The Collaborative International Dictionary of English v.0.48:

Lepidopterist \Lep`i*dop"ter*ist\, n. (Zool.)
   One who studies the Lepidoptera.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lepidopterist
    n 1: an entomologist who specializes in the collection and study
         of butterflies and moths [syn: lepidopterist,
         lepidopterologist, butterfly collector]