A synopsis of the genus Ethmia Hübner in Costa Rica: biology, distribution, and description of 22 new species (Lepidoptera, Gelechioidea, Depressariidae, Ethmiinae), with emphasis on the 42 species known from Área de Conservación Guanacaste.

We discuss 45 Costa Rican species of Ethmia Hübner, 1819, including 23 previously described: Ethmiadelliella (Fernald), Ethmiabittenella (Busck), Ethmiafestiva Busck, Ethmiascythropa Walsingham, Ethmiaperpulchra Walsingham, Ethmiaterpnota Walsingham, Ethmiaelutella Busck, Ethmiajanzeni Powell, Ethmiaungulatella Busck, Ethmiaexornata (Zeller), Ethmiaphylacis Walsingham, Ethmiamnesicosma Meyrick, Ethmiachemsaki Powell, Ethmiabaliostola Walsingham, Ethmiaduckworthi Powell, Ethmiasandra Powell, Ethmianigritaenia Powell, Ethmiacatapeltica Meyrick, Ethmialichyi Powell, Ethmiatransversella Busck, Ethmiasimilatella Busck, Ethmiahammella Busck, Ethmialinda Busck, and 22 new species: Ethmiablaineorum, Ethmiamillerorum, Ethmiadianemillerae, Ethmiaadrianforsythi, Ethmiastephenrumseyi, Ethmiaberndkerni, Ethmiadimauraorum, Ethmiabillalleni, Ethmiaehakernae, Ethmiahelenmillerae, Ethmiajohnpringlei, Ethmialaphamorum, Ethmiapetersterlingi, Ethmialesliesaulae, Ethmiaturnerorum, Ethmianormgershenzi, Ethmianicholsonorum, Ethmiahendersonorum, Ethmiarandyjonesi, Ethmiarandycurtisi, Ethmiamiriamschulmanae and Ethmiatilneyorum. We illustrate all species and their male and female genitalia, along with distribution maps of Costa Rican localities. Immature stages are illustrated for 11 species, and food plants are listed when known. Gesneriaceae is added as a new food plant family record for Ethmia. CO1 nucleotide sequences ("DNA barcodes") were obtained for 41 of the species.