Butterflies and moths generally feed on nectar that is full of sugar, but low in sodium. So they have to get sodium (aka salt) elsewhere. This behaviour is called “puddling” - male butterflies and moths drink anything salty and collect salt to be given to females as a nuptial gift. The females lay eggs that are higher in salt, which results in better function of excitable tissues - muscles and nerves - providing a selective survival advantage. (Source: insect physiologist here).
... in my research lab we just use water with sugar and a tiny bit of salt - add a teaspoon of sugar to a cup of water, sprinkle in some table salt, mix and that’d be good enough. But don’t leave an open container - they’ll drown. I’d suggest a closed container (like a mason jar) with a paper towel leading out of it like a wick. (video) (source)
