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Pine voles (Microtus pinetorum) cause economic losses to orchardists in the eastern United States by gnawing on the roots of fruit trees. Although they are small in body size, their impact on orchards can be quite substantial: in 1979 half of the annual mortality of apple trees in Henderson County, North Carolina, was attributed to vole damage (Sutton et a1 1981). Rodenticide application integrated with cultural management is currently regarded as a good combination for controlling vole populations in orchards. However, this solution to the vole problem is incomplete because poisons may unintentionally harm non-target organisms (Hegda1, Gatz & Fite 1981, Merson & Byers 1981) and there is evidence of genetic plasticity in pine vole populations since some have developed resistance to the rodenticide endrin (Webb & Horsfall 1967). Clearly, we need to continue working towards a safe and effective program for controlling pine voles. One approach is to examine the biological factors that regulate vole reproduction.