Caterpillars eating my Birch Tree

[Suninfrance]

Can anyone help me identify these caterpillars which are busy munching my weeping birch tree  into a bare branched stick:

[IMG]http://i12.photobucket.com/albums/a231/suninfrance/P1100173.jpg[/IMG]

[IMG]http://i12.photobucket.com/albums/a231/suninfrance/P1100125.jpg[/IMG]

[IMG]http://i12.photobucket.com/albums/a231/suninfrance/P1100123.jpg[/IMG]

[IMG]http://i12.photobucket.com/albums/a231/suninfrance/P1100131.jpg[/IMG]

... and then any ideas how I can get rid of them.  The hairy bunched ones are just one "cell", there are about 30 of then in all.

Look forward to your replies.

Jan

xx