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The transitional zone between prairie and boreal forest in Manitoba and Saskatchewan is more suited to the needs of forestry than to agriculture. Tree production is difficult in this zone for a number of reasons, one of which is the depredations of small mammals, especially the meadow vole, Microtus pennsylvanicus (Ord.). Vole populations peak every 3 to 5 years and on the average irruptions of serious importance occur about every 10 years. Populations in the transitional zone are generally higher than those in the treed areas of adjacent zones: areas of extreme populations coincide with areas of greatest forestry concern. Key factor analysis indicates that a high degree of damage predictability may be achieved by measuring juvenile vole survival. Spruce, jack pine, white pine, red pine and Scotch pine are increasingly vulnerable to rodent damage. Seed and stand density also influence degree of impact. Strategy of vole damage control must involve prediction surveys coupled with cultural, operational and baiting tactics.