Andrew Daw andrewd at redac.co.uk
Fri Jun 5 08:23:46 EDT 1998

>      I was wondering what drives butterflies and moths to 
>      migrate (in this case northwards into southern UK), 

Here are my thoughts (they may not necessarily be correct though):

As with most species of animal, it would normally be pressure on
resources (usually lack of food/foodplants) that forces a species
to look for new habitats.
Even our most sedentary species are known to fly off for long
distances (by their standards) in bumper years.

In Europe for exmple, the Painted Lady (Cynthia cardui) is resident
in North Africa.  By early spring the local population has grown to a
point that the local habitat cannot support them, and they head off for
pastures new.

Along with the annual spring migration of birds from Africa into Europe
every spring there is also a migration of insects too.  One of the natural
points for them to cross the Mediterranean sea is at Gibralta, and head up
through Spain. The next obstacle into Europe is then the Pyrenees mountains
between Spain and France.  Here the butterflies get funnelled through the
mountain passes making the migration more obvious to observers (or so I am

To reach the UK there is one more barrier, the English Channel.  With
favourable winds, Painted Ladies make it to the UK virtually every year,
but in varying numbers.  We would expect them to reduce in numbers as they
head North as less and less feel the urge to carry on having found suitable

>      into areas in which they cannot survive the winter.  Do 
>      some of the later/autumn generation migrate south to 
>      redress the balance?

Yes, there is a southerly migration every year too.  This is not so
pronounced as the Northerly migration as most of the butterflies are close
to the end of their natural adult lifespan, and failing weather can catch
them out too.  Being an African species a cold snap (overnight frost) can kill
them off quite easily compared to our more hardy resident species.  Note:
The Clouded Yellow (Colias croceus) is even less tolerant of the cold.

* Andrew Daw                   tel  :  +44 (0)1454 207 800        *
* Redac Systems Ltd            email:  andrewd at redac.co.uk        *
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