I caught some live butterflies for experiment and never wanted them die promptly. I needed to feed them artificially and what came readily to mind was pure honey. I don't mind getting contributions on better artificial diet for butterflies.
Honey is very concentrated, it would have to be diluted a great deal to use for this. Butterflies need salts, and maybe other substances to survive. That is why you sometimes see them at puddles and on dead fish.
The instructions at
http://www.berkeleyswallowtails.com/homefed.shtml
might work.
But, as for everything in biology, it varies with the species.
As suggested in previous answer, you can go for diluted honey with water or sugar solutions. You can keep a piece of cloth or cotton dipped in sugar solution.