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.     

Response to photoperiod during diapause development in the spider mite Tetranychus urticae

To study the question whether photoperiodic time measurement in the spider mite Tetranychus urticae is based on a qualitative or quantitative principle, the duration of diapause development was determined in individual females at various constant photoperiods at 19 degrees C. Diapause duration at all four long-night treatments fluctuated around 64.5 days, varying from 62.2 at LD 12:12h to 66.4 at LD 10:14h. The within-treatment variation in diapause duration of the long-night groups appeared to be significantly correlated to the nightlength of the photoperiods used; the longer the nightlength, the higher the within-treatment variation. Frequency distributions of females completing diapause under the two regimes with nightlengths near the critical nightlength were skewed to the right. Mean diapause durations at these regimes, LD 13:11h and LD 14:10h, were 25.4 and 11.9 days, respectively. Mites completed diapause rapidly and synchronously under the three short-night photoperiods tested; within two weeks after transfer from cold storage at 4 degrees C to the diapause terminating regimes at 19 degrees C all females started reproduction. Mean diapause durations were 8.1, 6.4 and 6.5 days for the short-night treatments LD 15:9h, LD 17:7h and LD 19:5h, respectively. The coefficients of variation of diapause duration (variability within groups relative to the mean) of the short-night and the long-night groups varied from 18 to 42%; the coefficients of the two intermediate groups were 69and 81%. There was a clear difference in diapause duration between long-night and short-night groups, but no significant difference was present in this characteristic between different long-night groups on the one hand and only a small difference between different short-night groups on the other. These results support the hypothesis that photoperiodic time measurement in the spider mite is based on a qualitative principle; photoperiods are classified as either 'long' or 'short' in relation to a 'critical' photoperiod. However, around the critical nightlength, intermediate responses were observed which might hint at the quantitative nature of the underlying mechanism. Therefore, although most results are in agreement with the hypothesis of a qualitative mechanism, it cannot be excluded that photoperiodic time measurement in the spider mite is based on a quantitative principle.     

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.     

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.     

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.     

The effect of temperature on the photoperiodic 'counters' for female morph and sex determination in two clones of the black bean aphid, Aphis fabae

Experiments were performed on two clones of the black bean aphid, Aphis fabae Scopoli – one from Aberdeen, Scotland (57°N), the other from Cambridge, England (52°N) ? to determine the number of long- or short-night cycles required for 50% induction of winged versus wingless females on the one hand and males versus females on the other (i.e. required day number, RDN), at three temperatures, 12.5, 15 and 17.5°C. In the case of female morph determination, the RDN for long-night cycles was temperature compensated, whereas that for short-night cycles was highly temperature dependent. For sex determination, the RDN for long-night cycles was again temperature compensated, whereas, due to the mechanism of sex determination, male production was close to 100% in our protocol, even with a maximal number of short-night cycles, and the RDN could therefore not be assessed. Model-generated response curves, using the recently developed ‘double circadian oscillator model’ for photoperiodic time measurement in insects and mites, closely resembled the observations. It could also be shown that differences observed between response curves of female morph and sex determination in the Scottish clone were due, according to the model, to differences in their photoperiodic ‘counters’, rather than to differences in their clocks.     

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.     

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.     

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.     

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.     

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.     

Photoperiodic response of diapause induction in the pine caterpillar, Dendrolimus punctatus

The pine caterpillar, Dendrolimus punctatus (Walker) (Lepidoptera: Lasiocampidae), is a multivoltine pest of pine trees in China, overwintering as larvae. Winter diapause was induced by short day length. The critical night length was about 10 h 40 min at 25, 28, and 31 °C in the field, showing a temperature‐compensated diapause induction. Transfer experiments from a short night (L16:D8) to a long night (L12:D12) or vice versa at different times after hatching showed that sensitivity to day length was restricted to the first 14 days; the required day number for a 50% response at 25 °C was about 3.5 days for short nights but 7.5 days for long nights, indicating that short nights are photoperiodically more effective. When four successive short nights (L16:D8) were used to interrupt the long‐night regime (L12:D12) at different development stages and vice versa, the results showed that the highest sensitivity to photoperiod occurred on the 4th?8th day, corresponding to the second larval instar. Experiments of alternating short‐night (L16:D8) and long‐night (L12:D12) cycles during the larval period showed that the information of short nights as well as long nights could be accumulated. By rearing the larvae under conditions other than 24‐h light–dark cycles, we clearly showed that the dark period (scotophase) played a major role in the determination of diapause. The Nanda‐Hamner and Bünsow experiments failed to reveal rhythmic fluctuations with a period of about 24 h in the occurrence of diapause. Therefore, the photoperiodic clock in D. punctatus is an hourglass timer or a damped circadian oscillator.     

Photoperiodic induction of winged females in the black bean aphid. Aphis fabae

Abstract. The Photoperiodic of winged females (alatae) in the black bean aphid, Aphis fabae Scop. (Homopetera: Aphididae), is investigated in detail with emphasis on the interaction of the maternal and embryonic/young larval photoperiodic clocks. Previous work had shown that in uncrowded conditions the induction of gynoparae (winged females that produce sexual females) requires both prenatal and postnatal exposure to long-night (12 h) Photoperidic cycles: present results show that sole postantal exposure to long nights of any lenght does not induce wing formation in early-born aphids.
When aphids were exposed to experimental light-dark cycles postanatally only, their daughters developed as alate in long nights and as apterae in short nights: the critical night lenght (CNL) was 11:1 h. Additional prenatal exposure to experimental regimes resulted in a significantly shorter CNL (10.6 h). This difference could be accounted for by the fact that more experimental light-dark cycles were experienced in the latter case.
Apterous aphids transferred from LD 16:8 h to LD 12:12 h as either third-or fourth-stadium larvae, or young adults, switched for aptera-production to alata-production. The transition form aptera- to alata-production was rather abrupt in third-stadium transfers but more gradual when transfers occurred as fourth-stadium larvae and adults. Moreover, s the number of days required for 50% of the aphids to become alata-producers increased from 7–8 in third-stadium transfers, to 9–10 and 11–12 in the later transfers.     

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 Termination in the Small Brown Planthopper,Laodelphax striatellus (Hemiptera: Delphacidae)

The small brown planthopper, Laodelphax striatellus (Fallén) enters the photoperiodic induction of diapause as 3rd or 4th instar nymphs. The photoperiodic response curves in this planthopper showed a typical long-day response type with a critical daylength of approximately 11 h at 25°C, 12 h at 22 and 20°C and 12.5 h at 18°C, and diapause induction was almost abrogated at 28°C. The third stage was the most sensitive stage to photoperiod. The photoperiodic response curve at 20°C showed a gradual decline in diapause incidence in ultra-long nights, and continuous darkness resulted in 100% development. The required number of days for a 50% response was distinctly different between the short- and long-night cycles, showing that the effect of one short night was equivalent to the effect of three long nights at 18°C. The rearing day length of 12 h evoked a weaker intensity of diapause than did 10 and 11 h. The duration of diapause was significantly longer under the short daylength of 11 h than it was under the long daylength of 15 h. The optimal temperature for diapause termination was 26 and 28°C. Chilling at 5°C for different times did not shorten the duration of diapause but significantly lengthened it when chilling period was included. In autumn, 50% of the nymphs that hatched from late September to mid-October entered diapause in response to temperatures below 20°C. The critical daylength in the field was between 12 h 10 min and 12 h 32 min (including twilight), which was nearly identical to the critical daylength of 12.5 h at 18°C. In spring, overwintering nymphs began to emerge in early March-late March when the mean daily temperature rose to 10°C or higher.     

Male production by apterous viviparae of the aphid Myzus persicae temporarily exposed to different scotoperiods

Male production by apterous viviparae of a holocyclic biotype of the green peach aphid, Myzus persicae, was induced by pre-natal and/or post-natal exposures to a long-night regime of 15 h dark per diem. When the apterae were exposed to three or more long nights immediately after birth and they subsequently developed under a short-night regime of 8 h dark per diem, they produced females (mostly alate viviparae) during the first 8–10 days of reproduction and a high proportion of males thereafter. When the apterae were exposed to two long nights immediately before their birth and to short nights thereafter, they produced relatively few females (mostly alate viviparae), and males were deposited already after 4–6 days of reproduction, i.e. 16–18 days after the first exposure to the long nights. The proportion of males among the progeny of these apterae was highest when the two prenatal exposures comprised scotophases of 11–15 h; under such long-night regimes many aphids switched to producing males exclusively. To achieve this effect, the two long scotophases had to be separated by a photophase of more than 1–2 h. Fewer males were produced and most of the apterae reverted to the production of females (apterous viviparae) when the duration of the two prenatal scotophases was 9 h 45 min-10 h 30 min, or 18 h and longer.One long night of 15–39 h could also induce temporary male production if the aphids were exposed to it late in the 4th larval instar or as teneral adults.     

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 coincidence of light and melatonin with a specific phase of the circadian pacemaker is important for the timing of seasonal breeding in the ewe

The timing of reproductive activity in seasonal breeding sheep relies on daily photoperiodic signals being relayed to provide information on the time of year. Although light and melatonin are involved, the exact mechanism is not understood. In this experiment, three groups of 6 Romney Marsh ewes, a highly seasonal breed, were provided with 8 weeks of short nights (9.6-9.8 h, by artificially advancing dawn) around the winter solstice, near the end of their natural breeding season. One group of animals was infused to a physiological level with melatonin for 5 h during the afternoon prior to the onset of dark, while a second group was identically infused but for 5 h from the time of lights on. A third group received the short-night treatment only. Following the short-night treatment, all groups were exposed to long nights (> 14 h, by delaying dawn) until the summer solstice. Ovarian activity, assessed by progesterone monitoring twice weekly, showed that the noninfused and the morning-infused groups displayed renewed reproductive activity in response to the short-night/long-night treatment. There was no renewed ovarian activity in the afternoon-infused group, indicating that the time of day that melatonin is present, rather than the duration of melatonin exposure, is an important signal in the control of reproductive timing. Measurements of a marker of the endogenous circadian pacemaker, by melatonin measurements under acutely extended darkness, revealed that the short-night treatments phase advanced the onset of the pacemaker in all groups such that the afternoon phase of the pacemaker was coincident with light. The results provide strong support for the model that proposes that an afternoon-located sensitive phase of the pacemaker is responsible for the relay of photoperiodic signals in the timing control of seasonal breeding. The model proposes that the reproductive axis be primed during short nights when the sensitive phase is coincident with light in the afternoon so ovarian activity can be induced when the sensitive phase is located within the longer nights of autumn and coincident with endogenous melatonin.     

Selection for high diapause incidence in blow flies (Calliphora vicina) maintained under long days increases the maternal critical daylength: some consequences for the photoperiodic clock

Using a population of Calliphora vicina from southern Scotland (55 degrees N), showing a critical day length for maternal induction of diapause of about 14.5 h per day, strains of flies were selected for a high incidence of larval diapause under long day length (LD 16:8 h). Diapause incidence was raised from under 10% to almost 100% within five or six generations. The selected flies showed an increase in their critical day length to over 16 h per day, and a high incidence of larval diapause under very long photophases. Selected flies, however, showed mean circadian periods for locomotor activity little different from the original stock, or from flies selected for high diapause under LD 12:12 h, and a Nanda-Hamner profile lacking peaks and troughs of diapause incidence at about 24 h intervals. These results are interpreted to show that the photoperiodic regulation of diapause and the control of overt behavioural rhythmicity are 'separate' physiological systems.