I have not raised the butterflies that lay their eggs on Willows Berger, but I will try to help. Documented use according to the list I depend upon from this site:

Red Spotted Purples use:

1. Salix nigra - black willow
2. Salix caroliniana - Carolina willow

Viceroys use:
1. Salix nigra - black willow

2. Salix caroliniana - Carolina willow
3. Salix caprea - goat willow

Mourning Cloaks use:
1. Salix matsudana - corkscrew willow

French Pussy Willow = Salix caprea
Arctic Blue Willow= Salix purpurea

I believe that several moth species also use Willows, and I suppose that any Willow might potentially be used by the butterflies that use them. HTH
Angie

http://faq.gardenweb.com/faq/lists/butterfly/2004041551026161.html

Thanks I read on some sites that viceroys really like it so ill will find out next year hope it works:)