Abstract: Latitudinal clines are widespread in Drosophila melanogaster, and many have been interpreted as adaptive responses to climatic variation. However, the selective mechanisms generating many such patterns remain unresolved, and there is relatively little information regarding how basic life-history components such as fecundity, life span and mortality rates vary across environmental gradients. Here, it is shown that four life-history traits vary predictably with geographic origin of populations sampled along the latitudinal gradient in the eastern United States. Although such patterns are indicative of selection, they cannot distinguish between the direct action of selection on the traits in question or indirect selection by means of underlying genetic correlations. When independent suites of traits covary with geography, it is therefore critical to separate the widespread effects of population source from variation specifically for the traits under investigation. One trait that is associated with variation in life histories and also varies with latitude is the propensity to express reproductive diapause; diapause expression has been hypothesized as a mechanism by which D. melanogaster adults overwinter, and as such may be subject to strong selection in temperate habitats. In this study, recently derived isofemale lines were used to assess the relative contributions of population source and diapause genotype in generating the observed variance for life histories. It is shown that although life span, fecundity and mortality rates varied predictably with geography, diapause genotype explained the majority of the variance for these traits in the sampled populations. Both heat and cold shock resistance were also observed to vary predictably with latitude for the sampled populations. Cold shock tolerance varied between diapause genotypes and the magnitude of this difference varied with geography, whereas heat shock tolerance was affected solely by geographic origin of the populations. These data suggest that a subset of life-history parameters is significantly influenced by the genetic variance for diapause expression in natural populations, and that the observed variance for longevity and fecundity profiles may reflect indirect action of selection on diapause and other correlated traits.

Abstract: This review highlights a number of reasons why insect diapause merits attention. Knowledge of diapause is essential for understanding the seasonal biology of an insect species, and such information is also required for the development of effective pest management strategies; manipulating domesticated species used in pollination and silk production; developing accurate predictive models used to forecast periods of pest abundance; and increasing the shelf-life of parasitoids and predatory mites used in the biological control industry. Mechanisms used by diapausing insects to survive low temperature may provide tips for cryopreserving insect stocks, a vital need within the research community. Diapause also presents an interesting model for probing fundamental questions in development, and we are indebted to diapause studies for early insights into insect hormones. In addition, I argue that insect diapause may provide insights into questions on aging, obesity and disease transmission, and diapausing insects offer a potentially rich source of pharmaceutical agents that may contribute to improvement of human health.

Abstract: Johnson, C. L., Leising, A. W., Runge, J. A., Head, E. J. H., Pepin, P., Plourde, S., and Durbin, E. G. 2008. Characteristics of Calanus finmarchicus dormancy patterns in the Northwest Atlantic. - ICES Journal of Marine Science, 65: 339-350.Demographic time-series from four fixed stations in the Northwest Atlantic Ocean demonstrate variable timing of entry into and emergence from dormancy in subpopulations of the planktonic copepod Calanus finmarchicus. A proxy for timing of entry was established as the date each year when the proportion of the fifth copepodid stage (CV) in the subpopulation rose to half its overall climatological maximum CV proportion at that station. The proxy for timing of emergence at each station was set as the first date when adults were more than 10% of the total abundance of copepodid stages. An alternate emergence proxy date was determined by back-calculating the spawning dates of the first early copepodid stages appearing in spring, using a stage-structured, individual-based model. No single environmental cue (photoperiod, surface temperature, or average surface-layer chlorophyll a concentration) consistently explained entry or emergence dates across all stations. Among hypotheses put forward to explain dormancy in Calanus species, we cannot eliminate the lipid accumulation window hypothesis for onset of dormancy or a lipid-modulated endogenous timer controlling dormancy duration. The fundamental premise of these hypotheses is that individuals can only enter dormancy if their food and temperature history allows them to accumulate sufficient lipid to endure overwintering, moult, and undergo early stages of gonad maturation.

Abstract: To survive long periods of low food availability, some calanoid copepods have a life his- tory that includes a diapause phase during which copepodids delay development to adulthood, migrate to depth, reduce metabolism, and utilize stored lipids for nourishment. While seasonal pat- terns in diapause have been described, the environmental and physiological regulation of diapause has not been elucidated. We collected Calanus finmarchicus C5 copepodids from surface (0 to 39 m) and deep (157 to 201 m) waters in the Gulf of Maine, and both morphological and biochemical mea- surements indicated that these copepodids were from active and diapausing populations, respec- tively. Two complementary molecular techniques were used to compare gene expression in these 2 groups: (1) suppressive subtractive hybridization (SSH) was used to identify genes that may be differ- entially expressed, and (2) quantitative real-time RT-PCR was used to characterize patterns of gene expression in individual copepodids. Three genes associated with lipid synthesis, transport and storage (ELOV, FABP, RDH) were upregulated (more highly expressed) in active copepods, particu- larly those with small oil sacs. Expression of ferritin was greater in diapausing copepods with large oil sacs, consistent with a role of ferritin in chelating metals to protect cells from oxidative stress and/or delay development. Ecdysteroid receptor (EcR) expression was greater in diapausing cope- pods, highlighting the need for further investigation into endocrine regulation of copepod develop- ment. This study represents the first molecular characterization of gene expression associated with calanoid copepod diapause and provides a foundation for future investigations of the underlying mechanisms that regulate diapause.

Abstract: Life history, aggregation and dormancy of rubber plantation litter beetle Luprops tristis Fabricius, (Tenebrionidae: Coleoptera) is described from rubber plantation belts in the western slopes of Western Ghats from the south Indian state of Kerala. The life cycle lasted 12 months, including the 5 larval instars lasting 1 month, the 3 day pupal stage, and the adult stage that can last 11 months. The adult stage includes an inactive dormancy phase of 9 months in shelters and 1 month each of active pre-dormancy (feeding) and post-dormancy (feeding and reproduction) phases that occur in rubber plantation litter. Reproductive activities are confined to the post-dormancy phase. With the onset of summer rains, huge aggregations of adults invade residential buildings and enter into a state of dormancy for 9 months. Beetle aggregations were in the range of 0.5 million to 4.5 million individuals per residential building. Dormancy in L. tristis is best classified as oligopause, which is intermediate between quiescence and diapause. Adults and larvae feed preferentially on wilted rubber tree leaves. Age-specific variation in mortality during dormancy is distinct with higher survivability for adults that have a longer pre-dormancy period. Generations are non-overlapping.

Abstract: Using host plant quality as a diapause-inducing stimulus may be adaptive for herbiv- orous insects that use host plants whose phenology is unpredictable based on photoperiod or temperature. We examined the effects of leaf toughness of the woody vine Aristolochia kaempferi Willd. on larval performance and pupal diapause induction of the swallowtail buttersy Byasa alcinous Klug (Lepidoptera: Papilionidae). We also conducted Þeld observations to assess whether human disturbance affects leaf phenology of A. kaempferi. Laboratory experiments showed that tough leaves decreased the survival rate of young larvae and lengthened the larval period, although pupal weight was not affected by leaf toughness. The incidence of pupal diapause was greater with tough leaves than with soft leaves and increased with larval duration. In the Þeld, leaf age composition in A. kaempferi was greatly insuenced by human mowing, which led to changes in leaf quality. These results suggest that B. alcinous has a plastic diapause strategy using host plant quality as a diapause-inducing stimulus, which may have evolved in frequently disturbed environments.

Abstract: 1. Seasonality is a prime selective factor expected to result in local adaptation of life cycles and dormancy. Genetic differentiation in diapause response was investigated along a European latitudinal cline in the dung fly Scathophaga stercoraria (Diptera: Scathophagidae). Such differentiation may be mediated by additive or dominance genetic and/or maternal effects, which need to be distinguished. 2. Replicate sibships from five European populations (Lugano, Switzerland: 46.00°N; Zurich, Switzerland: 47.37°N; Oxford, U.K.: 51.75°N; Lund, Sweden: 55.70°N; Reykjavik, Iceland: 64.15°N) were raised in a common laboratory environment known to induce pupal winter diapause (12 °C and 12 h light), revealing a genetic latitudinal cline in both the proportion of individuals entering diapause and diapause duration in response to winter length estimated from weather data. 3. Populations from the extremes of the cline (Lugano and Reykjavik) were further reciprocally crossed to investigate the underlying genetics. This experiment revealed evidence for diapause induction at 12 °C being dominant (i.e. not merely additive) and clearly rejected maternal effects as the primary source of this between-population variation.

Abstract: The effects of day-length and temperature on pre-adult growth and induction of reproductive diapause are studied in Orius sauteri and Orius minutus (Heteroptera: Anthocoridae) from northern (43.0°N, 141.4°E) and central (36.1°N, 140.1°E) Japan. In the north, at 20 °C, pre-adult growth is slower under an LD 14 : 10 h photoperiod than under shorter or longer photophases. At 24 and 28 °C, the longer photophases result in shorter pre-adult periods. Acceleration of nym- phal growth by short days in autumn appears to be adaptive. In the central region, this response is less pronounced, suggesting that timing of adult emergence is less critical than in the north. Day length also influences the thermal requirements for pre-adult development. The slope of the regression line representing temperature dependence of pre-adult development is significantly smaller and the lower devel- opment threshold (LDT) is significantly lower under an LD 12 : 12 h photoperiod than under long-day conditions. The weaker dependence of nymphal growth on temperature and the lower LDT in autumn might be adaptive. In the north, increased temperature shifts the critical day length of diapause induction and suppresses the photoperiodic response in O. sauteri but not in O. minutus . Further south, the inci- dence of diapause in both species is low even under short-day conditions but the same interspecific difference is observed (i.e. increase of temperature affects the response in O. sauteri but not in O. minutus ). This suggests seasonally earlier dia- pause induction with weaker temperature dependence in O. minutus than in O. sauteri .

Abstract: The white grub Dasylepida ishigakiensis has a 2-year life cycle and spends approximately 9 months as a nonfeeding larva, pupa and adult on a subtropi- cal island. Evidence is presented indicating that this beetle has two diapauses that appear to synchronize this long life cycle with the seasons. Larvae exposed to 20, 22.5, 25 and 27.5 °C late in the third (last) stadium pupate rapidly except for some individuals kept at the highest temperature. The latter pupate upon transfer to 22.5 °C, indicating that larval diapause is maintained at high temperature but terminates upon transfer to a lower temperature. Pupal development is directly temperature-dependent in the range 20 - 30 °C. Adults develop reproductive organs (i.e. the ovary in females and the seminal vesicles and accessory glands in males) rapidly at 15 and 20 °C, whereas those kept at 25 °C take a long time to do so. Ovarian development is completely suppressed at 30 °C but initiated upon transfer to 20 °C. In the laboratory, males with well-developed reproductive organs mate even with sexually immature females , whereas females with undeveloped ovaries show no sexual behaviour. Although the two diapauses of this species are thermally regulated (i.e. a characteristic commonly expressed by insects in summer diapause), adults of this beetle emerge from pupae late in the autumn and remain in the soil for 2 months. Adult diapause effectively serves to synchronize the time of sexual matu- ration with the coldest month of the year.

Abstract: Developing larvae of the apple maggot Rhagoletis pomonella are frequently exposed to summertime apple temperatures that exceed 40 °C and, during their overwintering diapause, pupae are exposed to sub-zero soil temperatures for prolonged periods To investigate the potential involvement of heat shock proteins (Hsps) in response to these environmental extremes, the genes encoding Hsp70 and Hsp90 in R pomonella are cloned and expression monitored during larval feeding within the apple and during overwintering pupal diapause Larvae reared in the laboratory at constant temperatures of 25, 28 or 35 °C express Hsp90 but very little Hsp70 Larvae do not survive rearing at 40 °C The temperature cycles to which larvae were exposed inside apples in the field, ranging 16–469 °C over a 24-h period, elicit strong Hsp70 and Hsp90 expression, which begins at mid-day and reaches a peak in late afternoon, coinciding with peak air and apple temperatures Heat shock proteins are also expressed strongly by pupae during their overwintering diapause Hsp70 is not expressed in nondiapausing pupae but is highly expressed throughout diapause Hsp90 is constitutively expressed in both diapausing and nondiapausing pupae Rhagoletis pomonella thus strongly expresses its Hsps during pupal diapause, presumably as a protection against low temperature injury, and during larval development to cope with natural temperature cycles prevailing in late summer

Abstract: To investigate geographic adaptation of the migratory locust Locusta migratoria in China, locusts were collected from six localities, ranging from 47.4°N to 19.2°N. Using offspring from the various populations, we compared embryonic diapause, reproductive traits, cold-hardiness and adult body size. The incidence of embryonic diapause was influenced by the genetic makeup, parental photoperiod, and incubation temperature of the eggs. The northern strain (47.4°N) produced diapause eggs under all photoperiodic conditions, whereas the other strains produced a higher proportion of diapause eggs when exposed to a short photoperiod. The incubation temperature greatly influenced diapause induction. At a low temperature, all eggs entered diapause, even some of those from a tropical strain (19.2°N) in which no diapause was induced at high temperatures. Photoperiodic changes during the parental generation affected the incidence of embryonic diapause. Diapause intensity decreased with decreasing original latitude. Cold hardiness was compared by exposing eggs in diapause to either -10 or -20°C for various periods; the northern strain was more cold-hardy than the southern strain, although some eggs in the tropical strain were probably not in a state of diapause. Adult body size and head width showed a complicated pattern of variation along the latitudinal gradient, whereas egg pod size (egg pod width and egg number) and hatchling weight tended to decrease with decreasing latitude. These results reveal that L. migratoria has adapted to local environments and that the latitudinal gradient appears to play an important role in shaping L. migratoria life cycle and development.

Abstract: Photoperiod is a major factor in the induction of pupal diapause in the ovoviviparaous flesh fly, Sarcophaga similis. This species entered diapause when kept under short-day conditions throughout the embryonic and larval stages. Exposure to long-day conditions for the whole embryonic or whole larval stage, however, prevented diapause. Moreover, most insects did not enter diapause when they were exposed to long-day conditions for only 2 days in the embryonic stage just before larviposition. When the uterus containing embryos was removed from the mother's body and kept in vitro under long-day conditions, most of these embryos became nondiapause pupae even under subsequent short-day conditions. When postfeeding larvae were exposed to long-day conditions for 3 days, 50% entered diapause. From these results, it is concluded that sensitivity to photoperiod is highest in the later embryonic stage and embryonic sensitivity to photoperiod is independent of the mother.

Abstract: The morphology and functions of the brain neurons projecting to the retrocerebral complex were examined in terms of photoperiodic control of adult diapause in the bean bug, Riptortus pedestris. Backfills through the nervi corporis cardiaci stained 15–20 pairs of somata in the pars intercerebralis (PI) with contralateral axons, and 14–24 pairs in the pars lateralis (PL) with ipsilateral axons to the nervi corporis cardiaci. In the PL, two clusters of somata, PL-d and PL-v, were found. Forwardfills showed neurons in the PI terminated in the aorta, and those in the PL at the corpus cardiacum, corpus allatum, and aorta. Removal of the PI did not cause effects on diapause incidence both under short-day (12 h:12 h, light:dark) and long-day conditions (16 h:8 h, light:dark) at 25°C. Under short-day conditions, diapause incidence was significantly lower than the controls after removal of the PL. Either removal of PL-d or PL-v did not reduce diapause incidence. It decreased only when both the PL-d and PL-v were ablated. The PI is not indispensable for diapause in R. pedestris, and both PL-d and PL-v neurons are suggested to be involved in photoperiodic inhibition of ovarian development.

Abstract: The cabbage beetle, Colaphellus bowringi Baly (Coleoptera: Chrysomelidae), which is widely distributed in China, undergoes an imaginal diapause in the soil. In this study, we investigated the influences of parental geographic origin, host plant, and rearing density on the diapause incidence of progeny in this beetle. In studies conducted at 25 °C and L12:D12 photoperiod, the proportions of adults entering diapause were significantly different among latitudes from which the parents had been collected. The incidence of diapause increased with increasing latitude. Reciprocal cross tests between post-diapause adults from different latitudes showed that there were significant differences in diapause incidence between pure-bred and hybrid adults, suggesting that diapause incidence among progeny was determined by both the female and male, although the female appeared to have a stronger effect than the male. These results revealed that parental origin has an important influence on progeny diapause. Parental host plant species not only affected diapause incidence of the current generation, but also affected the incidence of diapause in the progeny produced by the non-diapause parents; yet, the parental host plant had no influence on the incidence of diapause in the progeny produced by the post-diapause parents. Rearing density of the parental generation had no significant effect on the incidence of diapause in the progeny.

Abstract: Different kinds of molecules were identified as antifreezing agents in the body fluids of cold tolerant invertebrates: sugars, polyols and proteins. While none of the active arthropods were so far reported to accumulate polyols, these compounds are present in the haemolymph of species that hibernate in a passive stage such as diapause. In this work we investigated insect species that are active during winter and we demonstrated the ability of the mecopteran Boreus hiemalis (Mecoptera, Boreidae), the wingless fly Chionea sp. (Diptera, Limoniidae) and cantharid larvae (Coleoptera, Cantharidae) to accumulate sugars in their haemolymph to survive during winter. We report, for the first time, that for snow-active insects, trehalose comprises an important haemolymph component, its concentration changing as a function of the season, suggesting that the same adaptive strategies against cold conditions have evolved both in winter active and winter diapausing insects.

Abstract: The successful use of the predatory bug Orius spp. (Hemiptera:Anthocoridae) for the biological control of pests, namely western flower thrips Frankliniella occidentalis (Pergande), in greenhouses during winter depends on overcoming the obstacles of short photoperiods and low temperatures which limit the efficacy of the predators through diapause induction, slowed development, reduced survival and reproduction. Thus, research has focused on determining the insect species least vulnerable to these problems. This study investigated the effect of varying day-lengths (9, 11, 13 and 16 h) at 26°C as well as constant (18, 22, 26 and 30°C) and fluctuating (10/22°C) temperatures on biological characteristics of Orius niger (Wolff). Photoperiod did not induce reproductive diapause and did not significantly affect development, survival of eggs and nymphs, longevity, fecundity or sex ratio; with the exception of the 16 h day-length which led to a shorter duration of nymphal development. Fluctuating temperatu...

Abstract: Summary 1. Factors explaining the end of diapause include environmental conditions such as daily photoperiod, temperature and humidity. However, because all these factors are almost constant inside deep caves, they would hardly affect diapause termination in many animal taxa that use such habitats in which to aestivate or hibernate (such as bats and some insects, snakes or frogs). 2. An innate biological signal, based on body reserves (mass), could determine the end of diapause in cave-dwelling animals. Another possibility is that the diapausing animals may use as a stimulus convective air circulation, produced by a fall in temperature outside the cave to a value below that inside (i.e. temperature inversions). Here, we explored these two non-exclusive hypotheses explaining the end of diapause and the start of migration in the caddis Mesophylax aspersus. This species aestivates as an adult in caves, as a physiological adaptation to seasonal drought and stream drying. 3. The variation in body mass of individuals entering and leaving the cave was similar, suggesting no role for the ‘reserve level’ hypothesis in breaking diapause. However, the onset of a temperature inversion in autumn coincided with the breaking of diapause followed after a few days by the migration of the caddisflies. 4. We conclude that the seasonal air currents, produced by thermal inversions, could be the migration stimulus for caddisflies and many other animals that spend a period of diapause in caves.

Abstract: It is known that the low temperature is the most important factor inducing the pre-pupal diapause in Trichogramma species. The position of the thermosensitive period over the life cycle and temporal variation of the degree of responsiveness were investigated in T. embryophagum Htg. by transferring pre-imaginal stages between ‘neutral’ temperature of 15� C and ‘diapause-inducing’ temperature of 10� C. Our experiments showed that 6 days long exposure at 10� C significantly increased the percentage of diapausing pre-pupae when started during rather large part of development: from embryo up to early pre-pupa. The highest thermosensitivity was recorded during the embryo and the larval stages, with some decrease during the hatching period. Treatments with shorter cold exposures (2–3 days) gave similar results. Even 24 h long exposure at 10� C increased the percentage of diapausing pre-pupae when applied during egg or early larval stage. Being started at the same stage of development, longer cold exposures caused stronger increase in the percentage of diapausing individuals. The experiments did not reveal any significant daily changes in thermosensitivity: at 12 : 12 h light : dark, larvae subjected to the low temperature during six photophases showed practically the same percentage of diapausing individuals as those subjected to the low temperature during six scotophases, and as those subjected to the 3 days long uninterrupted cold exposure. Hence, in natural conditions even occasional short-term cold periods could be accumulated. J. Appl. Entomol.

Abstract: Cold and desiccation tolerance was investigated in the eggs of the band-legged ground cricket Dianemobius nigrofasciatus in relation to embryonic diapause. Diapause eggs were more tolerant to both desiccation and cold than non-diapause eggs. In addition, diapause-destined eggs on day zero (0–12 h after being laid) already showed high tolerance to these stresses before entering diapause. This clearly indicates that stress tolerance, like diapause, is controlled by photoperiod, but is not directly associated with diapause itself. Because the acquisition of stress tolerance predates the onset of diapause, it is plausible that diapause programming during some period before the onset of diapause is involved in the acquisition of stress tolerance. Weights and sizes were nearly identical in short-day and long-day eggs until day five. Sorbitol, a major sugar alcohol in eggs of D. nigrofasciatus, was accumulated at the same level in short-day and long-day eggs on days zero and five. These results indicate that the surface-to-volume ratio as well as the accumulation of sugar alcohol is not involved in the acquisition of stress tolerance. Maternal factors are clearly involved in the acquisition of stress tolerance in D. nigrofasciatus eggs, but the physiological mechanisms underlying the tolerance are still unclear.

Abstract: The expression of metabolic enzyme genes and heat-shock protein genes (Hsp) during early embryogenesis in diapause and non-diapause eggs of the silkworm Bombyx mori was quantified by semi-quantitative RT-PCR. The trehalase gene (Tre) was expressed in non-diapause eggs up-to nine days, while in diapause eggs was not up regulated. The glycogen phosphorylase gene (GPase) was expressed in non-diapause eggs, whereas in diapause eggs a high level was observed in early stage, but down regulated in later stage. The phosphofructokinase gene (PFK) and sorbitol dehyrogenase-2 gene (SDH-2) expression was fluctuated in non-diapause eggs, whereas in diapause eggs these were expressed only at early stage and not observed in later stage. The glucose-6-phosphate dehydrogenase gene (G6P-DH) in non-diapause eggs was highly expressed during the differentiation phase and decreased in the organogenesis phase. In contrast to this, expression in diapause eggs was of low level during differentiation phase and of high level observed in the organogenesis phase. In the tissues, PFK and SDH-2 were selectively expressed in cuticle and midgut, whereas Tre expression was high in midgut and ovary of larvae incubated at 15°C. The Hsp (20.4, 20.8, 40, 70, and 90) were expressed in both diapause and non-diapause eggs. Their expression was, however, selective in tissues with Hsp20.4 in midgut and ovary, Hsp40 in head, Hsp70 in cuticle and Hsp90 in ovary and head in high amounts at 15°C. These results suggest that the metabolic enzyme genes studied except Hsp play a major role during embryogenesis of diapause and non-diapause silkworm.

Abstract: The genetic control of the photoperiodic larval diapause of Chymomyza costata was studied by hybridization, backcross and selection tests. The strains used in the experiment, normal photoperiodic-diapause strains from Finland (VK) and Japan (SP) and a mutant non-photoperiodic-diapause strain (NP), which was selected from the Japanese population, differed from each other with respect to critical daylength and critical temperature measured at long-day or short-day conditions. The loss of photoperiodic response in NP was shown to be associated with the lowering of the short-day critical temperature to the level of the long-day critical temperature. A recessive allele at a single autosomal locus is responsible for the trait. The differences between VK and SP, or VK and NP, in the critical daylength are due to at least two loci, one on the X chromosome and one on an autosome. The critical daylengths of F1 females obtained from reciprocal crosses between SP and VK were intermediate between those of the parental strains. SP and VK responded quantitatively to photoperiod in diapause induction; a shorter photoperiod was more effective in inducing diapause than a longer photoperiod.

Abstract: The Indian meal moth Plodia interpunctella Hubner (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.