Photoperiodic induction of diapause in the spider mite Tetranychus urticae: qualitative or quantitative time measurement?

Abstract. Insects and mites may measure photoperiods eitfier by classifying them as long or short relative to a critical value (qualitative time measurement) or by using the absolute value (quantitative time measurement). The spider mite Tetranychus urticae is thought to use a qualitative mechanism of time measurement. In this paper we present the results of experiments with an inbred line of the spider mite (to keep genetic variation in photoperiodic responses small), to test whether quantitative aspects also play a role. Differences in diapause incidence in different long-night photoperiods at different temperatures may be an indication of quantitative responses to photoperiod. The effect of temperature on the photoperiodic response curve was studied at 16^oC, 19^oC and 22^oC. The response curves appeared to be similar at 16^oC and 19^oC, with a critical nightlength between 10 and 11 h. At 22^oC, diapause induction was less than 100% in all long-night regimens and die critical nightlength had shifted to 12 h. Maximum diapause induction (93%) occurred in a light-dark cycle with a 16 h dark phase (LD 8:16 h). Diapause induction was lowest in long-night photoperiods with dark phases of 20 h and longer. The number of light-dark cycles needed for 50% diapause induction at 19^oC varied. between 12.1 and 14.7 for LD 6:18 h, between 10.9 and 12.5 for LD 8:16 h, between 10.6 and 11.6 for LD 10:14 h, and between 10.1 and 10.7 for LD 12:12 h. Independent of die light-dark regimen, diapause induction took place in some individuals after receiving 8 cycles and virtually all individuals entered diapause after 16 cycles. No effect was found of the photoperiodic treatment during prediapause development (LD 6:18 h, LD 8:16 h, LD 10:14 h, LD 12:12 h) on diapause duration. The average diapause duration at LD 10:14 h and 19^oC was 61 days over all four treatments. We explained the results by hypothesising that nightlengths are assessed qualitatively and mat the photoperiodic clock operates more accurately near the critical nightlength.     

Photoperiodic counter of diapause induction in Pseudopidorus fasciata (Lepidoptera: Zygaenidae)

Induction of larval diapause is a photoperiodically controlled event in the life history of the moth Pseudopidorus fasciata. In the present study, the photoperiodic counter of diapause induction has been systematically investigated. The required day number (RDN) for a 50% response was determined by transferring from a short night (LD 16:8) to a long night (LD 12:12) or vice versa at different times after hatching, The RND differed significantly between short- and long-night cycles at different temperatures. The RDN for long-night cycles at 20, 22, 25 and 28 degrees C was 11.5, 9.5, 7.5 and 8.5 days, respectively. The RDN for short-night cycles was 3 days at 22 degrees C and 5 days at 20 degrees C indicating that the effect of one short night was equivalent to the effect of 2-3 long nights effect. Night-interruption experiments of 24h photoperiods by a 1 h light pulse showed that the most crucial event for the photoperiodic time measurement in this moth was whether the length of pre-interruption (D(1)) or the post-interruption (D(2)) scotophases exceeded the critical night length (10.5 h). If D(1) or D(2) exceeded 10.5 h diapause was induced. The diapause-averting effect of a single short-night cycle (LD 16:8) against a background of long nights (LD 12:12) showed that the photoperiodic sensitivity was greatest during the first 7 days of the larval period and the highest sensitivity was on the fourth day. Both non-24 and 24 h light-dark cycle experiments revealed that the photoperiodic counter in P. fasciata is able to accumulate both long and short nights during the photosensitive period, but in different ways. The information from short-night cycles seems to be accumulated one by one in contrast to long-night cycles where six successive cycles were necessary for about 50% diapause induction and eight cycles for about 90% diapause. These results suggest the accumulation of long-night and short-night cycles may be based on different mechanisms.     

Induction and inhibition of diapause by the same photoperiod: experimental evidence for a "double circadian oscillator clock"

On the southern Iberian Peninsula, the seasonal life history of the large white butterfly, Pieris brassicae, comprises 2 different photoperiodically induced developmental arrests: a hibernation diapause at photophases < 11 h and an estivation diapause at photophases > 14 h. At intermediate photophases (12 h to 13 h), the butterfly responds with a nondiapause. Combined with the experimental setup to determine photosensitivity in insects, the different photoperiodic responses at long-, intermediate-, and short-night conditions were examined to gain more insight into the time measurement mechanism in P. brassicae. The study reveals evidence for a "double circadian oscillator clock" mechanism that is based on 2 submechanisms, a "short-night determining system" and a separate "long-night determining system." This conclusion was drawn from the facts that an LD 9:15 long-night induces a hibernation diapause but inhibits an estivation diapause and, conversely, that an LD 16:8 short-night inhibits a hibernation diapause but induces an estivation diapause. This opposite effect of the same photoperiod supports the argument for the existence of 2 independent targets for light-dark cycles, interpreted as 2 antagonistic time measurement systems. The existence and independence of 2 systems was further shown by differences in long-night versus short-night responses regarding photosensitivity, temperature dependence, and heritable factors. The long-night measurement system is most effective in the 5th larval stage, is highly affected by temperature, and is easy to manipulate by selective inbreeding. The short-night measurement system is most effective in the 4th larval stage, is largely temperature compensated, and is not affected by experimental manipulation of the longnight measurement system.     

Diapause induction and clock mechanism in the cabbage beetle, Colaphellus bowringi (Coleoptera: Chrysomelidae)

Photoperiodic control of diapause induction was investigated in the short-day species, Colaphellus bowringi, which enters summer and winter diapause as adult in the soil. Photoperiodic responses at 25 and 28 degrees C revealed a critical night length between 10 and 12 h; night lengths > or =12 h prevented diapause, whereas night lengths <12 h induced summer diapause in different degree. Experiments using non-24-h light-dark cycles showed that the duration of scotophase played an essential role in the determination of diapause. Night-interruption experiments with T=24 h showed that diapause was effectively induced by a 2-h light pulse in most scotophases; whereas day-interruption experiments by a 2-h dark break had a little effect on the incidence of diapause. The experiments of alternating short-night cycles (LD 16:8) and long-night cycles (LD 12:12) during the sensitive larval period showed that the information of short nights as well as long nights could be accumulated. Nanda-Hamner experiments showed three declining peaks of diapause at 24 h circadian intervals. Bünsow experiments showed two very weak peaks for diapause induction, one being 8 h after lights-off, and another 8 h before lights-on, but it did not show peaks of diapause at a 24 h interval. These results suggest that the circadian oscillatory system constitutes a part of the photoperiodic clock of this beetle but plays a limited role in its photoperiodic time measurement.     

Photoperiodic control of diapause in Pseudopidorus fasciata (Lepidoptera: Zygaenidae) based on a qualitative time measurement

In the zygaenid moth, Pseudopidorus fasciata, both larval diapause induction and termination are under photoperiodic control. In this study, we investigated whether photoperiodic time measurement (with a 24-h light-dark cycle) in this moth is qualitative or quantitative. Photoperiodic response curves, at 22, 25, and 28 degrees C indicated that the incidence of diapause depended on whether the scotophases exceeded the critical night length (CNL) or not. All scotophases longer than the CNL-induced diapause; all scotophases shorter than the CNL-inhibited diapause. The CNL was 10.5h at 25 and 28 degrees C, and 10h at 22 degrees C. By transferring from various short photoperiods (LD 8:16, LD 9:15, LD 10:14, LD 11:13, LD 12:12, and LD 13:11) to a long photoperiod (LD 16:8) at different times, the number of light-dark cycles required for 50% diapause induction at 25 degrees C was 7.14 at LD 8:16, 7.2 at LD 9:15, 7.19 at LD 10:14, 7.16 at LD 11:13, and 7.13 at LD 12:12, without showing a significant difference between the treatments. Only at LD 13:11 (near the CNL), the number of light-dark cycles was significantly increased to 7.64. The intensity of diapause induced under different short photoperiods (LD 8:16, LD 9:15, LD 10:14, LD 11:13, and LD 12:12) at 25 degrees C was not significantly different with an average diapause duration of 36 days. The duration of diapause induced under LD 13:11 was significantly reduced to 32 days. All results indicate that the night-lengths are measured as either "long" or "short" compared with some critical value and suggest that photoperiodic time measurement for diapause induction in this moth is based on a qualitative principle.     

Induction and termination of prepupal summer diapause in Pseudopidorus fasciata (Lepidoptera: Zygaenidae)

The seasonal life cycle of the zygaenid moth, Pseudopidorus fasciata is complicated by two different developmental arrests: a winter diapause as a fourth larval instar and a summer diapause as a prepupa in a cocoon. Both larval diapause induction and termination are under photoperiodic control. Short days induce larval diapause with a critical daylength of 13.5h and long days terminate diapause with a critical daylength of 14h. In the present study photoperiodic control of summer diapause was investigated in Pseudopidorus fasciata. Under long photoperiods ranging from LD 14:10 to LD 18:6, only part of the population entered summer diapause, the rest continued to develop. The lowest number of prepupae entered diapause at LD 14:10, followed by LD 16:8 and LD 17:7. The highest incidence of diapause occurred with photoperiods of LD 15:9 and LD 18:6. By transferring the diapausing prepupae induced by various long photoperiods (LD 14:10, LD 15:9, LD 16:8, LD 17:7, LD 18:6) to LD 13:11, 25 degrees C, the duration of diapause induced by LD 14:10 was significantly shorter than those induced by longer photoperiods. By keeping aestivating prepupae induced by LD 15:9, 28 degrees C or by natural conditions at short photoperiods (LD 11:13 and LD 13:11) and at a long photoperiod (LD 15:9), the duration of diapause at LD 15:9 was more than twice as long as than those at LD 11:13 and LD 13:11. Moreover, adult emergence was highly dispersed with a high mortality at LD 15:9 but was synchronized with low mortality at LD 11:13 and LD 13:11. When the naturally induced aestivating prepupae were kept under natural conditions, the early aestivating prepupae formed in May exhibited a long duration of diapause (mean 126 days), whereas the later-aestivating prepupae formed in July exhibited a short duration of diapause (mean 69 days). These results indicate that aestivating prepupae require short or shortening photoperiod to terminate their diapause successfully. By transferring naturally induced aestivating prepupae to 25, 28 and 30 degrees C, the duration of diapause at the high temperature of 30 degrees C was significantly longer than those at 25 and 28 degrees C, suggesting that high temperature during summer also plays an important role in the maintenance of summer diapause in Pseudopidorus fasciata. All results reveal that summer diapause can serve as a "bet hedging" against unpredictable risks due to fluctuating environments or as a feedback mechanism to synchronize the period of autumn emergence.     

Diapause‐inducing signals prolong nymphal development in the two‐spotted spider mite Tetranychus urticae

Female two‐spotted spider mite Tetranychus urticae are grown under different photoperiods and the photoperiodic regulation of diapause is examined. The photoperiodic response curve for diapause induction was of the long day–short day type, with critical day lengths (CDLs) of 2 and 12.5 h; diapause was induced between these CDLs. The preimaginal period is significantly longer in diapausing females than in non‐diapausing females; moreover, a significant positive correlation is detected between diapause incidence and deutonymphal period. Diapause incidence is high when long‐night photoperiods are applied against a background of continuous darkness in the stages including the deutonymph; this stage appears to be the most sensitive to photoperiod. These observations suggest that diapause‐inducing conditions inhibit nymphal development, particularly in the deutonymphal stage when photoperiodic time measurement for determination of reproduction or diapause is carried out.     

Predation risk affects diapause induction in the spider mite Tetranychus urticae

Whenever diapause induction triggers movement into another microhabitat or the development of protective morphological structures, this may also alter predation risk. If the risk of being eaten is lower in the diapause phase, then there may be selection favouring diapause induction in response to predators or their cues. In this article, we studied the effect of the predatory mite Typhlodromus pyri on diapause induction in the spider mite Tetranychus urticae. We used a Greek strain because under long-night photoperiods and low temperature only part of the population enters diapause, thereby leaving room for the impact of another factor. In spider mite groups under predation, the percentage diapause induction increased whenever night-lengths were such that diapause was induced (13–16 h of night). Given this diapause induction in response to predation risk, the question arises whether entering diapause helps spider mites to escape from predation and contribute more offspring to the spring generation next year.     

Geographical variation in photoperiodic induction of diapause in the spider mite (Tetranychus urticae): a causal relation between critical nightlength and circadian period?

The photoperiodic response of 10 strains of the two-spotted spider mite (Tetranychus urticae), originating between 40.5 degrees and 60 degrees N in Western and Central Europe, was found to be highly variable. The critical nightlength for photoperiodic induction of diapause was strongly correlated with latitude for the lowland populations and varied from 7.75 hr in the north to 13.25 hr in the south. The length of the circadian period, taken as the peak-to-peak interval in response curves of resonance experiments done with T. urticae, varied between 17.75 and 21.5 hr and appeared weakly correlated with latitude. Only a very weak correlation was observed between critical nightlength and circadian period. These results do not provide evidence in favor of a circadian-based photoperiodic clock in T. urticae. On the other hand, they also do not refute this possibility, as there may be other circadian or noncircadian factors affecting the critical nightlength, which could mask the influence of circadian period.     

Experimental evidence for a non-clock role of the circadian system in spider mite photoperiodism

In the spider mite Tetranychus urticae photoperiodic time measurement proceeds accurately in orange-red light of 580 nm and above in light/dark cycles with a period length of 20 h but not in 'natural' cycles with a period length of 24 h. To explain these results it is hypothesized that the photoperiodic clock in the spider mite is sensitive to orange-red light, but the Nanda-Hamner rhythm (a circadian rhythm with a free-running period tau of 20 h involved in the photoperiodic response) is not and consequently free runs in orange-red light. To test this hypothesis a zeitgeber was sought that could entrain the Nanda-Hamner rhythm to a 24-h cycle without inducing diapause itself, in order to manipulate the rhythm independently from the orange-red sensitive photoperiodic clock. A suitable zeitgeber was found to be a thermoperiod with a 12-h warm phase and a 12-h cold phase. Combining the thermoperiod with the long-night orange-red light/dark regime, both with a cycle length of 24 h, resulted in a high diapause incidence, although neither regime was capable of inducing diapause on its own. The conclusion is that the Nanda-Hamner rhythm is necessary for the realization of the photoperiodic response, but is not part of the photoperiodic clock, because photoperiodic time measurement takes place in orange-red light whereas the rhythm is not able to 'see' the orange-red light. It is speculated that the Nanda-Hamner rhythm is involved in the timely synthesis of a substrate for the photoperiodic clock in the spider mite.     

The effect of temperature on the photoperiodic 'clock' and 'counter' of a Scottish clone of the vetch aphid, Megoura viciae

Photoperiodic response curves were determined for a Scottish clone of the vetch aphid, Megoura viciae Buckton, at three temperatures: 12.5, 15, and 17.5 degrees C. Critical night lengths (CNLs) for ovipara (sexual female) induction were 6 h, 7 h and 8 h, respectively. High incidences of ovipara production were observed in all night lengths longer than the CNL including continuous darkness (DD), as well as in continuous light (LL) at 12.5 and 15 degrees C. At the same three temperatures, the number of long- or short-night cycles required for half of the experimental aphids to be ovipara producers (i.e. the required day number, RDN) was determined. The RDN for long-night cycles (LD12:12) could not be determined at 12.5 degrees C, but was temperature compensated between 15 and 17.5 degrees C. The RDN for short-night cycles (LD20:4) could not be determined at any temperature. However, as induction of oviparae was always 100% in 12.5 degrees C, 94-100% in 15 degrees C and dropped from 100% to between 47 and 71% in 17.5 degrees C, it seems that short-night accumulation was temperature dependent. When fourth-stadium larvae were transferred from LD20:4 at 20 degrees C to the same light-dark cycle at 15 degrees C, the aphids, when adult, switched to the production of oviparae after about 4 weeks. First-born progeny kept in LD20:4 and 15 degrees C switched to the production of oviparae about 7 days after the moult to adult. Thus, the photoperiodic response can be directly affected by temperature, irrespective of photoperiod. Model-generated response curves using the 'double circadian oscillator model' for photoperiodic time measurement (Vaz Nunes, M., 1998. A double circadian oscillator model for quantitative photoperiodic time measurement in insects and mites. Journal of Theoretical Biology 194, 299-311) closely resembled the observations. Differences between these data and the results of previous experiments with an English clone of M. viciae could be accounted for by differences in the photoperiodic clocks (damping rate and period) as well as the photoperiodic counters.     

A comparison of photoperiodic control of diapause between aestivation and hibernation in the cabbage butterfly Pieris melete

In the cabbage butterfly, Pieris melete, summer and winter diapause are induced principally by long and short daylengths, respectively; the intermediate daylengths (12-13 h) permit pupae to develop without diapause. In this study, photoperiodic control of summer and winter diapause was systematically investigated in this butterfly by examining the photoperiodic response, the number of days required to induce 50% summer and winter diapause and the duration of diapausing pupae induced under different photoperiods. Photoperiodic response curves at 18 and 20 degrees C showed that all pupae entered winter diapause at short daylengths (8-11 h), the incidence of diapause dropped to 82.3-85.5% at 22 degrees C without showing a significant difference between short daylengths, whereas the incidence of summer diapause induced by different long daylengths (14-18 h) was varied and was obviously affected by temperature. By transferring from various short daylengths (LD 8:16, LD 9:15, LD 10:14 and LD 11:13) to an intermediate daylength (LD 12.5:11.5) at different times after hatching, the number of cycles required to induce 50% winter diapause (7.28 at LD 8:16, 7.16 at LD 9:15, 7.60 at LD 10:14 and 6.94 at LD 11:13) showed no significant difference, whereas by transferring from various long daylengths (LD 14:10, LD 15:9, LD 16:8 and LD 17:7) to an intermediate daylength (LD 12.5:11.5) at different times, the number of cycles required to induce 50% summer diapause (5.95 at LD 14:10, 8.02 at LD 15:9, 6.80 at LD 16:8, 7.64 at LD 17:7) were significantly different. The intensity of winter diapause induced under different short daylengths (LD 8:16, LD 9:15, LD 10:14 and LD 11:13) was not significantly different with an average diapause duration of 87 days at a constant temperature of 20 degrees C and 92 days at a mean daily temperature of 19.0 degrees C, whereas the intensity of summer diapause induced under different long daylengths (LD 14:10, LD 15:9, LD 16:8 and LD 17:7) was significantly different (the diapause duration ranged from 75 to 86 days at a constant temperature of 20 degrees C and from 76 to 88 days at a mean daily temperature of 19.0 degrees C). All results suggested that photoperiodic control of diapause induction and termination is significantly different between aestivation and hibernation.     

Photoperiodic control of diapause termination, colour change and postdiapause reproduction in the southern green stink bug, Nezara viridula

Abstract.  The effects of day length on adult diapause development, associated with diapause body colour change as well as postdiapause reproduction are studied in Nezara viridula from Japan. Facultative diapause spontaneously terminates under three constant short-day and near-critical photoperiods at 25 °C without low temperature treatment. The period required for body colour change from russet to green and the precopulation and preoviposition periods differ significantly between the photoperiodic treatments, being shortest under LD 13 : 11 h, intermediate under LD 12 : 12 h and longest under LD 10 : 14 h. Photoperiodic conditions do not affect postdiapause reproductive performance: the total egg production, duration of the period of oviposition and other reproductive indices do not differ significantly between the photoperiodic conditions. The total egg production depends on the duration of the period of oviposition but not on how long females remained russet during diapause. It is concluded that diapause in N. viridula does not require low temperature for its successful completion and diapause duration affects winter survival but not postdiapause reproductive performance or longevity. Such independence of the postdiapause reproductive performance from the duration of diapause may have contributed to the continuous worldwide range expansion of this species into temperate zone.     

Response to nigh interruption in photoperiodic determination of wing form of the ground cricket Dianemobius fascipes

ABSTRACT. In the ground cricket Dianemobius fascipes (Walker), the occurrence of long- or short-winged form is determined by photoperiod during larval development. The pattern of this response is similar to that of diapause induction in other insects with a clearly defined critical nightlength (11.5 h). In symmetric skeleton regimes formed by a pair of 1 h light pulses, the critical nightlength is about 2 h longer than in the complete photoperiod. The long-night (short-wing) effect is eliminated by an interrupting light pulse. This response to night interruption shows only one peak in scotophases of various lengths and can be related to the critical nightlength before or after interruption. The light-reaction time required for the interruption effect changes during the night. In the early scotophase, 1 h of light is enough but 15 min fails to reverse the long-night effect, while a light pulse of only 1 min is effective about 1 h before the critical nightlength is reached. Several possible interpretations of this change are discussed.     

Responses to stepwise photoperiodic changes for the larval diapause of the Indian meal moth Plodia interpunctella

Abstract The Indian meal moth Plodia interpunctella Hübner (Lepidoptera: Pyralidae) diapauses as a last‐instar (fifth) larva. At 30 °C, no larvae enter diapause under any photoperiodic conditions; at 25 °C, the photoperiodic response curve is a long‐day type with a critical length of approximately 13 h light; at 20 °C, diapause is induced moderately even under long days (> 13 h). Cumulative effects of short days or long days on diapause induction are determined by alternate, stepwise and gradually changing regimes of photoperiod at 25 °C. When the larvae are repeatedly exposed to LD 16 : 8 h and LD 12 : 12 h photoperiods every other day, the incidence of diapause is 37%. When the larvae are placed under an LD 16 : 8 h photoperiod for 2 days and then under an LD 12 : 12 h photoperiod for 1 day, it is 38 %. Exposure to an LD 16 : 8 h photoperiod for 1 day and then to an LD 12 : 12 h photoperiod for 2 days induces only 15% diapause. This may indicate that the photoperiodic information is not accumulated in a simple fashion despite the generally accepted hypothesis (i.e. photoperiodic counter). Larvae exposed to an LD 16 : 8 h photoperiod for 5 days after oviposition express a very high incidence of diapause even under short days between an LD 2 : 22 h and LD 12 : 12 h photoperiod. After 10 days exposure to an LD 16 : 8 h photoperiod, however, the short day does not induce diapause strongly. On the other hand, an LD 12 : 12 h photoperiod in the early larval life is highly effective in the induction of diapause. A gradual increase or decrease of photoperiod (2 min day^?1) shows that the direction of photoperiodic change does not affect the diapause determination.     

Diapause induction and clock mechanism in the cabbage butterfly Pieris melete Ménétriés

Photoperiodic control of diapause induction was systematically investigated in the cabbage butterfly, Pieris melete, which enters summer and winter diapause as a pupa. Summer and winter diapause are induced principally by short and long scotophases, respectively; the intermediate scotophases (11-12 h) permit pupae to develop without diapause. Photoperiodic responses under 24-h light-dark cycles at 16.9, 18, 20 and 22 °C showed that the hibernation response was temperature compensated, whereas aestivation response was strongly temperature-dependent. The incidence of diapause for both aestivation and hibernation showed a decline at the ultra-short and ultra-long scotophases. Experiments using non-24-h light-dark cycles showed that the length of the scotophase played an essential role in the determination of diapause. The highest photosensitivity differed under hibernation and aestivation conditions. With a 3 × LD 12:12 interruption, a maximal inhibition of aestivation occurred in the L3/2 stage, and of hibernation it occurred in the L4/0 stage. A long-night of LD 10:14 induced hibernation diapause but inhibited aestivation diapause and, conversely, a short-night of LD 14:10 inhibited hibernation diapause but induced aestivation diapause. With a 1-h light pulse at LD 11:13, a maximal inhibition of hibernation occurred 3 h before lights-on (late scotophase), whereas, with a 1-h light pulse at LD 12.5:11.5, a maximal induction of aestivation occurred 2-3 h after the onset of darkness (early scotophase). Nanda-Hamner and Bünsow experiments failed to reveal the involvement of a circadian system, suggesting that the photoperiodic time measurement for diapause induction in this butterfly resembles an hourglass-like timer or a damped circadian oscillator.     

Photoperiodic clock of diapause induction in Pseudopidorus fasciata (Lepidoptera: Zygaenidae)

Pseudopidorus fasciata enters diapause as fourth instar larvae at short day lengths. Using 24-h light-dark cycles, the photoperiodic response curves in this species appeared to be similar with a critical night length of 10.5h at temperatures below 30 degrees C. At an average temperature of 30.5 degrees C, the critical night length had shifted to between 15 and 17h. In experiments using non-24-h light-dark cycles, it was clearly demonstrated that the dark period (scotophase) was the decisive phase for a diapause determination. In night interruption experiments using 24-h light-dark cycles, a 1-h light pulse at LD12:12 completely reversed the long night effect and averted diapause in all treatments. At LD 9:15 light pulses of 1-h, 30- or 15-min also averted diapause effectively when both the pre-interruption (D(1)) or the post-interruption scotophases (D(2)) did not exceed the critical night length. If D(1) or D(2) exceeded the critical night length diapause was induced. The most crucial event for the photoperiodic time measurement in this species is the length of the scotophase. A 10-min light pulse placed in the most photosensitive phase reversed diapause in over 50% of the individuals. Night interruption experiments under non-24-h light-dark cycles indicated that the photoperiodic clock measured only D(1) regardless of the length of D(2), suggesting that the most inductive cycles are often those in which L+D are close to 24h. In resonance experiments, this species showed a circadian periodicity at temperatures of 24.5 or 26 degrees C, but not at 30.5 and 23.3 degrees C. On the other hand, Bünsow and skeleton photoperiod experiments failed to reveal the involvement of a circadian system in this photoperiodic clock. These results suggest the photoperiodic clock in this species is a long-night measuring hourglass and the circadian effect found in the final expression of the photoperiodic response in the resonance experiments may be caused by a disturbing effect of the circadian system in unnatural regimes.     

Photoperiodic and temperature control of diapause induction and colour change in the southern green stink bug Nezara viridula

Abstract. The effect of photoperiod and temperature on the duration of the nymphal period, diapause induction and colour change in adults of Nezara viridula (L.) (Heteroptera: Pentatomidae) from Japan was studied in the laboratory. At 20 °C, the developmental period for nymphs was significantly shorter under LD 10 : 14 h (short day) and LD 16 : 8 h (long day) than under intermediate photoperiods, whereas at 25 °C it was slightly shorter under intermediate than short- and long-day conditions. It is assumed that photoperiod-mediated acceleration of nymphal growth takes place in autumn when day-length is short and it is unlikely that nymphal development is affected by day-length under summer long-day and hot conditions. Nezara viridula has an adult diapause controlled by a long-day photoperiodic response. At 20 °C and 25 °C in both sexes, photoperiodic responses were similar and had thresholds close to 12.5 h, thus suggesting that the response is thermostable within this range of temperatures and day-length plays a leading role in diapause induction. Precopulation and preoviposition periods were significantly longer under near-critical regimes than under long-day ones. Short-day and near-critical photoperiods induced a gradual change of adult colour from green to brown/russet. The rate of colour change was significantly higher under LD 10 : 14 h than under LD 13 : 11 h, suggesting that the colour change is strongly associated with diapause induction. The incidences of diapause or dark colour did not vary among genetically determined colour morphs, indicating that these morphs have a similar tendency to enter diapause and change colour in response to short-day conditions.     

Effects of thermoperiod on photoperiodic determination of larval diapause in Ostrinia nubilalis

Some effects of thermoperiods on the photoperiodic determination of larval diapause in the European corn borer, Ostrinia nubilalis are reported. Thermoperiods and photoperiods were shown to interact to a highly significant degree in the induction of diapause. Diapause determination was found to be sensitive to the duration and temperature of the cryoscotophase of the thermophotoperiod; the thermophotophase characteristics exerted only minor influence. Cryoscotophase temperatures of 10°C or lower caused an unexpected increase in the apparent critical nightlength for diapause determination. A 0°C cryoscotophase effectively prevented the photoperiodic determination of diapause. Experimental investigation of the effects of 0°C cryoscotophases produced evidence that such low-temperature cryoscotophases may suppress the biological clock functions regulating the determination of diapause. Photoperiodic regimes in which 0°C pulses were inserted at successive points in 24-h cycle evoked responses suggesting that the early scotophase (saturation phase) was more strongly influenced by the low temperature pulse than was the later scotophase (scotonon); low temperature had little effect on the light-dependent processes (photonon) associated with the photophase.     

Photoperiodism of diapause induction in Thyrassia penangae (Lepidoptera: Zygaenidae)

Thyrassia penangae enters winter diapause as a prepupa in a cocoon. Photoperiodism of diapause induction was systematically investigated in this moth. The photoperiodic response curves under 24-h light-dark cycles showed that this insect is a typical long-day species. The critical daylength was 13 h 30 min at 25 °C, 13 h at 30 °C and 12 h 20 min at 28 °C. Transferring experiments from a short day (LD 12:12) to a long day (LD 15:9) or vice versa indicated that photoperiodic sensitivity mainly occurs during the larval period. In experiments using non-24-h light-dark cycles, when the length of photophase exceeded the critical daylength (13.5 h), was diapause inhibited effectively, even when the length of scotophase exceeded the critical nightlength (10.5 h). Only when a long scotophase was combined with a short photophase, diapause was induced effectively. This result suggests that daylength measurement is more important than nightlength measurement in T. penangae. Night interruption experiments under 24-h light-dark cycles exhibited two points of apparent light sensitivity, but the photosensitive position was highly influenced by temperature and the length of scotophase. Nanda-Hamner experiments failed to reveal the involvement of a circadian system in this photoperiodic time measurement. All light-dark cycles from LD 12:12 to LD 12:72 resulted in a short day response, and all cycles from LD 14:4 to LD 14:72 resulted in a long day response, suggesting that photoperiodic time measurement in this moth is performed by a day-interval timer or an hourglass-like clock.