Andy
I think that there are effectively two dynamics here. There are genuine migrants/vagrants and there are colonists.
On the first category, a flick through Robineau (the French equivalent of Skinner) will give you a significant headache at the number of 'lookalike' species on the continent on things that you would simply not look at twice from an identification point of view on a busy night because there are no British confusion species - Broad-bordered Yellow Underwing; Annulet; Netted Pug; etc. On that front, occasionally, something begins to be highlighted such as The Feline (Puss Moth look-a-like) or The Sorcerer (Alchymist look-a-like) which brings things to people's radar but even on things like Flame Brocade and Clancy's Rustic there are confusion species which may be overlooked easily. For instance, on Beautiful Marbled, the 'first' for Britain was soon discovered not to be the first for Britain when an earlier collected Purple Marbled was reidentified. Most people had never previously heard of purpurina. So getting to grips with cryptic continental species which may otherwise be overlooked is a considerable task.
On the second category, insect populations move much more quickly than larger species. Conistra rubiginosa is a clasic example. I quote from a message on the migrant moths e-mail group:-
"Hi all
It may be of interest to you, that in my country Denmark this species is
not regarded as a true migrant but as a very clear expansionist species.
When I started being interested in moths 35-40 years ago it only occurred
in our eastern most island of Bornholm, then it invaded a small island in
Kattegat (probably from Sweden), and after year 2000 it has spread to all
the major islands and started to appear in Jylland (west) as well. Where I
live in Copenhagen it has been clearly the most abundant late flying
noctuid for at least 5 year easily outnumbering C. vaccinii and E.
transversa.
I think it is quite likely, that you will find that you have a new
colonizer rather than a vagrant.
By the way: It is much easier to find with sugar ropes than ligth.
best wishes
Jan F. Rasmussen"
In that type of situation, French, Dutch and other European moth-trappers may be able to provide an early warning system which is picked up. See Jon Clifton's prediction of this week's events in the Norfolk Moths Group Spring newsletter (pages 12, 13 and 15):-
http://www.norfolkmoths.co.uk/newsletters/79.pdf
All the best
