P BONELLO¹, TR GORDON¹, AJ STORER², WR MCNEE² and DL WOOD²

¹Department of Plant Pathology, Hutchison Hall, University of California, Davis, CA 95616, USA; ²Department of ESPM, Wellman Hall, University of California, Berkeley, CA 94720, USA

Background and objectives
It is common knowledge and experience that conifers infected by fungal pathogens are predisposed to attack by bark beetle. However, very little information is available on the physiological mechanisms underlying this predisposition. In this paper we investigate some of the changes in ponderosa pine (Pinus ponderosa) induced by the root pathogen Heterobasidion annosum which may influence the behaviour of mountain pine beetle (Dendroctonus ponderosae), western pine beetle (Dendroctonus brevicomis) and the California five-spined ips (Ips paraconfusus). This complex of host tree, disease and insect pests is common in the central Sierra Nevada in California.

In particular, we present data addressing the possibility that (i) pathogen-induced, elevated evolution of ethylene from the canopy may be a mechanism of primary attraction of bark beetles; and (ii) root infection may induce systemic accumulation of auto-toxic soluble phenolic compounds of host origin which may affect beetle feeding behaviour.

Results and conclusions
Preliminary data from a field experiment established in 1997 show that 4 months after inoculation, current-year needles of 40-year-old inoculated trees evolved on average more ethylene than corresponding needles from mock-inoculated and control trees. In a study using an olfactometer for walking insects, significantly more I. paraconfusus beetles walked upwind when ethylene was present in the test air than with air alone. In addition, male beetles (the sex initiating tunnelling in this species) appeared more attracted to the ethylene source than the females, although a hierarchical log-linear analysis showed no significant differences between the sexes. More tests will be conducted using the other beetle species.The trend in ethylene evolution, coupled with the olfactometer results, suggests that increased ethylene evolution may be a mechanism by which bark beetles locate diseased trees suitable for colonization.

In two independent greenhouse studies, it was found that systemic accumulation of the pine stilbenes pinosylvin and pinosylvin monomethylether was associated with symptom expression in artificially inoculated 4-year-old ponderosa pine trees. This confirms results from a previous study in which autotoxicity of pine stilbenes was demonstrated in 6-week-old Scots pine seedlings [1], and indicates that stilbene autotoxicity may be one of the steps that, directly or indirectly, render infected trees palatable to bark beetles.

Although these results are preliminary, certain phenomena are beginning to appear that may shed light on the complex physiological mechanisms underlying the relationships between root disease agents, hosts, and tree killing bark beetles. It is expected that a better understanding of these interactions will enable more directed efforts to identify control measures which will maintain forest productivity in the future.

References
1. Bonello P, Heller W, Sandermann H Jr, 1993. New Phytologist 124, 653-663.