Champignons phytopathogenes associes a deux coleopteres scolytidae du pin sylvestre (Pinus sylvestris L.) et etude preliminaire de leur agressivite envers l'hote

Francés. Nous nous sommes proposés de reconnaître les espèces de champignons susceptibles de jouer un rôle dans les relations entre le pin sylvestre et les scolytides Ips sexdentatus et Tomicus piniperda. L’étude a porté principalement sur les variations de la fréquence (pourcentage de présence) de chacun des champignons dans les divers relevés, au cours des phases d’évolution des insectes adultes, et sur une appréciation de leur agressivité pour le pin sylvestre. Ophiostoma brunneo-ciliatum et Ophiostoma ips peuvent peut-être jouer un rôle dans les mécanismes d’installation d’ips sexdentatus, par suite de leur agressivité moyenne, de la fréquence élevée de leur association avec l’insecte et des faibles variations de cette fréquence d’un prélèvement à l’autre. Chez Tomicus piniperda, c’est Leptographium wingfieldii qui s’est avéré posséder la plus grande chance de jouer ce rôle, grâce à une agressivité très forte, à une fréquence d’association dont les variations sont en concordance avec les stades de développement de l’insecte, et malgré les faibles valeurs de cette fréquence. Chacun des deux scolytides pourrait en outre correspondre à un type particulier de relations Pins-scolytides-champignons.

Inglés. Our aim was to determine the species of fungi which have a good possibility of playing a role in the relationships between scots pine and two of its harmful bark beetles, lps sexdentatus and Tomicus piniperda.Two species, Ophiostoma brunneo-ciliatum and Ophiostoma ips, for lps sexdentatus, and three species, Leptographium wingfieldii, Ophiostoma minus and Hormonema dematioides, for Tomicus piniperda, were first selected after the identification of various fungi isolated from the beetles and from their galleries. Their relations with the sapwood blue-stain were specified by an examination of the bark beetles’ attacks on trap-trees (table 4). The frequency of their associations (frequency of the presence of each fungus in the samples) with the insects was followed over a period of 3 years for I. sexdentatus, 4 years for T. piniperda, along the maturation stages of the adults. O. brunneo-ciliatum and O. ips had a high frequency of association with I. sexdentatus. L. wingfieldii had a low but uniform frequency, H. dematioides a high and uniform frequency and O. minus a very variable frequency of association with T. piniperda (table 1 and 2). The aggressiveness of these fungi for scots pine was assessed by inoculating trees with fungal cultures and measuring the intensity of the tree defense reaction and the extension of the fungus after 3 weeks. L. wingfieldii and O. minus have the highest aggressiveness. H. dematioides has little aggressiveness. O. brunneo-ciliatum and O. ips are intermediate (Fig. 1 and 2). It is concluded that O. brunneo-ciliatum and O. ips have a high probability of playing a role in the mechanisms of the establishment of I. sexdentatus in scots pine, because of their aggressiveness and their high constancy. L. wingfieldii has a good possibility of playing a role for T piniperda, because of its very high aggressiveness for scots pine, the steady variations of its frequency in synchrony with the development of the beetle and in spite of the low level of this frequency. The conditions in which the fungi could play such a role will have to be specified, but it seems that the two bark beetle species could each correspond to a different situation of pine-bark beetle-fungus relationship.