Abstract
Predation by small mammals has been reported as an important mortality factor for the cocoons of sawfly species. However, it is difficult to provide an accurate estimate of newly spun cocoons and subsequent predation rates by small mammals for several reasons. First, all larvae do not spin cocoons at the same time. Second, cocoons are exposed to small mammal predation immediately after being spun. Third, the cocoons of the current generation are indistinguishable from those of the previous generation. We developed a hierarchical Bayesian model to estimate these values from annual one-time soil sampling datasets. To apply this model to an actual data set, field surveys were conducted in eight stands of larch plantations in central Hokkaido (Japan) from 2009 to 2012. Ten 0.04-m2 soil samples were annually collected from each site in mid-October. The abundance of unopened cocoons (I), cocoons emptied by small-mammal predation (M), and empty cocoons caused by something other than small-mammal predation (H) were determined. The abundance of newly spun cocoons, the predation rate by small mammals before and after cocoon sampling, and the annual rate of empty cocoons that remained were estimated. A posterior predictive check yielded Bayesian P-values of 0.54, 0.48, and 0.07 for I, M, and H, respectively. Estimated predation rates showed a significant positive correlation with the number of trap captures of small mammals. Estimates of the number of newly spun cocoons had a significant positive correlation with defoliation intensity. These results indicate that our model showed an acceptable fit, with reasonable estimates. Our model is expected to be widely applicable to all hymenopteran and lepidopteran insects that spin cocoons in soil.
Highlights
Accurate estimates of animal abundance provide basic but important information for ecological issues
Ecology and Evolution published by John Wiley & Sons Ltd
It was difficult to estimate the number of spun cocoons and the predation rate of cocoons by small mammals for several reasons
Summary
Accurate estimates of animal abundance provide basic but important information for ecological issues. Wildlife managers use abundance to decide management options for pest populations or rare species. Obtaining accurate and reliable methods for estimating abundance is, critical for the success of a diverse array of ecological applications (Conn et al 2006). Sawflies are primitive Hymenoptera that are a major taxonomic group of the free-living defoliator guild (Wagner and Raffa 1993). The populations of some sawfly species sometimes reach outbreak level, causing the conspicuous defoliation of host plants. The cocoon stage of these species has received more detailed investigation than the adult, egg, and free-living larval stages for several a 2015 The Authors.
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