as well as fruiting trees where excess fruit drops to the ground.
In forests, there are fewer flowers, especially in summer and fall when the trees have leafed out. Therefore, various butterflies, including the Morning Cloak and the Question Mark, seek alternative sources of nutrients. They will feed on rotting fruit, sugar solutions, tree sap, animal dung and carrion. The easiest and perhaps the most palatable of these options is rotting fruit. Rotting fruit can ferment and it is possible that a butterfly could get drunk on this stuff.
It is necessary to ensure that the fruit actually rots and not just dries out. Use large chunks of fruit equivalent to about quarter of an apple. Place the fruit in shade. The fruit will bring in other insects that you may not want, so place the feeder away from door entrances to your house. You will probably be successful if you use apple, but you can also use pear, banana or pineapple.