Critical Night Length Research Articles

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 °C was 11.5, 9.5, 7.5 and 8.5 days, respectively. The RDN for short-night cycles was 3 days at 22 °C and 5 days at 20 °C indicating that the effect of one short night was equivalent to the effect of 2–3 long nights effect. Night-interruption experiments of 24 h 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.

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 °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 10.5 h at 25 and 28 °C, and 10 h at 22 °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 °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 °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.

Effect of light pulses in early scotophase on resetting of the night‐measuring diapause clock of the Indian meal moth, Plodia interpunctella

Abstract. The Indian meal moth, Plodia interpunctella Hübner (Lepidoptera: Pyralidae) may enter diapause in the last larval instar in response to the photoperiod during the preceding instars. An hourglass‐type photoperiodic clock may measure night length for this purpose. The present study describes the resetting of the hourglass by light pulse(s) in the early scotophase and by scanning the subsequent clock phase by another light pulse (P). When the lights‐off time of a first light pulse is fixed at 4 h after dusk under LD 4 : 20 h and LD 6 : 18 h photoperiods and its duration is increased from 1 to 3 h, the critical night length (CNL) from dawn is decreased, but that from dusk to P increases. A 3‐h first light pulse efficiently resets the time measuring system. If this 3‐h light pulse is split into two 1‐h light pulses (L1 and L2) by 1 h of darkness, the dark‐time measuring function appears to be impeded and CNL from P to dawn disappears, but that from L2 to P is expressed. This indicates that the receptivity to light pulses varies among individual insects.

Diapause induction and clock mechanism in the pine caterpillar Dendrolimus tabulaeformis (Lep., Lasiocampidae)

Abstract: Dendrolimus tabulaeformis overwinters as third to fourth instar larvae at short days in autumn. Using 24‐h light–dark cycles, the photoperiodic response curves were similar at 24 and 28°C. The critical night length was 9 h 20 min at 24°C and 9 h 50 min at 28°C. Under non‐24 h light–dark cycles, duration of scotophase proved crucial in the determination of diapause. In night interruption experiments using 24‐h light–dark cycle, a 1‐h light pulse falling 8 h in the darkness strongly averted diapause in comparison with other light pulses. Nanda–Hamner experiments showed two weak troughs of diapause inhibition, suggesting the possible involvement of the circadian system. However, Bünsow experiments did not support the evidence of the involvement of circadian oscillatory system in photoperiodic time measurement. These results suggest that photoperiodic time measurement in this moth shows a non‐oscillatory ‘hourglass‐like’ response model or a rapidly damping oscillator model.

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 °C revealed a critical night length between 10 and 12 h; night lengths ≥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.

Dependence of photoperiodic response of growth cessation on the stage of development in Picea abies and Betula pendula seedlings

Dependence of photoperiodic response of growth cessation on the stage of development was examined in seedlings of Norway spruce ( Picea abies (L.) Karst.) and silver birch ( Betula pendula Roth) in greenhouses with 20 °C day and 10 °C night temperatures. Different combinations of photoperiod and stage of development were created by repeating sowing five times with 2 week intervals during the summer. During the experiment light conditions were natural but the daily temperature sum accumulation was regulated to be constant. Eight origins of spruce and seven origins of birch from different latitudes (60–67°N) in Finland were used. In the first growing season both Norway spruce and silver birch seedlings from the first sowings required a longer time for growth cessation than seedlings from the later sowings. However, because the seedlings from the first sowings ceased their growth on an earlier calendar date, the night length at the time of growth cessation was shorter for the seedlings from the first sowings. The results suggest that the variation in the timing of growth cessation of Norway spruce and silver birch seedlings during the first growing season was explained jointly by night length and stage of development. Seedlings from northern origins stopped their growth with shorter night length than those from southern origins. The effects of latitude and average temperature sum of the original growing site on the critical night length of growth cessation in the first growing season were stronger in Norway spruce than in silver birch. In the second growing season the sowing time did not affect the timing of the formation of the terminal buds, but slightly affected the timing of height growth cessation of Norway spruce and silver birch seedlings.

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

Photoperiodic control of diapause termination was systematically investigated in Pseudopidorus fasciata. In 24 h light-dark cycles, the rate of diapause termination in this species depended on photoperiod. The critical night length (CNL) for diapause termination was 10 h, 0.5 h shorter than that for diapause induction. Night-interruption experiments with T = 24 showed that diapause was effectively terminated when the scotophases separated by light pulse were shorter than the critical night length (10 h); no developing individuals were found if the duration of the pre-interruption scotophase or the post-interruption scotophase exceeded the CNL. A 15-min light pulse was sufficient to reverse the effect of long night when it was placed 8 h after lights-off. Resonance experiments with a constant photophase of 12 h or 16 h and various scotophases of 4-80 h showed an hourglass-type photoperiodic response, where no rhythmicity was found. In another resonance experiment with constant scotophase of 8 h and various photophases of 4-72 h, all individuals developed into cocoons. In the BĂźnsow experiment, the response curve showed two apparent peaks for diapause termination, one being 8 h after lights-off, and another 8 h before lights-on. However, there was no periodic rhythmicity, which again indicates an hourglass principle. The results lead to the conclusion that the same photoperiodic clock mechanism (a long-night measuring hourglass) is involved in both diapause induction and termination.

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 10.5 h at temperatures below 30°C. At an average temperature of 30.5°C, the critical night length had shifted to between 15 and 17 h. 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 24 h. In resonance experiments, this species showed a circadian periodicity at temperatures of 24.5 or 26°C, but not at 30.5 and 23.3°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.

Circadian Timekeeping for the Photoperiodic Control of Budset in Picea abies (Norway Spruce) Seedlings

Romanian populations of Norway spruce are induced to set terminal buds by four inductive cycles of 8 h light/16 h darkness. To distinguish between circadian and hourglass timekeeping for the photoperiodic control of budset, seedlings were raised in continuous light at 300 µmol m-2s-1 at 20°C for 10 weeks. They were then exposed to an extended night regime consisting of three cycles of 8 h light/40 h dark with 4-h or 1-h nightbreaks (120 µmol m-2s-1) applied to groups of plants at intervals during the extended night. Following a final cycle of 8 h light/16 h dark to maximize budset, the plants were transferred to continuous light. Budset was delayed when the night-break was applied close to the critical nightlength (CNL) of 6–7 h or about 22–23 h later in the extended night, consistent with circadian rather than hourglass timekeeping. Confidence intervals were calculated for the times to maximum effect of the night-breaks.

Aphid photoperiodic clocks

An overview is given of photoperiodic research performed with English and Scottish clones of the vetch aphid, Megoura viciae, and the black bean aphid, Aphis fabae, during the last decade, with emphasis on the photoperiodic photoreceptor and the clock–counter mechanisms. Photoperiodic photoreceptor. The photoperiodic photoreceptors of the vetch aphid, M. viciae, are located in the brain. Immunocytochemical techniques have recently indicated that an anterior dorsal region of the protocerebrum possesses antigenic sites that are consistently labelled with a number of antibodies raised to invertebrate and vertebrate opsins and phototransduction proteins. Counter. In two clones of both M. viciae and A. fabae, long-night accumulation appears to be temperature compensated, whereas short-night accumulation is temperature sensitive. Clock. (1) The clocks of the English and Scottish clones of M. viciae can be modelled by a slowly damping long-night (LN) system and a rapidly damping short-night (SN) system. The LN system of the Scottish clone damps more slowly and has a shorter period than that of the English clone. The critical night length (CNL) of the English clone is highly temperature compensated, that of the Scottish clone is less so. (2) The clock in presumptive gynoparae of an English clone of A. fabae can be modelled by rapidly damping LN and SN systems; the SN system's damping rate being temperature dependent, but that of the LN system being temperature compensated. (3) The clocks in gynopara and male producers of the English clone of A. fabae are mimicked by rapidly damping LN systems and self-sustained SN systems, while in a Scottish clone they are assumed to consist of slowly damping LN systems and self-sustained SN systems. (4) The CNLs for gynopara and male production are temperature compensated in both the English and the Scottish clones of A. fabae.

On critical night lengths and temperature compensation in the photoperiodic response of two geographical clones of the black bean aphid, Aphis fabae

Summary Photoperiodic response curves were determined for two clones of the black bean aphid, Aphis fabae Scopoli, at three temperatures, 12.5, 15 and 17.5°C. Critical night lengths for the induction of winged females in an English clone (52° N) were 10.5, 11 and 11.5 h, respectively, and 10, 10.5 and 11 h in a Scottish clone (57° N). Critical night lengths for male induction were 10.5, 11 and 11 h at 12.5, 15 and 17.5°C in the English clone, and 10, 10.5 and 10.5 h, respectively, in the Scottish clone. High incidences of winged females and males were observed at all scotophases longer than the critical night length in both clones. In addition, in the English clone, the incidences of winged female and male producers in continuous darkness were 0% at 15 and 17.5°C, and 6% at 12.5°C. In the Scottish clone, however, continuous darkness resulted in high incidences of both winged female and male producers at 12.5 and 15°C, but 0% winged female producers and 6% male producers at 17.5°C. In scotophases shorter than the critical night length, including continuous light, no males or winged females were observed in either clone under the non-crowded rearing conditions used. The results are discussed in terms of the ‘double circadian oscillator model’ for photoperiodic induction.

The processes inhibited and promoted by abscisic acid in photoperiodic flowering of Pharbitis nil

Summary A dual role of abscisic acid (ABA) in photoperiodic flowering of Pharbitis nil , cv. Violet was studied. ABA given before an inductive dark treatment lowered the flowering response without influencing the critical night length and the most effective time of a night break. ABA given after a dark treatment accelerated the transport of flowering stimulus and enhanced flower initiation response at shoot apex. Endogenous ABA level in cotyledons was determined by gas chromatography-selected ion monitoring. There was no remarkable difference between ABA levels under short-day and long-day conditions. Fluridone, an inhibitor of ABA biosynthesis, given before and during an inductive dark treatment affected flowering response only slightly. The results indicate that exogenous ABA inhibits the early phase of the flowering without influencing the time-measuring process, and promotes both transport of the flowering stimulus and flower-initiation process, although endogenous ABA plays little role in the regulation of flowering.

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°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°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°C, but was temperature compensated between 15 and 17.5°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°C, 94–100% in 15°C and dropped from 100% to between 47 and 71% in 17.5°C, it seems that short-night accumulation was temperature dependent. When fourth-stadium larvae were transferred from LD20:4 at 20°C to the same light–dark cycle at 15°C, the aphids, when adult, switched to the production of oviparae after about 4 weeks. First-born progeny kept in LD20:4 and 15°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.

Thermoperiodic responses in insects and mites simulated with the double circadian oscillator clock.

The "double circadian oscillator model" for the photoperiodic clock has been used to simulate thermoperiodic responses in insects and mites. Two assumptions have been made: (1) the clock measures cryophase in a similar way to scotophase, and (2) temperature cycles are able to entrain the clock in a similar way to LD cycles. Simulations showed that Assumption 1 causes the "critical cryophase" to be of about equal duration as the critical night length. Assumption 2 is not always needed if diapause incidences in DD are high at low temperatures but low or zero at high temperatures. The latter assumption is needed, however, if high diapause occurs in thermoperiodic cycles in DD, whereas nondiapause occurs in DD with both high and low constant temperatures. The model accounts for the observation that the amplitude of the temperature cycle is important in some insects, whereas the temperature of the cryophase is crucial in others.

A Double Circadian Oscillator Model for Quantitative Photoperiodic Time Measurement in Insects and Mites

From studies on photoperiodic time measurement in insects it is known that different night lengths at the same side of the critical night length can have different inductive strengths. This means that nights of different length, and either longer or shorter than the critical night length, can have qualitatively different values. Nevertheless, few photoperiodic-clock models have been developed that are based on quantitative night-length measurement.In this paper a model is proposed that consists of two independent, circadian mechanisms. Both mechanisms determine the length of a night and give it a quantitative value, which is either zero or positive. One of the mechanisms (LN system) generally gives a scotophase a positive value when it is “long” (i.e. lights-on occurs when the LN oscillator is in its descending phase), whereas the other (SN system) gives a scotophase a positive value when it is “short” (i.e. lights-on occurs when the SN oscillator is in its ascending phase). In this particular context, therefore, “long” and “short” do not necessarily mean longer or shorter than the critical night length.The reasons for two time-measuring systems instead of one are: first, in some insects only long nights are accumulated, not short nights, or vice versa. Second, long nights are less sensitive to temperature than short nights. Third, in some cases it seems that long and short nights are determined in a different manner. These observations indicate that long and short nights could be determined by separate mechanisms.Responses generated by the proposed model parallel those observed experimentally with the spider mite,Tetranchus urticaeand the aphid,Megoura viciae. General properties of the model are discussed and compared with Zaslavski's quantitative clock model which shares some features.

Shifts in the semidian rhythm of flowering do determine night-break timing and critical night length in Pharbitis nil

There is a semidian (≈12 h) rhythm in the flowering response of the short-day plant Pharbitis nil Choisy following 90 min exposure to either far-red light/darkness or a temperature drop (27 °C to 12 °C) given at various times in constant conditions before an inductive dark period. This semidian rhythmic response to the temperature-drop pretreatments in the light is also evident through the inductive dark period without change of phase. Furthermore, those pretreatments which increase flowering also advance the time of maximum sensitivity to red light (R) interruptions of the dark period by up to 1.5 h and shorten the critical night length. Conversely, pretreatments which reduce flowering delay the time of maximum R inhibition by up to 1.5 h and increase the critical night length by the same amount. However the phase of a circadian rhythm of flowering response had no effect on either the time of maximum R inhibition or the critical night length. Thus, the semidian rhythm determines both the time of maximum R inhibition and the critical night length in Pharbitis.

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°C. Diapause duration at all four long-night treatments fluctuated around 64.5 days, varying from 62.2 at LD 12:12 h to 66.4 at LD 10:14 h. 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 13:11 h and LD 14:10 h, 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°C to the diapause terminating regimes at 19°C all females started reproduction. Mean diapause durations were 8.1, 6.4 and 6.5 days for the short-night treatments LD 15:9 h, LD 17:7 h and LD 19:5 h, 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 69 and 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.

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 16oC, 19oC and 22oC. The response curves appeared to be similar at 16oC and 19oC, with a critical nightlength between 10 and 11 h. At 22oC, 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 19oC 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 19oC 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.

PHOTOPERIODISM AND LIFE CYCLE PLASTICITY OF AN APHID, MACROSIPHUM EUPHORBIAE (THOMAS), FROM CENTRAL NORTH AMERICA

AbstractPhotoperiodic responses are described for five clones of the potato aphid, Macrosiphum euphorbiae (Thomas) (Homoptera: Aphididae), from Manitoba, Canada, a region with a strongly seasonal, continental climate. When exposed to long nights, parthenogenetic wingless females produce winged males and parthenogenetic winged females that subsequently produce mating females. The young adult males and winged females are restless and take flight readily, which is typical of a post-teneral migratory phase that could carry them to a winter host. These traits are characteristic of host-alternating species. However, wingless females also produce mating females, and winged females also produce parthenogenetic wingless females, traits which are characteristic of nonhost-alternating species. Clones differ in the frequencies of phenotypes they produce, some being more host-alternating than others. Critical night lengths are shortest and frequencies of males highest for clones that are closest to the host-alternating type of life cycle. The importance of phenotypic and genotypic plasticity in the life cycle is discussed.

Melatonin and the pea aphid, Acyrthosiphon pisum

Pea aphids, Acyrthosiphon pisum, were fed on artificial diet containing various concentrations of melatonin. Under long-day conditions (16 h light:8 h dark) their progeny included males and virginoparous/oviparous (asexual/sexual) intermediate females, which normally occur only in short days or around critical night-length. Endogenous melatonin in pea aphids was measured by radioimmunoassay and verified by parallelism with a melatonin standard curve and by thin layer chromatography. However, melatonin titres showed large variations and although they tended to be higher during the scotophase than during the photophase they were not significantly different. The possibility of melatonin being involved in photoperiodism is discussed.