Physiological reactions ofPinus radiatato attack by woodwasp,Sirex noctilioF. (Hymenoptera: Siricidae)

Abstract

Physiological studies ofPinus radiatadamaged artificially and of others attacked bySirex noctilioF. indicated that temporary disturbances of the normal physiology of the host trees produce potential breeding sites for the woodwasp. Different physiological processes within trees were monitored after treatment and attack; these included changes in trans-location, transpiration, phloem respiration and osmotic and moisture content. The response of theS. noctilioto trees baited with phloem extracts or monoterpene solutions was investigated. The results of these studies, together with the published findings of other workers on the effects of insect secretions on trees and the resistance phenomenon, were interpreted into a general hypothesis of the interaction of the insect with the host tree. It was found that damage impaired translocation and enhanced transpiration and phloem respiration. These changes resulted in changes in tissue permeability and increased the rates of monoterpene and water vapour loss through the bark. The attraction ofS. noctilioto trees was related to these changes, and the lowered osmotic status of the phloem favoured egg deposition. Drilling also was accompanied by the secretion of arthrospores of the symbiotic fungusAmylostereum areolatumand mucus, which inhibited translocation. High water tensions in stressed trees favoured fungus invasion, and low turgor resulting from the permeability changes minimised resinosis at oviposition sites. Successful establishment of the fungus, together with the effects of the mucus, resulted in tree death and the creation of a suitable milieu forSirexdevelopment. The hypothesis proposes that the outcome of any attack is related to the duration and effects of the initial stressor on the tree and the tree's ability to moderate these effects. The response in the physiology of individual trees to stress affects the probability of attack while the resultant interaction between insect and tree determines the intensity of drilling, arthrospore and mucus load, and tree susceptibility or resistance to that attack.