Survey Techniques

Nothing beats finding your own iris site (well, …).  We all need our own personal locality.  But finding new localities is hugely challenging and very time-consuming.  The scope is vast as this insect is undoubtedly very under-recorded, certainly within its core range.   Its national range boundaries are poorly known, and may well be fluid anyway.  These last two points largely explain why the Purple Emperor is not a BAP priority species – there is no evidence of any decline, rather the reverse as new sites are now steadily being discovered (e.g. recent discoveries in Hertfordshire and Kent, for example).  All this means that Butterfly Conservation has to have higher priorities than the Purple Emperor, though the society does categorise it as a Species of Conservation Concern.  Our own aim is simple: we don’t want it a SCC, let alone a BAP species; we want it to be common. 

Finding an indolent arboreal butterfly that occurs at very low population density is highly challenging.  For a start, we do not yet understand Purple Emperor population structures.  It may be that the butterfly occurs in clusters of tiny colonies within well-wooded (or well treed) landscapes, but we simply do not know.  Within such a matrix individual colonies flare up, peak, decline and die out.  He may well be moving around and moving on quite a bit, though with some longer term colonies. 

Recent Doings suggest, strongly, that iris  is by no means dependent of forests or large woods, and that a matrix of small woods, copses, tree-lined hedges, scrub and waste land may well suffice.  Certainly, the Emperor is not dependent on oak and is not an ‘ancient woodland’ species, occurring readily on neglected scrubbed-up heaths, downland, abandoned meadows etc. that have reverted to scrub woodland.  In such places sallow thickets develop, though they tend not to last for long. 

Essentially, iris is a butterfly of sallow thickets, not tall oak woodland.  Search, then, for places rich in middle aged or old sallows, especially where they occur in dense shady situations, or are shaded by tall trees to the south. 

There is no sound survey technique, and it may be that a combination of some or all of the following is necessary:-

  • Searching for exploring males in the mid to late morning.  At many (but not all) sites, the males frenetically search the tops of sallow brakes and of sallow-rich scrub along ride edges in mid to late morning.  This activity may be combined with visiting the ride surface for sustenance. 
  • Later, around noon, males sail around the crowns of trees.  We usually associate this behaviour with the upper reaches of oak trees, really because of the long history of oak plantation. 
  • 'Master Trees'.  These exist in and around woods on uneven ground and are essentially sheltered high points, particularly on the north-east or east side of stands of tall trees that provide shelter from dominant winds.  Some are along wood edges, especially north and east edges.  Often you are looking into the sun at 'Master Trees'.  Look at contours on 1:25000 scale maps and aerial maps on Google Earth, but activity may be highly localised.  Above all, search for areas of still air in glades surrounded by tall trees, and follow your intuition. ’  
  • ‘Oak-edging’.  Sometimes, males patrol, almost ceaselessly, along lines of tall oaks to the immediate west of young plantations rich in sallow scrub.  They do this from late morning through to about 2pm.  This activity may be a phenomenon of strong colonies.  Again, you are mainly looking into the sun. 
  • Females.  Hanging around in glades surrounded by tall sallows waiting for laying females can pay dividends.  Females can be seen laying on isolated sprays under the main canopy, and low on dangling sprays in heavy shade.  They are most active from 12.00-3pm. 
  • Searching for eggs and young larvae.  You need to look on the uppersides of sallow leaves that are mid green in colour, are soft, and have non-glossy upper surfaces.  Such leaf characteristics are found in quite heavy shade, from 4’ upwards.  Larvae are found almost exclusively on the tips of leaves, especially those that curl down a bit.  The feeding marks, on either side of the leaf tips, become prominent during September and October, making that a good survey period.  Concentrate on Goat Willow Salix caprea

If you find a new colony please report it to your local BC branch, and let the land owner know.