Cocoon Predation by Small Mammals, and Pine Sawfly Population Dynamics

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Abstract
Neodiprion sertifer (Hymenoptera: Diprionidae) cocoons were planted singly (50 cocoons per site), in groups of 10 (20 groups per site) and in groups of 50 (10 groups per site) at 21 mostly one-hectare sites representing a wide range of forest types. Small mammals destroyed 70% (range 28-99.6%) of the cocoons in one month at the 21 sites. In stands of barren pine forest on poor soil the numbers of predators were positively correlated with predation rate, but at more fertile sites, where predators were twice as numerous, there was no such correlation, perhaps because of more abundant alternative food. The masked shrew Sorex caecutiens and the bank vole Clethrionomys glareolus were the two most important predators. The shrew destroyed especially scattered cocoons while the vole destroyed especially clumped cocoons. The shrew and the vole memorized the locations of the cocoon groups within their home ranges, distinguished between small and large groups, and their movements were affected for (at least) one to five weeks by the previous presence of cocoon groups. We tested whether bird predation of cocoons in tree canopies would be less severe than small-mammal predation on the grond, where larvae spin cocoons. It would not: 85% in trees vs 70% on the ground. Small-mammal predation may contribute to two observations about pine sawfly outbreaks in northern Europe: the vulnerability of stands on poor soils (few predators) and the high outbreak frequency of N. sertifer the only northern European pine sawfly with a short cocoon period in summer (short predation period).