The dormancy process that results in entry into diapause. Diapause is a neurohormonally mediated, dynamic state of low metabolic activity. Associated characteristics of this form of dormancy include reduced morphogenesis, increased resistance to environmental extremes, and altered or reduced behavioral activity. Full expression develops in a species-specific manner, usually in response to a number of environmental stimuli that precede unfavorable conditions. Once diapause has begun, metabolic activity is suppressed even if conditions favorable for development prevail. Once initiated, only certain stimuli are capable of releasing the organism from this state, and this characteristic is essential in distinguishing diapause from hibernation.