Successful bark-beetle biocontrol. A review of the only case in history Releasing Rhizophagus grandis in Georgia, France, Britain and Türkiye against Dendroctonus micans

The bark beetle Dendroctonus micans (Kug.) (Coleoptera, Curculionidae, Scolytinae), a pest of spruce and pine, probably originates from eastern Siberia. Its closest relative is the North American D. punctatus (LeConte), on the other side of the Behring Strait. D. micans has largely spread westward and is presently established in most of the spruce forests of Eurasia. In newly colonised territories, it is considered as a tree-killing pest. A very specific predator, Rhizophagus grandis (Coleoptera, Monotomidae), has followed its prey in its territorial expansion, with temporal and spatial lags due to long distance passive transportation of D. micans with trade, and to habitat fragmentation. When both species coexist, D. micans is very sparce and only occasionally kills trees, because the high capacity of R. grandis to locate prey even at very low densities enables it to control the pest. This very particular relationship has been exploited for biological control. Mass production and inoculative releases of R. grandis have been undertaken in four countries: Georgia since 1963, France (1978), Great Britain (1983) and Türkiye (1985). Two main production techniques were used, rearing on D. micans -infested logs, and semi-artificial methods in polystyrene boxes containing live prey larvae. Four million R. grandis were produced and released in Georgia, 800,000 in France, 250,000 in Britain and 10,880,000 in Türkiye. The predators established wherever they were released, gradually colonizing high proportions of the pest brood chambers, and spreading at various rates depending on local conditions. Four to eight years after the releases, effective and stable control was recorded in all the sites treated. This is the only recorded case of fully successful classical biological control of a bark-beetle pest. The reasons and limitations of this success are discussed.