My colleagues and I have been looking for caterpillars in recent (<12 year-old) openings (0.5-2 ha clearings created by timber exploitation) in the forests north of Lausanne Switzerland. We randomly selected 19 such openings and looked for caterpillars for 2 hours in each. We found 5 occupied openings. It will probably come as no surprise to most of you, but Emperors seems to prefer young to middle aged openings. To young = no sallows, to old = sallows have been cut down to promote more profitable woods. Given the rythm of interventions in Swiss forests, the optimal stage seems to be around 6-8 years after timber exploitation (sory, the legends are in french...).
What's interesting for me is:
- we can find caterpillars in randomly selected sites, which suggest a relatively high larval density in these openings (there must be false absence in our dataset too)
- we have shown this (serious-looking) graph to foresters and it helped making them understand the importance of keeping sallows in regeneration patches and it increased there awareness of the woodland butterflies (most of them were unaware of the existence of these beauties at all, the less so in their own woods)
NB: we did not look for caterpillars in edges, lanes and other potential places, so these results are probably very local/regional and will differ depending on where you study them.
Any input, thoughts, similar results would be greatly appreciated.