. An exploratory investigation on the effect of the infestation of trees by the wood wasp and its associated fungus on the chemical composition of Pinus patula pulpwood was undertaken. Various tree classes representing different levels of physiological growth stress from this infestation were compared. Together with the above stress agents, fire damage to P. patula trees was also considered as a possible cause of changes in pulpwood chemical composition. Chemical analyses to determine the Seifert cellulose and Klason lignin contents as well as the levels of waterand solvent-soluble extractives were conducted using published Tappi standard methods. The results indicated only negligible differences between infestation levels (tree classes) with respect to Seifert cellulose and Klason lignin contents. In contrast, highly significant differences were observed for solventand water-borne extractives. It is evident from the results that P. patula trees engage a defence strategy to counter the effects of the infestation and the resulting physiological stress. The results of the chemical analyses suggest that trees should not be harvested any younger than the intended rotation age of 12 years and at the time of harvesting all the biomass, including the infected wood, should be sent to the pulp mill. Fire-damaged trees can be utilised in the same way as healthy trees when applying the TMP process provided the charcoal on the outer bark is removed. The usefulness of a biplot to simultaneously display the various tree classes and their chemical composition is illustrated.