Analysis of the operation of the photoperiodic counter provides evidence for hourglass time measurement in the spider miteTetranychus urticae

The circadian and photoperiodic clock of the pea aphid

The pea aphid, Acyrthosiphon pisum, is a paradigmatic photoperiodic species that exhibits a remarkable annual life cycle, which is tightly coupled to the seasonal changes in day length. During spring and summer, characterised by longer days, aphid populations consist exclusively of viviparous females that reproduce parthenogenetically. When autumn comes and the days shorten, aphids switch their reproductive mode and generate males and oviparous sexual females, which mate and produce cold-resistant eggs that overwinter and survive the unfavourable season. While the photoperiodic responses have been well described, the nature of the timing mechanisms which underlie day length discrimination are still not completely understood. Experiments from the 1960's suggested that aphids rely on an 'hourglass' clock measuring the elapsed time during the dark night by accumulating a biochemical factor, which reaches a critical threshold at a certain night length and triggers the switch in reproduction mode. However, the photoperiodic responses of aphids can also be attributed to a strongly dampened circadian clock. Recent studies have uncovered the molecular components and the location of the circadian clock in the brain of the pea aphid and revealed that it is well connected to the neurohormonal system controlling aphid reproduction. We provide an overview of the putative mechanisms of photoperiodic control in aphids, from the photoreceptors involved in this process to the circadian clock and the neuroendocrine system.

Circadian and Neuroendocrine Basis of Photoperiodism Controlling Diapause in Insects and Mites: A Review

The photoperiodic system is concealed in the highly complex black-box, comprising four functional subunits: 1) a photo/thermo-sensitive input unit, 2) a photoperiodic clock based on a circadian system, 3) a condenser unit counting the number of inductive signals, and 4) a neuroendocrine switch that triggers a phenotypic shift. This review aims to summarize the research history and current reach of our understanding on this subject to connect it with the molecular mechanism of the circadian clock rapidly being unveiled. The review also focuses on the mode of intersubunit information transduction. It will scan the recent advancement in research on each functional subunit, but special attention will be given to the circadian clock-endocrine conjunct and the role of melatonin signaling in the regulation of insect photoperiodism. Prothoracicotropic hormone (PTTH) probably plays the most crucial role in the regulation of pupal diapause, which is the simplest model system of diapause regulation by hormones investigated so far, particularly in the Chinese oak silkmoth (Antheraea pernyi). A search for the trigger to release the PTTH found some candidates, that is, indoleamines. Indolamine metabolism is controlled by arylalkylamine N-acetyltransferase (aaNAT). Indolamine dynamics and aaNAT enzymatic activity changed according to photoperiods. aaNAT activity and melatonin content in the brain showed not only a photoperiodic response but also a circadian fluctuation. aaNAT had multiple E-boxes, suggesting that it is a clock-controlled gene (ccg), which implies that cycle (cyc, or brain-muscle Arnt-like 1 = Bmal1)/Clock (Clk) heterodimer binds to E-box and stimulates the transcription of aaNAT, which causes the synthesis of melatonin. RNAi against transcription modulators, cyc, or Clk downregulated aaNAT transcription, while RNAi against repressor of cyc/Clk, per upregulated aaNAT transcription. Immunohistochemical localization showed that the circadian neurons carry epitopes of melatonin-producing elements such as aaNAT, the precursor serotonin, HIOMT, and melatonin as well as clock gene products such as cyc-ir, Per-ir, and dbt-ir, while PTTH-producing neurons juxtaposed against the clock neurons showed hMT2-ir in A. pernyi brain. Melatonin probably binds to the putative melatonin receptor (MT) that stimulates Ca2+ influx, which in turn activates PKC. This induces Rab 8 phosphorylation and exocytosis of PTTH, leading to termination of diapause. All the PTTH-expressing neurons have PKC-ir, and Rab8-ir. When diapause is induced and maintained under short days, serotonin binding to 5HTR1B suppresses PTTH release in a yet unknown way. RNAi against this receptor knocked out photoperiodism; short day response is blocked and diapause was terminated even under the short day condition. The result showed that a relatively simple system controls both induction and termination in pupal diapause of A. pernyi: the circadian system regulates the transcription of aaNAT as a binary switch, the enzyme produces a melatonin rhythm that gates PTTH release, and 5HTR1B and MT are probably also under photoperiodic regulation. Finally, we listed the remaining riddles which need to be resolved, to fully understand this highly complex system in future studies.

Light intensity influences feeding and fecundity of Tetranychus urticae (Acari: Tetranychidae) through the responses of host Cucumis sativus leaves

We investigated feeding and fecundity of the two-spotted spider mite, Tetranychus urticae (Acari: Tetranychidae), on leaves of cucumber (Cucumis sativus) seedlings that had been acclimatized to different light intensities. Based on these data, we analyzed the relationships between mite performance (feeding and fecundity) and leaf properties. The cucumber seedlings were grown in controlled-environment chambers under different light intensities at a photosynthetic photon flux density of 50, 100, 150, 300, or 450 µmol m^- 2 s^- 1 until the first true leaves had expanded. Adult females were released on the adaxial surfaces of excised leaf samples from the seedlings of each treatment group and held under standardized light intensity (200 µmol m^- 2 s^- 1). Fecundity and leaf damage area increased and decreased, respectively, as the acclimatization light intensity increased, indicating indirect effects of light intensity on feeding and fecundity through changes in the host leaf properties. Leaf mass per area (LMA) and photosynthetic capacity, which increased as the acclimatization light intensity increased, was positively related to the fecundity, but was negatively related to the leaf damage area. The higher LMA and photosynthetic capacity results in an increased amount of mesophyll per unit leaf area. This would allow the mites to feed efficiently from a limited area, which may explain the increased fecundity on these leaves.

Physiological and molecular mechanisms underlying photoperiodism in the spider mite: comparisons with insects

Photoperiodism is an adaptive, seasonal timing system that enables organisms to coordinate their development and physiology to annual changes in the environment using day length (photoperiod) as a cue. This review summarizes our knowledge of the physiological mechanisms underlying photoperiodism in spider mites. In particular, the two-spotted spider mite Tetranychus urticae is focussed, which has long been used as a model species for studying photoperiodism. Photoperiodism is established by several physiological modules, such as the photoreceptor, photoperiodic time measurement system, counter system, and endocrine effector. It is now clear that retinal photoreception through the ocelli is indispensable for the function of photoperiodism, at least in T. urticae. Visual pigment, which comprised opsin protein and a vitamin A-based pigment, is involved in photoreception. The physiological basis of the photoperiodic time measurement system is still under debate, and we have controversial evidence for the hourglass-based time measurement and the oscillator-based time measurement. Less attention has been centred on the counter system in insects and mites. Mite reproduction is possibly regulated by the ecdysteroid, ponasterone A. Prior physiological knowledge has laid the foundation for the next steps essential for the elucidation of the molecular mechanisms driving photoperiodism.

Environmental Engineering Approaches toward Sustainable Management of Spider Mites

Integrated pest management (IPM), which combines physical, biological, and chemical control measures to complementary effect, is one of the most important approaches to environmentally friendly sustainable agriculture. To expand IPM, we need to develop new pest control measures, reinforce existing measures, and investigate interactions between measures. Continued progress in the development of environmental control technologies and consequent price drops have facilitated their integration into plant production and pest control. Here I describe environmental control technologies for the IPM of spider mites through: (1) the disturbance of photoperiod-dependent diapause by artificial light, which may lead to death in seasonal environments; (2) the use of ultraviolet radiation to kill or repel mites; and (3) the use of water vapor control for the long-term cold storage of commercially available natural enemies. Such environmental control technologies have great potential for the efficient control of spider mites through direct physical effects and indirect effects via natural enemies.

Insect photoperiodic calendar and circadian clock: independence, cooperation, or unity?

The photoperiodic calendar is a seasonal time measurement system which allows insects to cope with annual cycles of environmental conditions. Seasonal timing of entry into diapause is the most often studied photoperiodic response of insects. Research on insect photoperiodism has an approximately 80-year-old tradition. Despite that long history, the physiological mechanisms underlying functionality of the photoperiodic calendar remain poorly understood. Thus far, a consensus has not been reached on the role of another time measurement system, the biological circadian clock, in the photoperiodic calendar. Are the two systems physically separated and functionally independent, or do they cooperate, or is it a single system with dual output? The relationship between calendar and clock functions are the focus of this review, with particular emphasis on the potential roles of circadian clock genes, and the circadian clock system as a whole, in the transduction pathway for photoperiodic token stimulus to the overt expression of facultative diapause.

Deciphering time measurement: the role of circadian 'clock' genes and formal experimentation in insect photoperiodism

This review examines possible role(s) of circadian 'clock' genes in insect photoperiodism against a background of many decades of formal experimentation and model building. Since ovarian diapause in the genetic model organism Drosophila melanogaster has proved to be weak and variable, recent attention has been directed to species with more robust photoperiodic responses. However, no obvious consensus on the problem of time measurement in insect photoperiodism has yet to emerge and a variety of mechanisms are indicated. In some species, expression patterns of clock genes and formal experiments based on the canonical properties of the circadian system have suggested that a damped oscillator version of Pittendrigh's external coincidence model is appropriate to explain the measurement of seasonal changes in night length. In other species extreme dampening of constituent oscillators may give rise to apparently hourglass-like photoperiodic responses, and in still others there is evidence for dual oscillator (dawn and dusk) photoperiodic mechanisms of the internal coincidence type. Although the exact role of circadian rhythmicity and of clock genes in photoperiodism is yet to be settled, Bünning's general hypothesis (Bünning, 1936) remains the most persuasive unifying principle. Observed differences between photoperiodic clocks may be reflections of underlying differences in the clock genes in their circadian feedback loops.

Controversial aspects of photoperiodism in insects and mites

This review examines several controversial aspects of photoperiodism in insects and mites including the role of the circadian system in night length measurement, the nature of apparent hourglass-like responses, and whether or not the circadian component in photoperiodism is the same as that in overt behavioural rhythms. These aspects of the phenomenon are discussed in terms of the entrainment of circadian oscillations by cycles of light and temperature. There is considerable variety of photoperiodic response within the insects (and other arthropods) to show, inter alia, circannual rhythms, internal and external coincidence night length timers, and in some species, non-circadian hourglass-like devices. Many apparent hourglass-like responses, however, could be circadian 'clocks' of the external coincidence type involving oscillations that dampen below threshold in extended periods of darkness. The review also concludes that there is little evidence in favour of the "Hourglass clock-oscillator counter" model proposed for the mite Tetranychus urticae by Vaz Nunes and Veerman (1982a). The responses of this species to complex light and temperature cycles may also be interpreted in terms of a damped oscillator version of external coincidence.

Artificial selection for responsiveness to photoperiodic change alters the response to stationary photoperiods in maternal induction of egg diapause in the rice leaf bug Trigonotylus caelestialium

Female adults of the rice leaf bug Trigonotylus caelestialium (Kirkaldy) (Heteroptera: Miridae) produce non-diapause eggs under long-day conditions, whereas they produce diapause eggs under short-day conditions. These egg-production modes change following a photoperiodic change from long-day to short-day conditions or vice versa, with individual variations in responsiveness shown in the time from the photoperiodic change to the mode change. Strains of this insect with higher or lower responsiveness to photoperiodic change were established after several generations of selection, indicating that the individual variation has a genetic basis. The selected strains that were more responsive and less responsive to one photoperiodic change were found to be less responsive and more responsive to the opposite photoperiodic change, respectively, indicating a significant negative correlation between responsiveness to reciprocal photoperiodic changes. The selected strains also had a significantly different incidence of diapause-egg producers in stationary photoperiods compared to a non-selected strain, showing that selections for responsiveness to photoperiodic change were essentially the same as selections for a higher or lower incidence of diapause-egg producers. These results indicate that responsiveness to photoperiodic change is one aspect of the tendency to produce diapause or non-diapause eggs.

Action spectrum for the suppression of arylalkylamine N-acetyltransferase activity in the two-spotted spider mite Tetranychus urticae

An action spectrum was obtained for the suppression of arylalkylamine N-acetyltransferase (NAT) activity in the two-spotted spider mite Tetranychus urticae by irradiating the mite with monochromatic lights of various wavelengths using the Okazaki Large Spectrograph at the National Institute for Basic Biology, Okazaki, Japan. Fluence-response curves were obtained for wavelengths between 300 and 650 nm by irradiating the mite for 4 h day(-1). The samples were frozen after the third exposure. A negative correlation between the logarithmic fluence rate and NAT activity was detected in the range of 0.01-1 micromol m(-2) s(-1) for wavelengths between 300 and 500 nm and in the range of 0.1-10 micromol m(-2) s(-1) for wavelengths between 550 and 650 nm. The constructed action spectrum indicated that the photoreceptors mediating the circadian and/or photoperiodic systems might be UV-A- and blue-type photoreceptors with absorption peaks at 350 and 450 nm.

Extrinsic light:dark cycles, rather than endogenous circadian cycles, affect the photoperiodic counter in the pitcher-plant mosquito, Wyeomyia smithii

A wide diversity of organisms use photoperiod (daylength) as an environmental cue to anticipate the changing seasons and to time various life-history events such as dormancy and migration. Photoperiodic time measurement consists of two main components, (1) the photoperiodic timer that discriminates between long and short days, and (2) the photoperiodic counter that accumulates and stores information from the timer and then induces the phenotypic output. Herein, we use extended night treatments to show that light is necessary to accumulate photoperiodic information across the geographic range of the mosquito, Wyeomyia smithii and that the photoperiodic counter counts extrinsic (external) light:dark cycles and not endogenous (internal) circadian cycles.

Effects of extending the light phase on diapause induction in a Japanese population of the two-spotted spider mite, Tetranychus urticae

Artificial lighting is a merit of a 'plant factory', which might be utilized to suppress an increase in pest population. We investigated the effects of extending the light phase on diapause induction in the two-spotted spider mite (TSSM), Tetranychus urticae. TSSM were reared at 18 degrees C under light phases ranging from 2 to 64 h combined with a constant dark phase of 16 h in aluminum bottles, with white light emitting diodes attached inside to minimize fluctuations in air temperature between the light and dark phases. Diapause was induced in adult TSSM females when the light phase was 24 h or shorter, and diapause induction was inhibited when the light phase extended over 32 h. The development of deutonymphs was delayed under a diapause-inducing photoperiod. Diapause inducing photoperiods may suppress an increase in the TSSM population, by slowing down development and reproduction.

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.

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.

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.

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 time measurement in insects and mites: a critical evaluation of the oscillator-clock hypothesis

The validity of the oscillator-clock hypothesis for photoperiodic time measurement in insects and mites is questioned on the basis of a re-interpretation of available experimental evidence. The possible role of the circadian system in photoperiodism in arthropods is critically reviewed. Apart from the outcome of kinetic experiments, based on diel and non-diel light/dark cycles, evidence from various genetic and physiological experiments is discussed in relation to the oscillator-clock hypothesis. The conclusion is that photoperiodic time measurement in insects and mites is performed by a non-circadian 'hourglass' clock. Experimental evidence suggests a non-clock role for the circadian system in the photoperiodic mechanism of insects and mites.

Photoperiodic time measurement in insects: a review of clock models

Based on analyses of responses of insects and mites to a wide range of diel and nondiel experimental light-dark schedules, a variety of models have been developed for the photoperiodic clocks in these species by nearly as many investigators. According to some of these models, the photoperiodic clock is based on a mechanism separate from the circadian system, that is, a so-called "hourglass." According to other models, the clock is based on one or more circadian oscillators that may be coupled to each other and that may or may not show a certain degree of damping. In this context, a rapidly damping oscillator could be regarded as an hourglass. The present article gives an overview of the many different clock models and their philosophies, and it makes comparisons among them to provide a better understanding about how these models are related, if at all, and why the double circadian oscillator model is the most favored model at present.

The same photoperiodic clock may control induction and maintenance of diapause in the spider mite Tetranchus urticae

In the spider mite Tetranychus urticae, both diapause induction (which takes place during the larval and nymphal stages) and diapause maintenance (in the adult female) are under photoperiodic control. The question of whether or not the same photoperiodic clock is involved in both photoperiodic reactions was investigated in eight strains of the spider mite, originating from different localities in Europe. The methods employed consisted of (1) determination of the relative importance of the photophase and scotophase in the two photoperiodic reactions; (2) comparison of photoperiodic response curves for diapause induction and diapause maintenance; and (3) determination of the effect of light breaks on the capacity of long nights to maintain diapause, and comparison with the effect of light breaks in diapause induction experiments. The scotophase appeared to be much more important than the photophase for both diapause induction and diapause maintenance. In all strains the critical daylength for diapause maintenance, measured at the moment of saturation of the response to long daylengths, was identical to the critical daylength for diapause induction. However, the critical daylength for diapause maintenance appeared to be labile; it shifted gradually to shorter values as the mites were kept in the cold for a longer period of time, or were kept at a higher temperature for a progressively longer period of time after their stay in the cold room. This seems to reflect a gradual loss of photoperiodic control of diapause maintenance as diapause development proceeds. Photoperiods close to the critical daylength appeared to be less strong with regard to diapause maintenance than shorter daylengths. Quantitative differences in the "strength" of different daylengths were found in all strains investigated. Interruption of the night by short pulses of light revealed either one or two peaks of sensitivity in the night, or one broad "trough" where the two peaks had merged. However, in each case maximal sensitivity to the light breaks occurred at the same position in the night for diapause induction and diapause maintenance. The many similarities found lead to the conclusion that most probably the same photoperiodic clock mechanism is involved in both diapause induction and diapause maintenance in T. urticae.

The effect of temperature on photoperiodic induction of diapause in insects and mites: a model for the photoperiodic "counter"

Existing models for the accumulation of photoperiodic information (the photoperiodic "counter") cannot explain satisfactorily the various effects of temperature on diapause induction in insects and mites, nor the observation that in several insect species the so-called "required day number" has a high degree of temperature compensation. In the model presented in this paper the effect of temperature has been incorporated in such a way that both the rate of increase of the "induction sum" (or "diapause titre") and the level of the "minimally required induction sum" (or "threshold level") are temperature dependent. The proposed model is in accordance with observed temperature effects, and gives an explanation for a temperature-compensated "required day number".

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.