I have been trying to net a Red Spotted Purple that has been visiting my rotting fruit tray without any luck. I remembered reading about butterfly traps so I ended up taking one of the large circular butterfly pavilions and cutting the bottom out. I hooked the top to a shepards hook. The open bottom is placed about 3-4 inches above the fruit tray. I finally got her today. I will keep her a few days in a container with willow so that she can lay some eggs in peace. There is also fruit in there for her to eat. I can't wait to watch the little ones grow--I hope anyway. Don't count your butterfly eggs before they hatch.
This is the pavilion that I am talking about. It is easy to unzip the top and take the butterfly out. They fly up to the top and then usually don't fly back down.
Apparently, you can also nab moths who nectar with this trap.