In order to evaluate the hazard posed by insects on windthrow timber preserved on alpine passes, spruce logs were placed on the Lukmanier Pass (1900 m a.s.l.) and in Sedrun (1350 m a.s.l.) in the adjacent Anterior Rhine Valley in the Canton of Graubünden. Over a period of two years, temperature, insect flight activity and log infestation were recorded.
On the Lukmanier Pass, phloem temperature of the upper, sun-exposed log halves exceeded ambient air temperature by as much as 23 °C as a result of the intense sun radiation. As long as a snow cover insulated the logs, phloem temperature constantly remained between –2 and 0 °C.
The fact, that the investigated logs had already been infested in the windthrow area and that large amounts of infested timber were stored nearby on the pass by the forest service, rendered the interpretation of the results rather difficult. However, the data made clear that an infestation of logs by detrimental insects was possible even at high elevations. Moreover, larvae were able to complete their development, though retarded by low temperatures. The insects preferred the lower, shaded parts of the logs. Infestation presumably originated from the pre-infested timber rather than from autochthonous populations. This was suggested by the low numbers of xylobiontic insects caught in flight traps placed at a distance from the timber yards. If non-infested logs were transported to high altitudes, infestation pressure would be significantly lower than in valleys.