In eight strains of the spider mite Tetranychus urticae Koch (Acari: Tetranychidae), originating from different localities in Europe, the critical daylength for diapause induction and termination was almost the same in each strain but varied with the latitudinal origin of the strains; critical daylength was shorter in strains originating from lower latitudes and longer in those from higher latitudes. Diapause intensity, measured as the period of chilling required for diapause termination under a short day photoperiod (LD 10:14) and 19°C, again varied with the latitudinal origin of each strain, being higher the more northern the origin of the strain. An exception were two mountain strains which showed a longer critical daylength and a deeper diapause than expected on the basis of their latitudinal origin. The number of long-day (LD 17:7) cycles required for 50% diapause termination after a certain period of chilling was higher in the northern and lower in the southern strains. These results indicate that geographic strains of T urticae may differ considerably in their diapause attributes, which may be explained as an adaptation to local climatic conditions. The great plasticity of the diapause response may, among other factors, have been responsible for the wide distribution of this mite species.

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 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.

Bumblebees are generally believed to be annual insects. However, here we will show that under laboratory conditions the bumblebee Bombus terrestris (L.) can produce a second generation without a period of cold storage (diapause) or CO 2 narcosis (a method to break diapause). It is also shown that this so‐called non‐diapause trait can be selected for. The percentage of non‐diapausing queens increased from 8% (minimum) to 97% (maximum) in two generations of selection. However, it was not possible to maintain isofemale non‐diapause lines. Colonies of the fourth and fifth generation remained small (expressed in worker number) and produced a small number of queens. Also the percentage of queens that started laying eggs (defined as the percentage non‐diapause) decreased in the fourth and fifth generations. To study whether this decline of the non‐diapause lines was caused by inbreeding, a control experiment was conducted. In this control experiment queens were mated with their brothers (full‐sib mating) for several generations and the number of queens that start egg laying was measured. This revealed that inbreeding can have a negative effect on the egg‐laying capacities of queens thus causing the decline of inbred (non‐diapause) lines.

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 12:12h 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°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 15:9h, 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.

Entomologia Experimentalis et ApplicataVolume 84, Issue 2 p. 195-197 'Bistability' experiments and the photoperiodic clock in the spider mite Tetranychus urticae Marlies Vaz Nunes, Marlies Vaz Nunes Aphid Biology Group, Department of Biology, Imperial College at Silwood Park, Ascot, Berks. SL5 7PY, UKSearch for more papers by this authorAlfred Veerman, Corresponding Author Alfred Veerman Institute for Systematics and Population Biology, Section Population Biology, University of Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The NetherlandsSearch for more papers by this author Marlies Vaz Nunes, Marlies Vaz Nunes Aphid Biology Group, Department of Biology, Imperial College at Silwood Park, Ascot, Berks. SL5 7PY, UKSearch for more papers by this authorAlfred Veerman, Corresponding Author Alfred Veerman Institute for Systematics and Population Biology, Section Population Biology, University of Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The NetherlandsSearch for more papers by this author First published: 20 October 2003 https://doi.org/10.1046/j.1570-7458.1997.00215.xCitations: 4 Author for correspondence AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume84, Issue2August 1997Pages 195-197 RelatedInformation

Behavioural responses were studied in mated and unmated females of a Dutch strain of the spider mite Tetranychus urticae which were destined to enter diapause. Observations were confined to the period between acquirement of the winter coloration by the females (a few days after the last moult) and their entering of artificial hibernation sites offered to the mites on the host leaves. Unmated females were found to stay longer on the leaf surface than mated females; they entered the hibernation sites significantly later than mated females. The delay in the search for hibernation sites shown by unmated females may be seen as a behavioural adaptation to enhance the chance of being fertilized before hibernation. Winter survival of mated and unmated diapausing females of the same strain of mites was studied both in the laboratory at a constant temperature of 2±1°C and outdoors under natural climatic conditions in Amsterdam during the winter of 1990–1991. Survival was high under both conditions for mated as well as unmated females; no significant differences in survival were found between both types of female. Observations on post-diapause females of Tetranychus atlanticus (a mite belonging to the T. urticae complex) sampled from strawberry fields near Moscow in spring, showed that at most 10% of the females of this natural spider mite population were unmated. Both mated and unmated females had survived winter temperatures of -28 to -30°C.

In the spider mite Tetranychus urticae, both diapause induction and diapause termination are under photoperiodic control. Resonance experiments with three strains from different localities in Europe revealed the involvement of the circadian system in the photoperiodic termination of diapause. In all strains the τ-value of the rhythm involved in diapause termination appeared to be shorter than that of the rhythm involved in diapause induction. This may be caused by a change in period length of a circadian oscillator involved in both induction and termination of diapause, or it may indicate that different circadian oscillators affect diapause induction and diapause termination in the spider mite. In previous experiments it has been demonstrated that the photoperiodic clock controlling diapause termination is most probably the same as the one controlling diapause induction. In eight strains of the spider mite, originating from widely different localities in Europe, the critical nightlengths for diapause induction and diapause termination appeared to be identical. Apparently some change occurs in the circadian system between diapause induction and diapause termination, without a concurrent change in photoperiodic time measurement. This makes it unlikely that the oscillator(s) involved in diapause induction and termination would constitute the photoperiodic clock. The results are interpreted according to the so-called non-clock or resonance hypothesis for the involvement of the circadian system in photoperiodic phenomena.

Photoperiodic control of diapause development was studied in three strains of the spider mite, Tetranychus urticae, originating from different latitudes in Europe. Diapause was almost fully maintained by a long-night regime during the first month of diapause, but was terminated rapidly and synchronously by a short-night regime. Diapause termination in continuous light was almost as rapid as in short nights; in continuous darkness diapause termination proceeded much slower, probably revealing the “spontaneous” rate of diapause development of these mites. The effect on diapause termination of successive short-night cycles appeared to be accumulated in a way comparable to cycle summation during diapause induction. The threshold for diapause termination by short nights, expressed as the number of cycles required for 50% diapause termination, was found to be lower the more southern the origin of the strain of mites. However, considerable differences in the number of cycles required for diapause termination were also present within strains: some mites needed only 3–4 short nights to terminate diapause, whereas others needed more than 10. In contrast with short-night cycles the effect of long-night cycles was not accumulative. Intensification of diapause under the influence of long-night cycles during its early stages appeared not to take place. Long nights were capable of maintaining the state of diapause if experienced before short-night cycles, but not afterwards. Short and long nights seem to act independently from each other, the former accelerating diapause development (“activation”), the latter slowing down diapause development (“diapause maintenance”). No antagonistic effect of long and short nights was found on the photoperiodic maintenance of diapause, in the sense that the effect of short nights might be diminished or even reversed by long nights, and vice versa, as found in photoperiodic induction of diapause in T. uriticae. Therefore diapause maintenance does not seem to involve a photoperiodic counter mechanism comparable with the counter involved in the photoperiodic induction of diapause in these mites.

Diapausing larvae of Eurytoma amygdali Enderlein (Hymenoptera, Eurytomidae) were collected in early August and late September. They were subjected to various photoperiod and temperature regimens for up to 20 weeks, then kept at L16:D8 and 19 degrees C for another 14 to 26 weeks for diapause to be terminated and pupation to take place.Photoperiod did not affect diapause completion. It was confirmed that the two morphologically distinct diapause stages have different temperature requirements for their completion. The first diapause stage was completed synchronously at temperatures between 16 and 19 degrees C. A higher temperature of 26 degrees C delayed diapause development. The second stage required lower temperatures between 4 and 10 degrees C. Spontaneous termination of diapause was observed at constant 19 degrees C.When applied to the first diapause stage for 20 weeks, low temperatures made the larvae refractory to subsequent intermediate temperatures. The first stage was thus maintained until a higher temperature of 26 degrees C made the larvae regain their ability to respond to the intermediate temperatures and complete this stage. Larvae grown in Retsou almonds had a higher diapause intensity than larvae grown in Truoito almonds.The results suggest that, in nature, the high temperatures of late summer and early autumn are likely to maintain the first diapause stage. Subsequently, the less warm temperatures of autumn allow the completion of the first stage by late autumn, and the low temperatures of late autumn and of winter allow the completion of the second diapause stage by mid winter.

Eight strains of the spider mite Tetranychus urticae, originating from different localities in western and central Europe, with latitudes ranging from 40.5 to 60-degrees-N, displayed marked differences in the period of chilling at 4-degrees-C required for diapause termination under a diapause-maintaining short-day photoperiodic regime at 19-degrees-C, to which the mites were transferred after the cold period. The higher the latitude from which the strains originated the longer was the period of chilling required for diapause termination, suggesting the presence of a gradient in diapause intensity, diapause being deeper the more northern the origin of the strains.Two strains originating from higher altitudes appeared to have a much deeper diapause than expected from their latitudinal origin. In addition, these two mountain strains showed mutual differences in diapause intensity, notwithstanding the fact that they originated from similar latitudes and altitudes; local climatic conditions probably act as strong selective forces with regard to diapause depth.All strains appeared to be sensitive to photoperiod during the period of diapause development. Diapause was quickly completed by a long-day photoperiod (LD 17:7 h), but was maintained by a short-day photoperiod (LD 10: 14 h). However, even under the latter regime sensitivity to photoperiod gradually diminished and eventually disappeared, thus leading to 'spontaneous' termination of diapause. The length of the period of diapause development, as measured by the sensitivity to photoperiod of diapausing mites, varied between strains; it was shorter in the southern strains and longer in the northern strains. The results indicate great variation in diapause intensity between strains, which is probably genetically determined and may have adaptive significance for this widespread species.When young females which had just entered diapause were kept for ever longer periods of time under the diapause inducing short-day regime at which they had been reared, before being transferred to the cold room, the duration of the period of chilling required for diapause termination was found to decrease proportionally in all three strains tested. These results suggest that intensification of diapause does not occur in T. urticae; diapause intensity seems to be highest at the beginning of diapause and to diminish gradually during diapause development.

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.

This review briefly describes characteristics and occurrence of diapause in phytoseiid mites. This is followed by a discussion of factors involved in induction, maintenance and termination of diapause (e.g. photoperiod, temperature and food availability), of the physiological mechanism of diapause induction (photoperiodic perception and comparison of photoperiodic and thermoperiodic induction mechanisms), and of applied aspects of diapause in phytoseiid mites.