They are supposed to use cherry (prunus); I think mine even ate some quaking aspen. (double-checked my book--it's listed as a possible larval food.) But it will probably prefer the willow.

Congratulations, btw, what a wonderful discovery! Did the kids get all excited?

My daughter, who is in her mid 30 was excited. All the years of raising cats when she was home-she was never impressed, now that she has kids of her own (2 & 4) she is finally starting to appreciate her mother's strange hobby.
The little ones love the big monarch cats and even released some adults for Grandma yesterday-but they won't touch the butterflies-"they have pricky feet grandma".
I guess I will offer both. I have more cherry than willow, but I probably have enough willow for one cat.

I'd use which ever cuttings last longest in water.

I'm starting to think RSP cats will eat just about anything the new cat finds itself on. I found one on my Norway maple this year. I have found numerous ones on my crabapples.

KC

Willow leaves don't hold up in water at all - I'd use the wild cherry for sure!
Sherry

Now that I think about it, red-spotted purple and viceroy cats stay absolutely glued to the leaf they were laid on for quite a time before they move on to other leaves. The only way I can think of you could transfer from willow to cherry is to cut off the leaf end where the egg is and put it on a cherry leaf in hopes that the cat would go to the cherry leaf to start its 'string on the end of the leaf thing'. It'd probably be easier to just cover the willow tree with mesh - I assume it's small - or sleeve the particular branch the cat is on.
Sherry

Thanks everyone for the input. I already have the leaf inside, so I may do as missherry has suggested and glue/pin the tip of the leaf onto a cherry and see if it will transfer.
I do a few demo's for the local schools, so I like to be able to take and show other cats beside monarchs.
Connie