Effects of growth and virulence of associated blue‐stain fungi on host colonization behaviour of the pine shoot beetles Tomicus minor and T. piniperda

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The pine shoot beetles Tomicus minor and T. piniperda are common in the Nordic countries. Of these, T. piniperda may attack and kill living but severely stressed trees, whereas T. minor has never been reported to be individually responsible for killing live trees. Both species are associated with blue‐stain fungi: T. minor with Ophiostoma canum and T. piniperda with Leptographium wingfieldii and Ophiostoma minus. The growth of these fungi was studied in phloem and sapwood of live Scots pine trees, on malt agar, and on malt agar under oxygen‐deficient conditions. Leptographium wingfieldii was more virulent (i.e. caused more extensive host symptoms) grew more quickly on malt agar, and was less affected by oxygen‐deficient growth conditions than either O. minus or O. canum. Ophiostoma canum was least virulent. In low‐density inoculations it induced lesions similar to those induced by sterile control inoculations; it grew very slowly on malt agar and stopped growing after ≈30 mm under oxygen‐deficient conditions. Ophiostoma minus was intermediate in all respects. The different virulence of the blue‐stain fungi associated with the two pine shoot beetles may explain the lower level of aggressiveness in T. minor.