In the northern part of its range in North America, the Hessian fly,Mayetiola destructor(Say), has one generation each year and overwinters in diapause. In the southern part of its range, it may have six generations each year, and does not overwinter in diapause; however, its summer development is interrupted by an aestival period. Between these two extremes, the number of generations varies with geographical location and prevailing climatic conditions, and the species usually diapauses and aestivates each year. Laboratory aestivation can be induced by high temperatures (daily thermocycles ranging from 17 to 36°C), and terminated by exposure to four or more consecutive days at 21.1°C. Diapause was terminated at 21.1°C in about the same number of days of exposure to daylengths of 8:16 or 16:8 (L:D), suggesting that photoperiod is less important than temperature. Extremely high Indiana temperatures during the summer of 1988 induced a more intense aestivation, and its termination was delayed in the field. Many eggs were laid on wheat that fall after the Hessian fly-free date, and a very mild winter followed, resulting in the largest spring fly infestation recorded in the last 24 yr. A large percentage of aestival puparia submerged under water for weekly intervals up to seven weeks delayed pupating by the duration of time that they were submerged, suggesting that a higher titer of oxygen might be necessary to break aestivation.