Narrow-bordered 5-spots or 5-spots?
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In the Bristol region lonicerae is now widespread (and probably increasing) whereas trifolii is now only known from one or two sites. Maybe the availability of suitable foodplants (and habitats) is a factor (it seems trifolii is more choosy requiring Greater Bird's Foot Trefoil). I don't know the situation in the Cotswolds but I believe trifolii is much more common.
I've been told that trifolii (5 spot) hasn't been recorded in Warwickshire (we're just north of the Cotswolds), so they're much more likely to be lonicerae (narrow-bordered 5-spot). I have seen the caterpillars, but haven't figured out the difference between them either.
Also, there was no Greater Bird's Foot Trefoil at the site. Curiously I went to another nearby site, just a couple of miles away, with a very similar wide range of flowers, and there, as far as I noticed, the burnets were all 6-spots - hundreds of them. There were no 6-spots at the site where the NB 5spots were.
If you are in an area where there aren't any 5 spots you are probably safe to assume it's a Narrow-bordered 5 spot (unless 5 spots show some sign of re-colonising).
and this is the main method I use to identify them on my site.
Lat/Lng: 52.1411, -1.615
OS grid ref: SP264492