Fungal flora associated with Ips typographus: frequency, virulence, and ability to stimulate the host defence reaction in relation to insect population levels

This study was aimed at determining the composition of Ips typographus L. (Coleoptera: Scolytidae) associated fungal flora in France, its virulence, and its ability to stimulate host defence reactions. The relationship between these parameters and the beetle population levels was also considered. The study was conducted in 2001, 2002, and 2003 in Norway spruce (Picea abies (L.) Karst.) stands, with different bark beetle damage levels. In each stand, the frequency of association between fungi and I. typographus was determined. The virulence of the most frequent species was assessed through mass inoculations on living spruce trees. The ability to stimulate the host defence reactions was estimated with low-density inoculations. The most frequent species, Ophiostoma bicolor Davids. & Wells, Ophiostoma piceaperdum Rumbold, and Ophiostoma tetropii Mathiesen, were all pathogenic. Ophiostoma piceaperdum also induced intense defence reaction zones, suggesting that it could play a role in I. typographus population establishment on living trees. However, significant correlations between fungal frequencies and damage of the current year were observed only with O. tetropii or O. bicolor, and no relationships between damage of the previous year and fungal frequencies were found. The effects of some fungal species on beetle population dynamics was suggested, but selection of species during epidemic condition was not confirmed.