Off-host stages of temperate zone ectoparasites must overcome two challenges: coping with unfavorable seasons and synchronizing their life cycles with host availability. In general, little is known about the seasonal cycles of insect ectoparasites of warm-blooded animals. the current study investigates the unusual phenology of a viviparous hippoboscid fly, the deer ked (Lipoptena cervi L.), that parasitizes boreal cervids. Despite months of asynchrony in offspring production, the adults emerge synchronously in mid-August across the northern boreal zone. We examined the role of diapause variation in the synchronization of life cycles by testing adult emergence success and time in relation to offspring birth month (October to April) and with respect to chilling time and photoperiod. Unexpectedly, we found that photoperiod had no role in regulating the life cycle, but diapause was maintained as long as pupae were exposed to cold. Pupae born before February needed a slightly longer exposure to high temperatures to terminate diapause if the cold period was short. Despite the apparent importance of a long period of chilling for life cycle synchrony, it was not required to terminate diapause. this finding of cold mainly preventing, rather than promoting, diapause termination is not novel among temperate insects, but it is rare. Slow diapause termination as a response to exceptionally long exposure to high, not low, temperatures seems to be a cornerstone for synchronizing the life cycle in the deer ked.