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Damage to trees in the Pacific Northwest ecosystems can occur at any point in a tree’s development. Of the three genera of voles, Microtus is the largest and most destructive to western forest ecosystems. Voles, like many small rodent species, have the ability to produce numerous litters in a single year and females are mature 30-40 days after they are born. Average litter size is three pups, and females can a produce a litter every 21 days. With this type of reproductive capability, it is easy to see why vole populations can rapidly expand in ideal habitats.