INTERACTIONS WITHIN AND BETWEEN SPECIES AT DIFFERENT DENSITIES OF THEBARK BEETLE IPS-TYPOGRAPHUS AND ITS PREDATOR THANASIMUS-FORMICARIUS

Interactions between the bark beetle Ips typographus and one of its pr edators, Thanasimus formicarius, were investigated in caged spruce log s containing both species in eight different density combinations. Pro ductively (offspring per female) of Ips was adversely affected by high Ips gallery density as well as high Thanasimus density. Thanasimus pr oductivity was enhanced by high Ips gallery density but negatively aff ected by high Thanasimus density. Ips productivity and Thanasimus deve lopmental rate differed between tree individuals, probably owing to tr ee-related differences in phloem thickness. Relative predator-caused I ps mortality was ca 20% higher at high gallery density (ca 300 egg gal eries per m(2)) than at low gallery density (ca 100 egg galleries per m(2)), indicating that mortality was density dependent. This differenc e was due to the fact that Thanasimus larval density was positively re lated to Ips gallery density. Mortality increased by ca 0.4% with each additional Thanasimus larva per m(2), independently of gallery densit y. Relative population levels of Ips and Thanasimus were monitored wit h pheromone traps in two regions differing in their Ips typographus ou tbreak history. Absolute catch as well as the Thanasimus/Ips catch rat io were ten times greater in the outbreak region than in the non-outbr eak region. Coupled with the results in the caging experiment, this in dicates that T. formicarius responds numerically to changes in I. typo graphus numbers per unit bark area as well as to Ips population change s at the regional level. The findings suggest that predation under bar k may be a significant factor in supressing I. typographus outbreaks.