Critical Photoperiod For Diapause Induction Research Articles

Diapause Induction, Color Changes, and Supercooling Point of Diapause Larvae of Tetrastichus septentrionalis Yang (Hymenoptera: Eulophidae).

The chalcid wasp Tetrastichus septentrionalis Yang (Hymenoptera: Eulophidae) is one of the dominant pupal parasitoids of Hyphantria cunea (Drury) (Lepidoptera: Erebidae). In this study, the photoperiod's effect on diapause induction in T. septentrionalis using the alternative host Tenebrio molitor was measured, revealing that T. septentrionalis is of the long-day type. The critical photoperiods for diapause induction in T. septentrionalis were estimated to be between photoperiods of 13:11 and 14:10 (L:D) h at 18 °C, and between photoperiods of 12:12 and 13:11 (L:D) h at 21 °C and 24 °C. We also found that T. septentrionalis diapausing larvae were grey-brown, while normally developed (non-diapausing) individuals were light yellow. The diapause-sensitive insect state was the larval stage, and the short light exposure treatment had a significant cumulative effect on diapause induction. The least squares method was used to calculate a lower developmental threshold of 13.34 ± 0.50 °C and an effective cumulative temperature of 184.46 ± 11.46 d·°C for post-diapause development. The average supercooling point of diapausing mature larvae was significantly lower than that of non-diapausing ones. Our research on T. septentrionalis provides a reference for the biological control of H. cunea and other leaf-eating pests.

Effects of photoperiod and temperature on the development and diapause of the endangered butterfly Zizina emelina (Lepidoptera: Lycaenidae)

We investigated the effects of photoperiod and temperature on the development of the endangered butterfly Zizina emelina under laboratory conditions, and its seasonal occurrence in Osaka Prefecture, central Japan in 2009. At 20 °C most individuals reared under conditions of <13 h of light/day entered diapause during the larval stage, whereas those reared under ≥14 h of light/day developed into adults without any developmental arrest. At 25 °C, none of the individuals entered larval diapause irrespective of the photoperiod. The critical photoperiod for diapause induction at 20 °C was 12 h 40 min. Almost all larvae that had been in diapause for 2 months (with or without chilling at 10 °C) developed into adults, and the mean pupation period after transfer to 25 °C under 16 h of light/day differed little between the two groups, suggesting that chilling is not necessary to terminate larval diapause in Z. emelina. Adults were continuously found from late April to late October, with a low population density during spring. We estimate that this species has 5–6 generations in Osaka, according to the developmental threshold, effective accumulated temperature, and critical photoperiod data obtained from the rearing experiment. The estimated seasonal life cycle was similar to the seasonal prevalence observed in the field.

Stable and variable life‐history responses to temperature and photoperiod in the beet webworm, Loxostege sticticalis

AbstractThe beet webworm, Loxostege sticticalis L. (Lepidoptera: Crambidae), unlike many temperate insects and despite its wide distribution range, has a geographically stable value of the critical photoperiod for diapause induction. It has thus been hypothesized that the species could adjust its life cycle to different climates in an alternative way, which should be reflected in geographical variation and/or environmental plasticity of some other ecophysiological trait. Three remote populations of the beet webworm were studied. The insects were reared from egg to adult at several combinations of temperature and photoperiod in order to measure development times of all the immature stages and pre‐pupal body weight, and to characterize the sensitivity of these life‐history traits to the two ecological factors. The thermal reaction norms for immature development appeared to be significantly different in the three populations. There was also a significant effect of photoperiod on development time as well as on the thermal sensitivity and lower temperature threshold for larval development. Pre‐pupae from the northernmost population were heavier and their body mass was more strongly affected by photoperiod than in the other two, but attainment of a greater weight under short‐day conditions, especially combined with higher temperatures, was common for all the three populations. Nevertheless, all the discovered geographical and environmentally induced differences in life‐history traits were very small and their adaptive significance remains problematic.

Phenological responses of Pseudomallada (Neuroptera: Chrysopidae): Comparative data from three Nearctic species and interspecific hybrids

Three Nearctic species - Pseudomallada macleodi (Adams & Garland), P. perfectus (Banks), and P. sierra (Banks) - share a suite of phenological characteristics with their Western Palearctic congeners. They overwinter as free-living larvae (primarily third instars) in a photoperiodically induced diapause. For these three species, the critical photoperiods for diapause induction fell between LD 16:8 and LD 14:10. In the laboratory, larvae remained responsive to daylength throughout diapause - short daylengths maintained diapause until diapause terminated spontaneously or death occurred. Transfer of diapausing larvae from short to intermediate or longer days accelerated diapause development, apparently in a quantitative response to daylength. A period of chilling appeared to delay, rather than hasten, diapause development. In general, the expression of diapause in F1 hybrids (P. perfectus × P. sierra) resembled that of the parents; there were minor differences in some features. Among populations of the three species under non-diapause conditions, the times for complete development (egg hatch to adult emergence) ranged between 40 and 50 days (23.9°C). Under long-day condi- tions, developmental rates of the P. perfectus egg and prepupal-pupal stages were linearly related to temperature; we provide thermal thresholds and heat-degree days for these stages. In contrast, developmental rates of free-living larvae did not have a linear relationship with temperature; photoperiod may interact with temperature to affect non-diapause development in these stages.

Diapause induction, color change, and cold tolerance physiology of the diapausing larvae of the Chouioia cunea (Hymenoptera: Eulophidae).

The chalcid wasp Chouioia cunea Yang (Hymenoptera: Eulophidae) is one of the most dominant pupal parasitoids of Hyphantria cunea (Drury) (Lepidoptera: Arctiidae), an invasive pest of many forestry trees and agricultural crops. For mass rearing C. cunea for biological control purposes, the pupae of Antheraea pernyi (Guérin-Méneville) (Lepidoptera: Saturniidae) have been widely used as a substitute host in China. In this article, photoperiodic effect on diapause induction in C. cunea within the pupae of A. pernyi was investigated, and the differences in cold tolerance physiology including supercooling point, water content, and activities of three protective enzymes (Peroxidase [POD], Catalase [CAT], and Superoxide dismuase [SOD]) between diapause and nondiapause mature larvae were comparatively determined. Our results revealed that C. cunea possess a short-day induced larval diapause. The critical photoperiods for diapause induction in C. cunea were estimated to be between a photoperiod of 13:11 and 14:10 (L:D) h at 18°C, or between a photoperiod of 12:12 and 13:11 (L:D) h at 21°C or 24°C. We also found that the color of C. cunea diapausing larvae was taupe, while the normally developed (nondiapausing) individuals were light yellow. This body color change can be used as an indicator of diapause entry of C. cunea larave. The average supercooling point of diapausing mature larvae were lower than those of nondiapausing ones. There were significant differences in the activity of three protective enzymes (POD, CAT, and SOD) between diapausing and nondiapausing mature larvae.

Geographic variation in diapause induction and termination of the cotton bollworm, Helicoverpa armigera Hübner (Lepidoptera: Noctuidae)

Overwintering diapause in Helicoverpa armigera, a multivoltine species, is controlled by response to photoperiod and temperature. Photoperiodic responses from 5 different geographical populations showed that the variation in critical photoperiod for diapause induction was positively related to the latitudinal origin of the populations at 20, 22 and 25°C. Diapause response to photoperiod and temperature was quite different between northern and southern populations, being highly sensitive to photoperiod in northern populations and temperature dependence in southern populations. Diapause pupae from southern population showed a significantly shorter diapause duration than from northern-most populations when they were cultured at 20, 22, 25, 28 and 31°C; by contrast, overwintering pupae from southern populations emerged significantly later than from northern populations when they were maintained in natural conditions, showing a clinal latitudinal variation in diapause termination. Diapause-inducing temperature had a significant effect on diapause duration, but with a significant difference between southern and northern populations. The higher rearing temperature of 22°C evoked a more intense diapause than did 20°C in northern populations; but a less intense diapause in southern population. Cold exposure (chilling) is not necessary to break the pupal diapause. The higher the temperature, the quicker the diapause terminated. Response of diapause termination to chilling showed that northern populations were more sensitive to chilling than southern population.All results demonstrate that H. armigera is not genetically homogeneous throughout its range, but rather is composed of distinct populations genetically adapted to local environmental conditions despite the potential for gene flow via seasonal migration of adults.

Adaptive latitudinal cline of photoperiodic diapause induction in the parasitoid Nasonia vitripennis in Europe

Living in seasonally changing environments requires adaptation to seasonal cycles. Many insects use the change in day length as a reliable cue for upcoming winter and respond to shortened photoperiod through diapause. In this study, we report the clinal variation in photoperiodic diapause induction in populations of the parasitoid wasp Nasonia vitripennis collected along a latitudinal gradient in Europe. In this species, diapause occurs in the larval stage and is maternally induced. Adult Nasonia females were exposed to different photoperiodic cycles and lifetime production of diapausing offspring was scored. Females switched to the production of diapausing offspring after exposure to a threshold number of photoperiodic cycles. A latitudinal cline was found in the proportion of diapausing offspring, the switch point for diapause induction measured as the maternal age at which the female starts to produce diapausing larvae, and the critical photoperiod for diapause induction. Populations at northern latitudes show an earlier switch point, higher proportions of diapausing individuals and longer critical photoperiods. Since the photoperiodic response was measured under the same laboratory conditions, the observed differences between populations most likely reflect genetic differences in sensitivity to photoperiodic cues, resulting from local adaptation to environmental cycles. The observed variability in diapause response combined with the availability of genomic tools for N. vitripennis represent a good opportunity to further investigate the genetic basis of this adaptive trait.

Diapause Induction in &lt;I&gt;Apolygus lucorum&lt;/I&gt; and &lt;I&gt;Adelphocoris suturalis&lt;/I&gt; (Hemiptera: Miridae) in Northern China

With widespread planting of Bt cotton and an associated reduction in the use of broad-spectrum insecticides, the mirid bugs Apolygus lucorum (Meyer-Dür) and Adelphocoris suturalis Jakovlev (Hemiptera: Miridae) become major pests of cotton in northern China in recent years. Both species overwinter as diapausing eggs that may enhance ability to survive the cold winter. The effects of photoperiod and temperature on diapause induction in A. lucorum and A. suturalis were investigated under laboratory conditions. Egg diapause was induced primarily by short photoperiod. Temperatures ranging between 17 and 26°C had little effect on diapause induction for both species. The impact of photoperiod (x) on diapause incidence (y) was described with modified Michaelis-Menten models y = 1.1(14 - x)/(14.82 - x) for A. lucorum and y = 1.07(14 - x)/(14.79 - x) for A. suturalis. The critical photoperiod for diapause induction in the first instars of parental A. lucorum and A. suturalis calculated from the models above was almost identical, at 13.3 h (13 h, 18 min) in a 24-h cycle. The parental nymphal stage was sensitive to short photoperiod for diapause induction in both species, with the first-instar nymphs the most sensitive. The sensitivity to short photoperiod decreased gradually as the A. suturalis nymphs developed, whereas the sensitivity dropped sharply at the second instar of A. lucorum. The adults of both species exhibited little sensitivity to photoperiod for diapause induction.

Geographic variation in photoperiodic diapause induction and diapause intensity in Sericinus montelus (Lepidoptera: Papilionidae)

Abstract Due to the risk of extinction and ornamental value of the swallowtail butterfly, Sericinus montelus Gray (Lepidoptera: Papilionidae) in China, knowledge about local adaptations is important for the conservation and economical utilization of the species. In the present study, photoperiodic diapause induction and diapause intensity of S. montelus populations from Jiamusi (46°37′N), Beijing (40°15′N), Zibo (36°48′N), Fangxian (32°36′N), Wuhan (30°33′N) and Huaihua (27°33′N) were characterized at 25°C. Logistic regression analysis revealed a significant population × hours of light interaction, confirming that photoperiodic responses varied among populations. The critical photoperiod was positively correlated with latitude and increased toward the north at a rate of about 1 h for each 6.67 degrees of latitude. Survival analyses indicated that survival time of diapausing pupae before adult eclosion differed significantly among populations at 25°C and 16 : 8 L : D h. The mean duration of pupal diapause was also positively correlated with latitude. Our study reveals geographic variation in the critical photoperiod for diapause induction and in diapause intensity of S. montelus. These results provide useful information for our general understanding about seasonal adaptation in insects and may also be used to predict how geographic populations respond to climate warming.

Effects of constant and changing temperature conditions on diapause induction inHelicoverpa armigera(Lepidoptera: Noctuidae)

The effects of photoperiod and temperature on the induction and termination of facultative pupal diapause in Helicoverpa armigera (Lepidoptera: Noctuidae) were investigated under laboratory conditions. Exposing H. armigera larvae to both constant and fluctuating temperature regimes with a mean of 25°C and 20°C resulted in a type-III photoperiodic response curve of a short-long day insect. The long-day critical daylengths for diapause induction were ten hours and 12 hours at the constant temperatures of 25°C and 20°C, respectively. Higher incidences of diapause and higher values both for the longer and the shorter critical photoperiods for diapause induction were observed at fluctuating regimes compared with the corresponding constant ones. At alternating temperatures, the incidence of diapause ranged from 4.2% to 33.3% and was determined by the temperature amplitude of the thermoperiod and by the interaction of cryophase or thermophase with the photoperiod. Helicoverpa armigera larvae seem to respond to photoperiodic stimuli at temperatures >15°C and <30°C; all insects entered diapause at a constant temperature of 15°C, whereas none did so at a constant temperature of 30°C under all the photoperiodic regimes examined. Although chilling was not a prerequisite for diapause termination, exposure of diapausing pupae to chilling conditions significantly accelerated diapause development and the time of adult emergence. Therefore, temperature may be the primary factor controlling the termination of diapause in H. armigera.

Photoperiodic response in the subtropical and warm‐temperate zone populations of the southern green stink bug Nezara viridula: why does it not fit the common latitudinal trend?

The photoperiodic response of diapause induction is studied in females of five subtropical and warm‐temperate zone populations of Nezara viridula (L.) (Heteroptera: Pentatomidae) in Japan (26.4–34.7°N; 127.4–135.7°E). Laboratory tests at 25 °C demonstrate that both warm‐temperate and subtropical populations have pronounced photoperiodic responses of adult diapause induction. Under short‐day conditions (LD 10 : 14 h), 73–100% of females enter diapause, whereas, under long‐day conditions (LD 15 : 9 h), 87–100% of females are nondiapause and reproduce. When the critical photoperiod for diapause induction is plotted against the latitude of origin of each population, the data points do not show the expected tendency of increasing critical photoperiod northwards but, instead, vary between 12 h 15 min and 13 h 30 min. It is suggested that adults from different populations of such a highly migratory species move often among subtropical islands (by themselves or being assisted by typhoons), thus constantly destabilizing the photoperiodic responses. Although important for general synchronization of seasonal development in the tropics and subtropics, winter diapause might not be so critical for survival in mild winters in these zones. Such circumstances might make the selective pressure over the photoperiodic response less severe in the tropics and subtropics than in the temperate zone. It is suggested that the current climate warming might contribute to this situation as well by promoting polewards migrations and lessening the selective pressure of overwintering conditions.

Rapid seasonal adaptation of an alien bruchid after introduction: geographic variation in life cycle synchronization and critical photoperiod for diapause induction

Whether alien insects that are introduced into temperate regions adapt to seasonally changing environmental conditions is an important question in evolutionary biology. If rapid evolution has occurred in a non-native environment, a latitudinal cline in critical photoperiod for diapause induction (i.e., the photoperiod at which half of the individuals enter diapause) and in life cycle synchronization with host plant phenology should be evident among locations. The alien bruchid Acanthoscelides pallidipennis (Motschulsky) (Coleoptera: Bruchidae) is native to North America and introduced into Japan with the host plant Amorpha fruticosa L. (Fabaceae) in the late 1940s. To examine whether seasonal adaptation has occurred in A. pallidipennis, we conducted a laboratory experiment and phenological observations using three latitudinally different populations. We bred F1 eggs at 22 °C and five photoperiodic regimens – L:D = 10:14, 13:11, 14:10, 15:9, or 16:8 hours – and examined whether diapause was induced. The estimated critical photoperiod for diapause induction was longest in the most northern population and shortest in the most southern population. Life cycle was found to be synchronized with host phenology in each location. Also voltinism varied geographically, from univoltine in the northern population to bivoltine in the southern populations. These results showed that A. pallidipennis rapidly adapted to seasonal environmental conditions in Japan after its introduction.

Life‐history traits related to diapause in univoltine and bivoltine populations of Ypthima multistriata (Lepidoptera: Satyridae) inhabiting similar latitudes

AbstractIn most temperate insects, diapause strategies and voltinism generally exhibit latitudinal clines, supporting the concept that they represent adaptations to climate. In contrast, in the satyrine butterfly Ypthima multistriata Butler, local populations with different voltinism patterns are geographically intermingled, suggesting that life‐history traits related to diapause may differ even between geographically and phylogenetically close populations. In this study, we experimentally examined the critical photoperiod for diapause induction and the larval developmental period in two univoltine and two bivoltine populations of Y. multistriata, all of which inhabit virtually the same latitude (34.652–34.750°N). We found that the critical photoperiod for diapause induction was longer in the univoltine populations than in the bivoltine populations. Moreover, the larval period under the long day length treatment was different among populations in both sexes, although significant differences were also detected between populations with the same voltinism. These results indicate that in Y. multistriata, life‐history traits related to diapause can not be attributed merely to climatic conditions such as temperature or day length, which depend largely on latitude. Therefore, we suggest that biotic elements, such as leaf toughness, as well as abiotic elements should be taken into account in attempts to explain the enigmatic pattern of geographic variation in the diapause strategies of Y. multistriata.

Insect overwintering in a changing climate

Insects are highly successful animals inhabiting marine, freshwater and terrestrial habitats from the equator to the poles. As a group, insects have limited ability to regulate their body temperature and have thus required a range of strategies to support life in thermally stressful environments, including behavioural avoidance through migration and seasonal changes in cold tolerance. With respect to overwintering strategies, insects have traditionally been divided into two main groups: freeze tolerant and freeze avoiding, although this simple classification is underpinned by a complex of interacting processes, i.e. synthesis of ice nucleating agents, cryoprotectants, antifreeze proteins and changes in membrane lipid composition. Also, in temperate and colder climates, the overwintering ability of many species is closely linked to the diapause state, which often increases cold tolerance ahead of temperature-induced seasonal acclimatisation. Importantly, even though most species can invoke one or both of these responses, the majority of insects die from the effects of cold rather than freezing. Most studies on the effects of a changing climate on insects have focused on processes that occur predominantly in summer (development, reproduction) and on changes in distributions rather than winter survival per se. For species that routinely experience cold stress, a general hypothesis would be that predicted temperature increases of 1 degree C to 5 degrees C over the next 50-100 years would increase winter survival in some climatic zones. However, this is unlikely to be a universal effect. Negative impacts may occur if climate warming leads to a reduction or loss of winter snow cover in polar and sub-polar areas, resulting in exposure to more severe air temperatures, increasing frequency of freeze-thaw cycles and risks of ice encasement. Likewise, whilst the dominant diapause-inducing cue (photoperiod) will be unaffected by global climate change, higher temperatures may modify normal rates of development, leading to a decoupling of synchrony between diapause-sensitive life-cycle stages and critical photoperiods for diapause induction. In terms of climate warming and potential heat stress, the most recent predictions of summer temperatures in Europe of 40 degrees C or higher in 50-75 years, are close to the current upper lethal limit of some insects. Long-term data sets on insect distributions and the timing of annual migrations provide strong evidence for 'positive' responses to higher winter temperatures over timescales of the past 20-50 years in North America, Europe and Asia.

Northerly shift in voltinism watershed in Hyphantria cunea (Drury) (Lepidoptera: Arctiidae) along the Japan Sea coast: Evidence of global warming?

The effects of climate change on the northern limit of the trivoltine area were clarified in the fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae). The Takaoka and Kanazawa populations used in the present study occur in locations to the north of Fukui (36.07°N), where H. cunea has recently shifted from a bivoltine to a predominantly trivoltine life cycle. The life-history traits of these populations were investigated and compared to those in the Fukui population. The lower threshold temperature for development and the thermal constant for one generation were 11.3°C and 674.5°d in the Takaoka population, and 11.2°C and 680.7°d in the Kanazawa population. The critical photoperiod for diapause induction at 25°C was 14 h 34 min in the Takaoka population and 14 h 28 min in the Kanazawa population. These critical photoperiods at 25°C were longer by 24 min and 18 min than the Fukui population, respectively. These results, together with climate data, suggest that the Takaoka population maintains a bivoltine life cycle, and a small part of the Kanazawa population has three generations per year, a predominantly bivoltine life cycle. Thus, the present northern limit of the trivoltine area lies around Fukui in districts along the Japan Sea.

Shifting of the life cycle and life‐history traits of the fall webworm in relation to climate change

AbstractThe effects of global warming on the life cycle and life‐history traits of the fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae), were investigated in Fukui, Japan. Our previous studies showed that the Fukui population had a predominantly bivoltine life cycle, but recently the life cycle has changed. In the present study, the life‐history traits of individuals collected in 2002 in Fukui were clarified and compared to those in the previous studies. The lower threshold temperature for development and the thermal constant for one generation were 10.6 °C and 724.4 degree days, respectively. Although these developmental parameters were not investigated in the previous studies, the difference of the developmental period between the present and previous results was negligible in the larval and pupal stages at 20 °C. The critical photoperiod for diapause induction was 14 h 29 min at 20 °C and 14 h 10 min at 25 °C. The critical photoperiod at 25 °C was shortened to 14 min from that of the previous studies for individuals collected in 1995. The incidence of pupal diapause in the second generation was investigated in individuals that were collected as fourth and fifth instars in the field. Some portion of individuals averted diapause even if they were kept under a short photoperiod of L14:D10 at 25 °C after collection. These results, together with climate data and field observations in Fukui, suggest that at least a part of the population has three generations per year at present. This shift of the life cycle occurred within 7 years and is probably related to global warming in recent years.

Seasonal adaptations of the fall webworm Hyphantria cunea (Drury) (Lepidoptera: Arctiidae) following its invasion of Japan

AbstractThe fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae), invaded Japan from North America about 60 years ago. Immediately after its invasion – and for the first three decades – its life cycle was bivoltine, two generations per year throughout its entire distribution in Japan. Thereafter, its life cycle shifted to trivoltine in the southwestern areas of Japan. In the present study we examined the life‐history traits proposed to be implicated in this event with the aim of clarifying the mechanism of this life‐cycle shift. The critical photoperiod for diapause induction, as defined by the photoperiod at which 50% of individuals enter diapause, was shorter in the trivoltine populations than in their bivoltine counterparts. The temperature sensitivity of the photoperiodic response, as defined by the difference in the critical photoperiod between 20 and 25°C, was greater in the trivoltine populations than in the bivoltine ones. The geographic variation in larval and pupal periods was positively correlated to the latitude of the original localities of the populations. The change in the number of larval instars would be one of the main factors accounting for the regional differences in the developmental period. These results suggest that some life‐history traits of H. cunea have changed following its invasion of Japan as an adaptive response to local climates.

Effects of photoperiod and temperature on preimaginal development and summer diapause of Chrysocharis pubicornis (Zetterstedt) (Hymenoptera: Eulophidae), a pupal parasitoid of leafminers (Diptera: Agromyzidae)

The preimaginal development of Chrysocharis pubicornis was studied at different temperatures and 14L:10D photoperiod. The lower developmental threshold was 7.4 and 7.2°C, and the thermal constant was 297.4 and 307.5 degree-days for males and females, respectively. The effects of combinations of different temperatures and photoperiods on larval diapause induction and termination were also examined. Larval development showed a short-day type response affected by temperature. The percentage of individuals entering diapause increased with the temperature and photoperiod. The critical photoperiod for diapause induction was between 12 and 14 h at 25°C, whereas at 15 h and 20°C the diapause rate did not reach 50% within the photoperiod range tested. Regardless of the photoperiod, diapause terminated at 15°C but was maintained at 25°C. At 20°C, 90% of individuals terminated diapause at 12 h photoperiod but only 16% at 16 h photoperiod. A significant effect of the photoperiod on the post-diapause emergence pattern of wasps was observed at 15°C. From these results, it is considered that summer diapause enables C. pubicornis to synchronize its occurrence with that of the garden pea leafminer, Chromatomyia horticola.

Photoperiodic Induction of Reproductive Diapause in the Predator Geocoris uliginosus (Hemiptera: Geocoridae)

Abstract The critical photoperiod for diapause induction in the predator Geocoris uliginosus (Say) and the stage/instar(s) sensitive to diapause-inducing photoperiods were determined in the laboratory. Six photoperiods were used for the critical photoperiod experiment (all at 24 ± 1°C): 10:14, 11:13, 12:12, 13:11, 14:10, and 15:9 (L:D) h. The critical photoperiod was ≈13.2 h of light d−1. At 12:12 and 11:13 h, 86–88% of the population entered diapause, whereas 20% or less entered diapause at longer daylength. To determine the stage/instar(s) sensitive to diapause-inducing photoperiods, cohorts of bugs were reciprocally switched between two photoperiods, 15:9 (not diapause-inducing) and 11:13 h (diapause-inducing), at the beginning of the second, third, fourth, and fifth instar, and at the beginning of the adult stage. A given cohort was switched only once during the experiment. Two additional cohorts were reared as controls under the respective photoperiods with no switching throughout the experiment. Exp...

Selection for discontinuous life‐history traits along a continuous thermal gradient in the butterflyAricia agestis

Abstract. 1. Voltinism may be conceptualised as the product of development rate and the timing of diapause – two components that together translate gradual environmental variation, through periods of growth and development, into ‘generational units’. This may result in very different selection pressures on diapause induction and development time in populations with different numbers of generations per year.2. Developmental data from univoltine and bivoltine populations of the butterflyAricia agestis(Lepidoptera: Lycaenidae) in North Wales were used to examine larval development time and the timing of diapause, and their contribution towards voltinism in populations that occur at the same latitude and in geographic proximity to one another along a thermal gradient.3. The critical photoperiod for diapause induction in univoltines and bivoltines from the same latitude differed by more than 1.5 h.4. Development time also differed significantly between these populations, in line with predictions that bivoltines would need to exhibit shorter development times in order to achieve two complete generations per year. Shorter development times for bivoltines result in lower pupal weights, suggesting a trade‐off exists between generation number and body size that may dictate the position of the transition zone between the two life‐history strategies. Analysis of development times in a third population, from southern England, with greater thermal availability than those from North Wales, further supports this hypothesised trade‐off.5. To achieve the conversion of a continuous thermal gradient into the binary biological response from univoltism to bivoltism, bivoltines speed up development, reduce adult body size and shift their diapause induction response.