Insects that remain in diapause for more than 12 months or skip at least one breeding season are in prolonged diapause (also called extended diapause, Hanski 1988). Both the parental generation and the current generation may play roles in determining the duration of diapause (i.e., diapause intensity). The parental generation plays a larger role in insects diapausing in the egg stage, particularly for eggs in which the embryo has not yet developed a nervous system (Hanski, 1988, Mousseau and Dingle, 1991). For some insects, transgenerational effects on diapause extend back to the grandparents and potentially further (e.g., Reznik et al. 2012), but the effects of previous generations are likely to be less than those of the immediate parents, which can directly transmit maternal RNA, hormones, or other cytoplasmic factors to the offspring (Denlinger 2002). Moreover, paternal nutrients or other factors may be transferred via nuptial gifts (e.g., Gwynne, 2008, Smith et al., 2017).

Although not confirmed in nature, laboratory-reared Mormon crickets Anabrus simplex Haldeman, 1852 (Orthoptera: Tettigoniidae) have a multiannual lifecycle (a.k.a. plurennial, Ingrisch 1986) in which eggs are capable of remaining in prolonged diapause for many years (Srygley, 2020a, Srygley, 2020b, Srygley, 2024a, Srygley, 2024b). Mormon crickets are native to rangeland in the western United States (Wakeland, 1959). Their propensity to aggregate into dense swarms, or bands, while immature, and to march directionally in search of nutrients, makes them a significant pest of crops and livestock forage (Cowan and Shipman, 1947, MacVean, 1987). Once immatures molt into adults, which lack functional wings for flight, Mormon crickets continue to march directionally, and in unison, stop daily to feed, mate, and lay eggs. Eggs are laid approximately 2.5 cm beneath the soil surface (Cowan 1929). In the laboratory, eggs laid by a single female can vary from all developing directly that same growing season after being laid to some or all of them arresting development at a microscopic blastula stage (Srygley 2020a). The extremely low metabolism of this stage allows the eggs to remain in the soil for multiple years in diapause (called ‘egg diapause’, Ingrisch 1987). In laboratory environments, eggs from a single female typically emerge from egg diapause at a steady rate over time (Srygley 2024a,b). Eggs that terminate diapause each growing season develop until the embryo fills the egg to then enter an obligate diapause (called ‘embryonic diapause’), in which it will overwinter and hatch the following spring. The factors influencing the timing and coordination of Mormon cricket egg development are not well understood, but in laboratory settings, parental photoperiod, body temperature, and nutrition, and the egg’s soil environment, including temperature and moisture, all affect the duration of egg diapause (Srygley, 2014, Srygley, 2020a, Srygley, 2024a).

In this study, I investigated the effects of parental photoperiod and offspring environment on prolonged egg diapause, embryonic development and hatching in natural settings populated by Mormon crickets in the Bighorn Mountains of Wyoming. The Bighorn Mountain population was particularly useful for investigating the relative role of the parental and offspring environments in determining diapause duration because, historically, it was the only one documented to have a biennial lifecycle (Cowan and Shipman 1940), whereas all other populations were conventionally deemed ‘annual’ (Cowan, 1929, Wakeland, 1959, Hartley, 1990).

As with other mountain ranges, higher elevation sites in the Bighorn Mountains are typically cooler and wetter than lower elevations, and the growing season tends to begin later and end earlier at higher elevations. Because the growing season is longer at lower elevations, I predicted that eggs at the lower elevation would develop and hatch faster than those at higher elevation (Srygley 2024a). On the other hand, the annual drying of the soil at the lower elevation desert sites might prolong diapause or cause greater egg mortality (Srygley 2020b).

In addition, parental photoperiod might serve as a reliable cue for the time that remains for eggs to successfully complete development before the growing season ends (Bradshaw et al. 2004). Hence, I predicted that a long-day, mid-summer photoperiod would serve as a temporal cue favorable for eggs to complete development the summer that they were laid, whereas a short-day, end of summer photoperiod would cue insufficient time delaying embryonic development until the following year (also see, Srygley, 2020a). As a result, Mormon cricket parents in a long-day photoperiod would lay eggs that were more likely to develop in the first growing season and hatch the following spring relative to eggs from parents in a short-day photoperiod, which should delay development to at least the second growing season. In addition, the greater likelihood of eggs of long-day photoperiod parents completing development in the first growing season should be most evident at lower elevations where the growing season is longer. This experiment in a natural setting was set up at the same time and with eggs from the same parents as a laboratory experiment (Srygley, 2020a).