The story from Yorkshire: Searching for the eggs of White-letter Hairstreak by Martin Greenland

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Martin Greenland as a member of Yorkshire Branch was their White-letter Hairstreak species co-ordinator until moving to Norfok. He has helped change the picture of White-letter Hairstreak distribution in Yorkshire and the north east of England.

The story from Yorkshire: Searching for the eggs of White-letter Hairstreak

Yorkshire is a large county in which to establish the distribution of this elusive butterfly. Considerable progress has been made through searching for eggs (and larvae) on elms in targeted squares.

Pros and cons
The main advantage of looking for eggs rather than adults is that there is much more time in which to do it. While the flight season may last 7 or 8 weeks, the eggs are on the twigs (whatever the weather!) from July to March, and eggshells can remain in situ for quite a while after that. It's certainly easiest after the leaves have fallen (and nettles and brambles have died down!) and before prolific seed clusters develop in April. Eggs strongly indicate a breeding presence, whereas a sighting of an isolated individual may not.

One drawback is that not everybody has acute enough eyesight: you need to be able to spot the eggs with the naked eye - although a hand lens is useful for checking if you're in any doubt. Also, not all sites lend themselves to this method. Trees growing tall and thin in woodland, or those flailed at field edges lack the necessary low branches. Temptingly suitable trees often overhang water, or are next to dangerous roads, or are just out of reach. In these cases it's better to scan with binoculars in June / July.

Where to look
First find your elm! In Yorkshire the species appears to breed almost exclusively on Wych elm, but this isn't the case further south. The species prefers flowering trees (the 1st-instar larva likes to hide and eat in a flower bud) - but if there isn't much alternative it is sometimes worth checking less mature trees: I've found a pupa on a non-flowerer, and eggs on several unpromising specimens. The problem comes if there are plenty of trees to choose from - where do you start? Remember the butterfly likes warmth and shelter: which tree offers the best prospects? Start at the sunny end of a row, or look for a tree set back a little from a woodland edge, creating an attractive bay.

Look for twigs with the rounded flower buds. Robust twigs are better than either the spindly growth you get in shade, or youthful, over-vigorous, whippy growth. Higher branches are usually better than lower: carry a hooked stick (umbrella or walking pole) to bend them down within reach (conveniently, 'wych' means flexible!).

On wych elm, the butterfly lays almost exclusively in specific places:-

• the underside of the girdle scar, where the most recent growth meets the older wood (often on older side-shoots rather than the leading stem);
• at the base of side shoots;
• on old leaf scars;
• at the base of buds.

Freshly laid eggs © photo: Martin Greenland	Egg on girdle scar © photo: Martin Greenland
Egg at base of side-shoot beginning to hatch © photo: Martin Greenland	Eggshell of hatched egg © photo: Martin Greenland

I tend to ignore buds: there are too many! It's relatively quick to twist the branch and scan the underside, checking girdle scars and forks. The eggs are usually described as being 'flying-saucer shaped'. Until they hatch, it's usually the contrast between the pale rim and the dark dome which catches my eye. After hatching, they are translucent, with a smooth hole in the dome. I also find various moth eggs, including those of the Brick, which feeds on wych elm seeds - but these are smaller, and lack the distinctive shape of white-letter.

Other signs
1st-instar larvae are tiny and flowers tend to disintegrate if searched, so I usually leave them alone.

1st instar larvae photo taken 3rd March 2007 © photo: Martin Greenland	1st instar larvae photo taken 3rd March 2007 © photo: Martin Greenland
2nd instar larvae (2) in developing seed cluster © photo: Martin Greenland	2nd instar larva (and feeding damage) © photo: Martin Greenland
3rd instar larva among unfurled leaves (note black head) © photo: Martin Greenland	4th instar larva in typical position under a wych elm leaf © photo: Martin Greenland

Seed clusters with eaten-out seeds are worth searching for 2nd-instar larvae. Large oval patches of feeding damage (sometimes with veins intact), especially at the base of a leaf, may betray the presence of a mature larva - look for silhouettes from below, against the light. The same technique applies to pupae - but there aren't many of them!

4th instar larva © photo: Martin Greenland	Corpse of parasitised 4th instar larva © photo: Martin Greenland
Final stage larva in position prior to pupation: it has darkened in colour and secured itself by its silk girdle, but hasn't yet shed its larval skin. © photo: Martin Greenland	Larva of Brick moth and feeding damage © photo: Martin Greenland

Pupa photographed end May 2007 in Derbyshire © photo: Martin Greenland

Results
In Yorkshire, targeted searching for eggs has been effective in establishing just how widespread the species can be. In my home area, egg records show a continuous distribution across whole blocks of tetrads, in a way that the records from casual sightings of adults do not (see maps).

Map: 2004 - 2006 White-letter Hairstreak Distribution (2km square) - South Yorkshire Ref: Martin Greenland Butterfly Conservation Yorkshire Branch

Map: 2004 - 2006 White-letter Hairstreak Distribution (10km square) - all Yorkshire Ref: Martin Greenland Butterfly Conservation Yorkshire Branch

Countywide, egg-searches in 'empty' 10km-squares have significantly contributed to increasing the number of occupied 10km-squares from 58 at the end of 2002 to 124 at the end of 2006.

Martin Greenland
January 2007