High Incidence Of Diapause Research Articles

A Comparative Study of Circadian Rhythmicity and Photoperiodism in Closely Related Species of Blow Flies: External Coincidence, Maternal Induction, and Diapause at Northern Latitudes.

This review compares adult locomotor activity rhythms and photoperiodic induction of diapause in 3 common species of blow fly, Calliphora vicina, Lucilia sericata, and Protophormia terraenovae. Activity rhythms were broadly similar in all 3 species, although P. terraenovae is much less sensitive to constant light inducing arrhythmicity. Photoperiodic induction of diapause, on the other hand, varies more widely between species. C. vicina and L. sericata overwinter in a larval diapause induced by autumnal short days (long nights) acting both maternally and directly upon the larvae. P. terraenovae, on the other hand, shows an adult (reproductive) diapause induced by short daylength and low temperature experienced by the larvae. In the Nanda-Hamner protocol, C. vicina shows 3 clear peaks of high diapause incidence in cycle lengths close to 24, 48, and 72 h, without dampening and therefore suggesting a photoperiodic mechanism based on a self-sustained circadian oscillator acting in a clock of the external coincidence type. Entrainment of the locomotor activity rhythm to extended Nanda-Hamner photocycles, as well as to LD cycles close to the limits of the primary range of entrainment, demonstrates that overt circadian rhythmicity may act as ‘hands’ of the otherwise covert photoperiodic system, as suggested by Bünning, nearly 8 decades ago. In 24 h LD cycles, both locomotor activity rhythms and the photoperiodic oscillator are set to constant phase (CT 12) at light-off, so that the photoperiodic clock measures changes in nightlength by the coincidence (or not) of dawn light with a ‘photoinducible phase’ late in the subjective night (at about CT 21.5 h) as photoperiod changes with the seasons. Apparent differences between quantitative and qualitative photoperiodic responses are discussed.

Temperature and Photoperiodic Response of Diapause Induction in Anastatus japonicus, an Egg Parasitoid of Stink Bugs.

Simple SummaryAnastatus japonicus Ashmead is an important egg parasitoid wasp and natural enemy of Tessaratoma papillosa. It is commercially used in biological control, and has effectively suppressed T. papillosa population for decades in China. In practice, medium temperature in spring always induces diapause in A. japonicus, leading to the delay of adult emergence and missing the best chance to control the pest. Accurately regulating the development of A. japonicus is a key technique for the production of biocontrol agents and field release applications. In this study, responsible environmental factors for the induction of diapause in A. japonicus were investigated. A short photoperiod and medium temperature led to diapause in A. japonicus. Second–third instar larva are the most sensitive stages to diapause stimuli. Diapausing mature larvae had a significantly higher survival rate after 180 days storage at 10 °C than that of nondiapausing mature larvae. Taken together, results suggest methods that could be exploited in the developmental regulation, field-release pretreatment technology, and long-term storage of A. japonicus.Anastatus japonicus Ashmead is a widely used biological control agent against stink bugs that can be successfully reared using the large eggs of the Chinese silkworm. In this study, environmental factors responsible for the induction of diapause in A. japonicus were investigated on host eggs of the Chinese silkworm. A. japonicus exhibited a facultative, mature larval diapause within its host eggs. Second–third instar larva are the most sensitive stages to diapause stimuli. The accumulation of diapause stimuli during all the larval stages maximized the diapause response. A short photoperiod of 10L:14D and temperature of 17 °C led to the occurrence of the highest diapause response, while a long photoperiod (14L:10D) and low temperatures (11 and 14 °C) prevented the diapause. A specific exposure period was required to reach high diapause incidence. Diapausing mature larvae had a significantly higher survival rate after 180 days storage at 10 °C than that of nondiapausing mature larvae. Taken together, results suggest methods that could be exploited in the developmental regulation, field-release pretreatment technology, and long-term storage of A. japonicus.

Diapause incidence and critical day length of Asian corn borer (Ostrinia furnacalis) populations exhibit a latitudinal cline in both pure and hybrid strains

The Asian corn borer, Ostrinia furnacalis, is a serious pest and is widely distributed in corn-producing regions throughout China. This insect enters facultative diapause as fully grown larvae in response to short day lengths. Here, we evaluate the changes in diapause incidence and critical day length (CDL) by crossing a tropical strain with five subtropical and temperate strains from a 2897-km latitudinal gradient to investigate the latitudinal variations in diapause induction and CDL and their heredity. Photoperiodic responses of diapause induction in the different geographical strains at different temperatures showed that diapause incidence and CDL increased with increasing latitude and decreased with increasing temperature. Similarly, diapause incidence and CDL for the hybrid strains also showed an increase with increasing latitude or decreasing temperature. Conversely, the diapause incidences were significantly lower and the CDLs were significantly shorter in hybrid strains than their parents with high diapause incidence. Furthermore, the males had significantly more influence than the females on the incidence of diapause in subsequent progeny. Diapause incidence and CDL in O. furnacalis are strikingly different among pure strains and hybrid strains. The photoperiodic response controlling diapause is strongly heritable, with diapause being a partial dominant character over nondiapause, with the incidence of diapause mainly determined by the male parent. These results can help us to predict the time course of diapause induction in nature and may provide a genetic means for pest management by providing information on the overwintering capabilities of the hybrids.

Effects of inter-subspecies hybridization of non-native Monochamus alternatus alternatus and native M. a. endai (Coleoptera: Cerambycidae) on the induction of larval diapause and adult body size

To evaluate the effects of Taiwanese Monochamus alternatus alternatus (Coleoptera: Cerambycidae) (T) on native M. a. endai populations (J) and pine wilt disease epidemics in Japan, the biological traits of reciprocally crossed lines between two subspecies were compared with parental lines. No significant differences were observed in egg viability among four crosses in the F1 generation. Hybridized lines had greater survival rates than inbred lines of the J × J cross. The incidence of larval diapause was lower in the inbred lines of M. a. alternatus (0.176 and 0.361) than in the inbred lines of M. a. endai (1.000). Hybridized lines showed a high diapause incidence of 0.896–1.000 in the F1 and F2 generations, indicating that diapause induction was dominant over nondiapause induction. Large numbers of ovarioles dominated over small numbers of ovarioles. Hybridized lines revealed greater adult body sizes than the inbred lines of parental subspecies in the F1 generation (heterosis), but not in the F2 generation. Heterosis in adult body size may contribute to pine wilt disease epidemics because of the greater oviposition rate and initial load of pathogenic nematodes. Two unlinked loci are estimated to control diapause induction. Gene introgression and the dynamics of nondiapause-inducing alleles are discussed.

Sex‐linked inheritance of diapause induction in the butterfly Pieris napi

AbstractMany temperate insects survive harsh environmental conditions, such as winter, by entering a state of developmental arrest. This diapause state is predominantly induced by photoperiod. The photoperiod varies with latitude and has led to local adaptation in the photoperiodic induction of diapause in many insects. To understand the rapid evolution of the photoperiodic threshold, it is important to investigate and understand the underlying genetic mechanisms. In the present study, the genetic basis of photoperiodic diapause induction is investigated in the green‐veined white butterfly Pieris napi (Lepidoptera, Pieridae) by assaying diapause induction in a range of conditions for a Swedish and Spanish population. Furthermore, the inheritance of diapause induction is assessed in reciprocal F1 hybrids and backcrosses between the two populations. The southern population shows a clear photoperiodic threshold determining diapause or direct development, whereas the northern populations show a high incidence of diapause, regardless of photoperiod. The hybrid crosses reveal that the inheritance of diapause induction is strongly sex‐linked, and that diapause incidence in the genetic crosses is highly dependent on photoperiod. This emphasizes the importance of assaying a range of conditions in diapause inheritance studies. The results indicate a strongly heritable diapause induction with a major component on the Z‐chromosome, as well as a minor effect of the autosomal background.

Diapause and winter survival of two Orius species from southern Africa

The cold hardiness and overwintering potential of the southern African pirate bugs, Orius thripoborus (Hesse) and Orius naivashae (Poppius) (Hemiptera: Anthocoridae), were assessed in the laboratory. Diapause traits were studied by observing nymphal development and reproductive performance of adults at 18 °C and three photoperiods (10:14, 12:12 and 14:10 (L:D) h). A 12 h light regime was also tested at 23 °C. A 12 h photoperiod and 18 °C induced reproductive diapause in 84 and 42 % of O. naivashae and O. thripoborus females, respectively. Cold tolerance of adults was measured by determining the supercooling point (SCP, the temperature at which the insect’s body fluids freeze) and lethal time (LT50, the time required to kill 50 % of the population) at 0 and 5 °C. All observed SCPs ranged from −21 to −17 °C. Significantly lower SCP values were observed for acclimated (seven days at 10 °C) O. naivashae females. LT50-values averaged 6.4 and 4.4 days at 0 °C and 11.6 and 7.8 days at 5 °C, for adults of O. thripoborus and O. naivashae, respectively. The findings indicate that O. naivashae is less cold tolerant and has a higher diapause incidence compared with O. thripoborus. Therefore, the latter species may have better potential for use in biological control programmes in the cooler regions of southern Africa or elsewhere.

Geographic Variation of Diapause and Sensitive Stages of Photoperiodic Response inLaodelphax striatellusFallén (Hemiptera: Delphacidae)

Large numbers of the small brown planthopper Laodelphax striatellus (Fallén) (Hemiptera: Delphacidae) occur in temperate regions, causing severe losses in rice, wheat, and other economically important crops. The planthoppers enter diapause in the third- or fourth-instar nymph stage, induced by short photoperiods and low temperatures. To investigate the geographic variation in L. striatellus diapause, we compared the incidence of nymphal diapause under various constant temperature (20 and 27°C) and a photoperiod of 4:20, 8:16, 10:14, 12:12, 14:10, and 16:8 (L:D) h regimes among three populations collected from Hanoi (21.02° N, 105.85° E, northern Vietnam), Jiangyan (32.51° N, 120.15° E, eastern China), and Changchun (43.89° N, 125.32° E, north-eastern China). Our results indicated that there were significant geographic variations in the diapause of L. striatellus. When the original latitude of the populations increased, higher diapause incidence and longer critical photoperiod (CP) were exhibited. The CPs of the Jiangyan and Changchun populations were ∼12 hr 30 min and 13 hr at 20°C, and 11 hr and 11 hr 20 min at 27°C, respectively. The second- and third-instar nymphs were at the stage most sensitive to the photoperiod. However, when the fourth- and fifth-instar nymphs were transferred to a long photoperiod, the diapause-inducing effect of the short photoperiod on young instars was almost reversed. The considerable geographic variations in the nymphal diapause of L. striatellus reflect their adaptation in response to a variable environment and provide insights to develop effective pest management strategies.

Development of neuropeptide analogs capable of traversing the integument: A case study using diapause hormone analogs in Helicoverpa zea

Diapause hormone and its analogs terminate pupal diapause in Helicoverpa zea when injected, but if such agents are to be used as effective diapause disruptors it will be essential to develop simple techniques for administering active compounds that can exert their effect by penetrating the insect epidermis. In the current study, we used two molecules previously shown to have high diapause-terminating activity as lead molecules to rationally design and synthesize new amphiphilic compounds with modified hydrophobic components. An assay for diapause termination identified 13 active compounds with EC50's ranging from 0.9 to 46.0 pmol per pupa. Three compounds, Decyl-1963, Dodecyl-1967, and Heptyl-1965, selected from the 13 compounds most active in breaking diapause following injection, also successfully prevented newly-formed pupae from entering diapause when applied topically. These compounds feature straight-chain, aliphatic hydrocarbons from 7 to 12 carbons in length; DH analogs with either a short-chain length of 4 or an aromatic phenethyl group failed to act topically. Compared to a high diapause incidence of 80–90% in controls, diapause incidence in pupae receiving a 10 nmole topical application of Decyl-1963, Dodecyl-1967, or Heptyl-1965 dropped to 30–45%. Decyl-1963 and Dodecyl-1967 also remained effective when topically applied at the 1 nmole level. These results suggest the feasibility of developing DH agonists that can be applied topically and suggest the identity of new lead molecules for development of additional topically-active DH analogs. The ability to penetrate the insect epidermis and/or midgut lining is critical if such agents are to be considered for future use as pest management tools.

Photoperiodism of diapause induction and diapause termination in Ostrinia furnacalis

AbstractThe Asian corn borer, Ostrinia furnacalis (Guenée) (Lepidoptera: Crambidae), enters facultative diapause as fully grown larva in response to short‐day conditions during the autumn. Photoperiodic response curves showed a stable critical daylength of ca. 13.5 h at 25, 28, and 30 °C, which is nearly identical with the critical daylength between 13 h 22 min and 13 h 27 min in the field. The incidence of diapause obviously declined during ultra‐long nights (14–22 h scotophases) and DD (i.e., constant darkness). The required day number (RDN) for a 50% response differed significantly between short‐ and long‐night cycles at different temperatures, indicating that the effect of one short night was equivalent to the effect of three long nights at 25 and 28 °C. The third instar was the stage most sensitive to the photoperiod. Night‐interruption experiments with a 1‐h light pulse at L9:D15 and L12:D12 exhibited a broad trough of diapause inhibition. In experiments using non‐24‐h light‐dark cycles, only the cycles containing a long night of 12 h resulted in high incidence of diapause, regardless of the duration of the accompanying light period, indicating that the duration of the dark period was of greater importance than that of the light period. The Nanda‐Hamner and Bünsow experiments in O. furnacalis did not show any rhythmic fluctuations with a period of ca. 24 h in their photoperiodic response curves; the incidence of diapause was low when the scotophase length exceeded 16 h. The rearing daylengths of 11 and 12 h evoked a greater intensity of diapause than did 13 h at 25 and 28 °C. By transferring naturally overwintering larvae from natural conditions to a short photoperiod of L12:D12 or a long photoperiod of L15:D9 combined with 25 °C, photoperiod appeared to have a significant influence on diapause development during the early phase of diapause. Field observations for 2 years revealed that pupation and adult emergence of overwintering individuals (50% responses) occurred in early May and mid‐May, respectively.

Optimization of Orius majusculus release: photoperiodic sensitivity at different temperatures and storage of diapausing adults

The photoperiodic sensitivity and storage of Orius majusculus (Reuter) from Antalya was investigated to obtain data for the optimization of its release. In the current study, it was not possible to induce a high (up to 100%) incidence of diapause unless all the nymphal instars and adults were successively exposed (i.e. full exposure) to a short day length (11.5 h) at constant temperatures of 18 and 26 °C, and fluctuating temperatures of 10/26 °C. Hence an intermittent exposure to long day lengths, even for short periods of time (only during instars IV–V or adult stage), would avoid a high incidence of diapause at 10/26 °C and 26 °C. The response of O. majusculus nymphal instars and adults to a short day length with a low incidence of diapause under fluctuating temperatures suggests that greenhouses, where such temperatures prevail, provide an environment that is conducive to diapause prevention. The release of adults is a better approach than the release of nymphs, since the adults and their nymphs, until the eclosion of new adults, are expected to respond to short day lengths with a low incidence of diapause in greenhouses. When nymphs are released, on the other hand, a high incidence of diapause would be inevitable. Increasing the constant temperature to 30 °C at full exposure did not prevent diapause and consequently did not change the incidence of diapause. Storage of diapausing O. majusculus adults was possible for 1 month with a higher survival rate and fecundity than in a study based on the storage of nondiapausing adults at a lower temperature.

Involvement of circadian oscillation(s) in the photoperiodic time measurement and the induction of reproductive diapause in a northern Drosophila species

An ability to predict forthcoming changes in environmental conditions and get prepared for them in advance is crucial for the survival and reproduction of organisms living in a seasonally changing environment. We have studied the possible involvement of circadian oscillator(s) in the photoperiodic timer controlling seasonal responses by tracing Drosophila montana females’ diapause induction in constant darkness and in a classical Nanda–Hamner experiment. Nearly all females developed ovaries in continuous darkness, which shows the direct development to be their default developmental pathway in the absence of photoperiods. In Nanda–Hamner experiment the females’ diapause incidence was close to zero in light:dark cycle 12:4 (photoperiod 16h) and increased to nearly 100% in 12:8 and 12:12 (photoperiods 20 and 24h). In longer photoperiods (28–72h) the females’ diapause percentages decreased gradually along with an increase in the length of the dark period, showing no peaks of high diapause incidence in the multiples of 24h. These findings suggest that the photoperiodic timer of D. montana is based on heavily damping circadian oscillator(s) or that it lacks strong oscillators. Damping of the photoperiodic timer under prolonged nights and constant darkness fits well with our earlier finding that these flies lose their locomotor activity rhythm in constant darkness, and suggests that the mechanisms underlying females’ photoperiodic diapause response and their free-running locomotor activity rhythm may be partly based on same oscillators.

Diapause induction and photoperiodic clock in Chilo suppressalis (Walker) (Lepidoptera: Crambidae)

AbstractThe mature larvae of the rice stem borer, Chilo suppressalis Walker (Lepidoptera: Crambidae) enters facultative diapause in response to short‐day conditions in the autumn (August–September). Diapause induction and photoperiodic clock mechanism were investigated in C. suppressalis larvae reared on an artificial diet in the present study. The critical night length for diapause induction was about 9 h 53 min to 10 h 39 min at 22 to 28°C. The third‐instar larvae were found to be relatively sensitive to diapause induction. Photoperiodic response under non‐24‐h light–dark cycles showed that scotophase length played an essential role in the induction of larval diapause in C. suppressalis, and consecutive exposure to long‐night cycles was necessary for a high diapause incidence. In the Nanda–Hamner experiment, diapause incidence peaked at scotophase of 12 h and dropped rapidly at scotophases > 24 h. In the Bünsow experiment, diapause incidence was clearly suppressed, especially at the light pulse located 8 h in the scotophase. Both the Nanda–Hamner and Bünsow experiments showed no rhythmic fluctuations with a period of about 24 h; thus the photoperiodic clock in C. suppressalis is a non‐oscillatory hourglass timer or a rapidly damping circadian oscillator.

Effect of host plants on diapause induction in immature and adult Tetranychus kanzawai (Acari: Tetranychidae)

I determined the difference in sensitivity of immature and adult mites to host plants with regard to diapause induction. I used immature and adult polyphagous spider mites, Tetranychus kanzawai Kishida (Acari: Tetranychidae), and two host plants, Orixa japonica Thunb. (Rutaceae) and Phaseolus vulgaris L. (Fabaceae). The diapause incidence was investigated in two adult mite-host and two immature mite-host combinations of treatments. The incidence of diapause induction was much higher in mites developing on O. japonica than those on P. vulgaris, whichever host plants they fed on in adulthood. The host plants affected adult mites to a much lesser extent than the immature mites, though the mites feeding on O. japonica showed a consistently higher diapause incidence. These results suggest that host plants affect diapause induction to a greater extent in immature mites than in adult mites.

Genetic and physiological variation among sexual and parthenogenetic populations of Asobara japonica (Hymenoptera: Braconidae), a larval parasitoid of drosophilid flies

We studied variations in genetic, physiological, and ecological traits, and the phylogenetic relationship among sexual and parthenogenetic populations of Asobara japonica, a larval parasitoid of drosophilid flies, in order to understand how they adapt to local environments and have differentiated. The strain from Iriomote-jima (IR) differed from other Japanese strains in the nucleotide sequences of its cytochrome oxidase subunit I (COI) and in not undergoing diapause and having a shorter preimaginal period and a higher adult tolerance of cold. The strains other than IR showed a low level of nucleotide variation in COI but varied in their mode of reproduction; the strains from the Ryukyu Islands were sexual, whereas those from the main islands of Japan and Ogasawara were parthenogenetic. In addition, strains from higher latitudes generally showed a high incidence of diapause, although there were some exceptions. On the other hand, preimaginal period and adult cold tolerance varied little among the strains excluding IR, and pupal cold tolerance, oviposition preference and incidence of parasitism varied little among the strains including IR. Evolution and environmental adaptations in this species are discussed, particularly focusing on parthenogenetic populations.

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.

Geographic variation in diapause induction under constant and changing conditions in Helicoverpa armigera

AbstractVariation in the incidence of diapause in local populations of Helicoverpa armigera (Hübner) and Helicoverpa assulta (Guenée) (Lepidoptera: Noctuidae) was examined in relation to changes in photoperiod and/or temperature during the larval period. Temperate zone populations of H. assulta, a native species in temperate Japan, showed a high incidence of diapause induction when only the photoperiod was decreased during the larval period, even at favorable temperatures. This photoperiod‐dependent response may allow H. assulta to foresee the beginning of autumn well in advance in the temperate zone, where temperature conditions are severe. In contrast, temperate zone populations of H. armigera, an invasive and polyphagous species mainly distributed in the subtropics, showed a high incidence of diapause only when both photoperiod and temperature decreased, whereas subtropical populations showed a very low incidence of diapause under the same conditions. Furthermore, both temperate zone and subtropical populations of H. armigera enter diapause under constant low temperatures at short‐day photoperiod. Thus, there is geographic variation in sensitivity to diapause‐inducing stimuli (changes in photoperiod and temperature) in H. armigera. This variation may be a part of the climatic adaptation achieved by H. armigera in Japan.

Geographic variation in photoperiodic response for the induction of pupal diapause in the Aristolochia-feeding butterfly Atrophaneura alcinous

The photoperiodic response curve for pupal diapause induction of the butterfly Atrophaneura alcinous was determined in several temperate and subtropical populations of the Japan Archipelago at 20°C, using Aristolocia debilis leaves as larval food. Altitude and host plant use also differed among them. For the temperate populations of Kiire, Kashihara, Fuchu, Yokosuka and Yamagata, the critical photoperiod increased with latitude, and the subtropical Ishigaki population showed the shortest critical photoperiod. Further, a change in the shape of photoperiodic response curve, but no shift in the critical photoperiod, was found in a mountain population of Kiire, giving a high incidence of diapause in long photoperiods. These results are discussed in relation to latitude, altitude, and host plant.

Selection for high diapause incidence in blow flies ( Calliphoravicina) 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 °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.

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 τ 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.

Variation in the life history pattern of Tetranychus urticae (Acari: Tetranychidae) after selection for dispersal.

The spider mite Tetranychus urticae shows variation in its dispersal capacity (i.e., the leaf quality at which a female decides to disperse). We were able to artificially select mites that had either a high or a low dispersal capacity, indicating that this trait was genetically controlled. We then compared correlated responses to this selection. Mites with a genetically high dispersal capacity ('HD' strains) had a higher diapause incidence and a lower performance compared to mites with a low dispersal capacity ('LD' strains). A possible effect of random genetic drift during the selection was negligible. Our results suggest that differential dispersal capacity is associated with contrasting life history patterns as a result of natural selection.