Fully grown larvae of Prodoxus y-inversus (Riley) undergo diapause in hardened cysts in the sterile tissue of Yucca baccata Torrey fruits. Dormancy normally lasts from early summer until the following May, but when optimal climatic cues, particularly winter chilling, are not received, the diapause is maintained. Larvae of the 1969 generation were collected in April 1970 in southern Nevada and held in winter conditions that varied but were warmer than those to which the population was adapted in Nevada. More than 180 individuals emerged synchronously following the 16th and 17th winters. Temperature data suggested that the intensity of winter chilling is the primary factor initiating diapause development and that after many years in diapause larvae are conditioned to respond to temperature regimes that were not acceptable for development in the early years of diapause. To test these hypotheses, larvae were held in constant warm temperatures for 4–5 yr, then exposed to a variety of winter conditions. Additional large groups eclosed following their 20th (151 adults) and 25th (125 adults) winters. X-rays before the 30th winter indicated that few larvae remained, and 14 moths emerged following 30 yr in diapause.