Terry Johnson, a wildlife specialist with Georgia's DNR, has reported his findings while researching this topic:
The Entomology Department of Penn State University conducted a study to determine butterfly use of boxes from 1995-97. They monitored 40 boxes built by an Eagle Scout. The boxes were erected along a woodland trail known to be used by wintering butterflies.
At the end of the first winter after the boxes were up inspections revealed that spider silk was discovered in eight boxes. When the boxes were checked after they had been in place for two years the biologists found nothing in eight structures; however, 32 were used by an assortment of other critters. The abandoned nests of umbrella wasps were found in seven boxes; two contained dead stinkbugs; pupating gypsy moths were discovered in two boxes; a colony of ants had staked a claim to one box and three boxes harbored overwintering cluster flies; spider webs were found in 26 boxes, and a white-footed mouse's nest in another. None of the boxes had been used by a single butterfly.
The results of the study prompted Robert Snetsinger, one of the entomologists conducting the research, to write, "I have yet to see evidence to support the notion that butterflies actually need or use butterfly houses. My suggestion is, if you want to do something useful for butterflies, build them a mud puddle."