For insect species in temperate environments, seasonal timing is often governed by the regulation of diapause, a complex developmental programme that allows insects to weather unfavourable conditions and synchronize their life cycles with available resources. Diapause development consists of a series of distinct phases including initiation, maintenance, termination and post-diapause development. The evolution of insect seasonal timing depends in part on how these phases of diapause development and post-diapause development interact to affect variation in phenology. Here, we dissect the physiological basis of a recently evolved phenological shift in Rhagoletis pomonella (Diptera: Tephritidae), a model system for ecological divergence. A recently derived population of R. pomonella shifted from specializing on native hawthorn fruit to earlier fruiting introduced apples, resulting in a 3-4 week shift in adult emergence timing. We tracked metabolic rates of individual flies across post-winter development to test which phases of development may act either independently or in combination to contribute to this recently evolved divergence in timing. Apple and hawthorn flies differed in a number of facets of their post-winter developmental trajectories. However, divergent adaptation in adult emergence phenology in these flies was due almost entirely to the end of the pupal diapause maintenance phase, with post-diapause development having a very small effect. The relatively simple underpinnings of variation in adult emergence phenology suggest that further adaptation to seasonal change in these flies for this trait might be largely due to the timing of diapause termination unhindered by strong covariance among different components of post-diapause development.
The fall webworm, Hyphantria cunea (Drury), enters facultative diapause as a pupa in response to short-day conditions during autumn. Photoperiodic response curves showed that the critical day length for diapause induction was 14 h 30 min, 14 h 25 min and 13 h 30 min at 22, 25 and 28°C, respectively. The photoperiodic responses under non-24 h light–dark cycles demonstrated that night length played an essential role in the determination of diapause. Experiments using a short day length interrupted by a 1-h light pulse exhibited two troughs of diapause inhibition and the effect of diapause inhibition was greater in the early scotophase than in the late scotophase. The diapause-inducing short day lengths of 8, 10 and 12 h evoked greater intensities of diapause than did 13 and 14 h. Diapause can be terminated without exposure to chilling, but chilling at 5°C for 90 and 120 d significantly accelerated diapause development, reduced mortality, and synchronized adult emergence. Additionally, the potential for H. cunea from the temperate region (Qingdao) to emerge and overwinter under field conditions in subtropical regions (Nanchang) of China was evaluated. Pupae that were transferred to Nanchang in early July showed a 60% survival rate and extremely dispersed pupal period (from 12 to 82 days), suggesting that some pupae may undergo summer diapause. Diapausing temperate region pupae that were moved out-of-doors in Nanchang during October showed approximately 20% overwintering survival; moreover, those pupae that overwintered successfully emerged the next spring during a period when their host plants would be available. The results indicate that this moth has the potential to expand its range into subtropical regions of China.
The Asian corn borer, Ostrinia furnacalis (Guen ee) (Lepidoptera: Crambidae), enters facultative diapause as fully grown larva in response to short-day conditions during the autumn. Photoperiodic response curves showed a stable critical daylength of ca. 13.5 h at 25, 28, and 30°C, which is nearly identical with the critical daylength between 13 h 22 min and 13 h 27 min in the field. The incidence of diapause obviously declined during ultra-long nights (14-22 h scotophases) and DD (i.e., constant darkness). The required day number (RDN) for a 50% response differed significantly between short-and long-night cycles at different temperatures, indicating that the effect of one short night was equivalent to the effect of three long nights at 25 and 28°C. The third instar was the stage most sensitive to the photoperiod. Night-interruption experiments with a 1-h light pulse at L9:D15 and L12:D12 exhibited a broad trough of diapause inhibition. In experiments using non-24-h light-dark cycles, only the cycles containing a long night of 12 h resulted in high incidence of diapause, regardless of the duration of the accompanying light period, indicating that the duration of the dark period was of greater importance than that of the light period. The Nanda-Hamner and B€ unsow experiments in O. furnacalis did not show any rhythmic fluctuations with a period of ca. 24 h in their photoperiodic response curves; the incidence of diapause was low when the scotophase length exceeded 16 h. The rearing daylengths of 11 and 12 h evoked a greater intensity of diapause than did 13 h at 25 and 28°C. By transferring naturally overwintering larvae from natural conditions to a short photoperiod of L12:D12 or a long photoperiod of L15:D9 combined with 25°C, photoperiod appeared to have a significant influence on diapause development during the early phase of diapause. Field observations for 2 years revealed that pupation and adult emergence of overwintering individuals (50% responses) occurred in early May and mid-May, respectively.
The intensity of pupal diapause in the cotton bollworm, Helicoverpa armigera (Hübner) was investigated under both laboratory and natural conditions. By transferring diapausing pupae induced under LD 11:13, LD 12:12 and LD 13:11 at 20, 22 and 25 °C to 25 °C combined with LD 15:9 to terminate diapause the rearing day length of 11 h evoked greater intensity of diapause than did 12 and 13 h at 25 °C; whereas the rearing temperature of 25 °C evoked more intense diapause than did 20 and 22 °C under LD 11:13. By transferring diapausing pupae induced under LD 12:12 at 20 and 22 °C to six temperatures of 18, 20, 22, 25, 28 and 31 °C combined with LD 15:9 to terminate diapause, the duration of diapause was significantly shortened from 146 days at 18 °C to 24 days at 31 °C, showing that high temperatures significantly accelerate diapause development. Furthermore, the duration of diapause was significantly longer at the rearing temperature of 22 °C than that at 20 °C when the diapause-terminating temperatures were 20 and 22 °C. Chilling at 5 °C did not shorten the duration of diapause but lengthened it when chilling period was included. However, chilling plays an important role in synchronizing adult emergence. Rearing temperature of 22 °C also evoked more intense diapause than did 20 °C in most chilling treatments. When the overwintering pupae were transferred at different times from natural temperatures to 25 °C, it was found that the earlier the transfer took place, the earlier the adults emerged when the time spent under natural conditions was included. However, cool temperatures before March showed an enhanced effect on diapause development at 20 °C, suggesting that the high diapause-terminating temperature can offset the effect of chilling on diapause development. The result of diapause termination under natural conditions suggests that the developmental threshold for post-diapause development in H. armigera should be around 17.5 °C.
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