Hi
If they're
mason bees and not leaf cutter
bees, they are already formed in their cocoons when you received them and can stay in the fridge hibernating as long as they have
enough moisture, but I am not sure how viable they are over a year. I put mine out once our daytime temperature is consistently 50ยบ or above.
I don't have any
experience using them to pollinate melons - they tend to like fruit
trees and berry bushes. There are limits to the type of flowers they are able to pollinate because of their physiology and their ability to travel; they don't travel as far as European
honey bees, but they pollinate more flowers. If there isn't sufficient food, they'll try to find a place that has them that's not your
yard .
I'm sure the good folks at the Xerces Society
http://xerces.org/fact-sheets/ can lend a hand.
I love, love, love these bees!
Have fun!