The Effect of Temperature and Photoperiod on Diapause Induction in Pupae of Scrobipalpa ocellatella (Lepidoptera: Gelechiidae)

The Effect of Temperature and Photoperiod on Diapause Induction in Pupae of Scrobipalpa... Abstract Scrobipalpa ocellatella (Boyd) (Lepidoptera: Gelechiidae) is one of the most important pests of sugar beet that causes quantitative and qualitative yield loss in the late summer. To locate the position for diapause induction, combinations of constant temperatures at 15, 18, 20, and 25°C and day lengths of 8, 10, 12, 14, 16, and 24 h were studied from egg to adult emergence. The incidence of diapause peaked at 15 and 18°C, with the day lengths of 12 and 11 h, whereas low temperatures did not improve the effects of short photoperiods (day lengths of 8 and 10 h) in diapause induction. The results showed that the critical day length for diapause induction was 12.8 h at overall 15 and 18°C. It was observed that the third instar larvae were the most sensitive stage to the inductive photoperiod (12:12 [L:D] h). The non-24-h light-dark experiment showed that the nigh length is more important than the day lengths measurement. In a set of 24-h light-dark cycles at 2:12 (L:D) h, a 1-h light pulse declined diapause induction markedly 1 h after scotophase. Field monitoring of the S. ocellatella for 2 yr (2015 and 2016) showed that the 50% of larvae enter winter pupal diapause in early September and this proportion increases in response to a decrease in the day lengths and temperature. From this study, it was concluded that low temperature acts in conjunction with short-day photoperiod in diapause induction of S. ocellatella. photoperiod, temperature, Scrobipalpa ocellatella, diapause induction, sugar beet Scrobipalpa ocellatella (Boyd) (Lepidoptera: Gelechiidae) became one of the serious and destructive agro-economic pests in temperate regions in recent years (Amin et al. 2008). They are scattered across the Middle and South Europe, Northern Africa, Middle East, and some Asian countries such as Pakistan, Syria, China, Iraq, and Iran (Al-Keridis 2016). This species was first found in Karaj, Iran in 1936, and now it is widely distributed in sugar beet fields in Iran (Kheiri et al. 1980). Further information on the bioecology of this moth is still unknown, although some of its biological aspects have been investigated (Kheiri et al. 1980). In the field, the adult moths emerge early in the spring and lay their eggs under the young leaves or stems, which are sometimes laid in clusters (Esmaili et al. 1996). Early instars feed on leaves for a few days; thereafter, they tunnel into the roots, which results in damaging the roots. As a result of this damage, the yield and sugar content reduce (Evaristo 1983, Bassyouny et al. 1993, Rashidov and Khasanov 2003, Naseri et al. 2016). This pest is a multivoltine; meaning, each moth has three to six generations per year. They pass the winter as larvae with various instars and their habitat is cut head sugar beet Beta vulgaris L. residues (Kheiri 1991). The living larvae of different instars in residues have a broad range of supercooling point which is from −6 to −25°C during autumn and winter (Ganji and Moharramipour 2015). The present studies showed that full-grown larvae leave the food source and go into the soil for pupation. The cold hardiness, in freeze-intolerant insects, is usually increased as supercooling point decreased (Lee 2010). However, this relationship was not observed in overwintering larvae of S. ocellatella (Ganji and Moharramipour 2015). Since the sugar beet residues are destroyed in winter, it is possible for overwintering larvae to die at subzero temperatures. This hypothesis concludes that most larvae that complete their development pupating in the soil will have a chance to pass severe winter. It has been shown that S. ocellatella are in a state of diapause pupae over winter (Ahmadi et al. 2017). Diapause is a primary mechanism used by insects to adapt to their life cycles with local environmental changes (Denlinger 2002). In this case, three ecophysiological phases, including induction, maintenance, and termination, are mostly expressed in response to photoperiod and temperature (Taylor 1980, Danks 1987, Koštál 2006). It is important to study the overwintering features, since the density of the pest in the overwintering generation influences the following year’s population. Coming to the point of diapause initiation and termination in the field condition will help us to improve the prediction and management of this pest. Unfortunately, there is no information about the diapause induction of S. ocellatella with respect to temperature and photoperiod. First, the aim of this study was to clarify the role of photoperiod and temperature on diapause induction in overwintering pupae and also, to provide the information about how the moth initiates and terminates diapause. Moreover, the relationship between the duration of diapause and these environmental factors was also examined. To understand the complex life history of the moth, some challenges were encountered. Materials and Methods Insect Rearing Condition The laboratory colony of S. ocellatella originated from fields of the Sugar Beet Research Institute (35° 83ʹ 96″ N, 50° 86ʹ 63″E), Karaj, Iran, in late June, 2014. The adult insects, which emerged from field-collected larvae, were used to start the colony by ovipositing the eggs on the sugar beet var Aria at the sixth-leaf stage. Plants containing ca. 50 eggs (<48 h) were transferred into a ventilated transparent box (30 × 15 × 15 cm) with 3-cm sand in the bottom for pupation of wandering larvae. Insect culture was maintained in a growth chamber (3000 liters, Jal Tajhiz Co., Karaj, Iran) set at 25 ± 1°C, 65 ± 5% relative humidity, and 16:8 (L:D) h. But all laboratory experiments were conducted in double low temperature incubators (40 × 2 liters, Iran-Khodsaz Co., Tehran, Iran) equipped with controlled illumination by electric timers. In average, five larvae were reared on each plant. Diapause Induction Under 24-h Light–Dark Cycles The effect of photoperiod and temperature on diapause induction was investigated by rearing the newly oviposited eggs of the same age (300 to 400 eggs and <48 h) at temperatures 15, 18, 20, and 25°C in combination with 4:20, 8:16, 10:14, 11:13, 12:12, 13:11, 14:10, and 16:8 (L:D) h until adult emergence. To estimate the critical day lengths in 50% of the population, the incidence of diapause was determined. The time required for pupal development was 10 to 13 d at 18°C and 16:8 (L:D) h. Thus, the pupae with a development time of more than 22 d were considered to be in diapause. Sensitive Stage for Diapause Induction It has been shown that the larval stage is the most sensitive to the photoperiodic signal for diapause induction (Danks 1987). In this study, two experiments were used to locate the photosensitive stage as described by Spieth (1995). Each experiment was carried out at 18°C with cohorts of 50 to 100 newly hatched larvae (<12 holds). In the first experiment, larvae at the onset of each instar were switched from 12:12 to 16:8 (L:D) h and the rearing was continued until the emergence of adult insects or vice versa. In the second experiment, the larvae of each instar were exposed from 12:12 to 16:8 (L:D) h, and then after treatment they returned to their main condition where the rearing was continued until the adult emergence. Furthermore, the reverse experiment was conducted by exposing each instar from 16:8 to 12:12(L:D) h. Non-24-h Light-Dark Cycles Photoperiodic Responses An experiment was conducted in a 12:12 (L:D) h and temperature at 18°C to determine the preference of light or dark duration. In this experiment, the larvae were exposed to non-24-h light-dark cycles instar immediately after egg hatching with scotophases of 8, 10, 12, and 14 h and varying photophases of 4 to 24 h. On the contrary, the first instars were reared under constant photophases of 16, 14, 12, and 10 h and varying scotophases of 4 to 24 h (Fig. 1). At least cohorts of 60 to 100 individuals were used in each treatment. Fig. 1. View largeDownload slide Photoperiodic responses under non-24-h light-dark cycles. (A) The first-instar larvae (<12 h) were exposed to constant scotophases of 8, 10, 12, and 14 h, and varying photophases of 4 to 24 h. (B) The first instars were exposed to constant photophases of 16, 14, 12, and 10 h, and varying scotophases of 4 to 24 h. Fig. 1. View largeDownload slide Photoperiodic responses under non-24-h light-dark cycles. (A) The first-instar larvae (<12 h) were exposed to constant scotophases of 8, 10, 12, and 14 h, and varying photophases of 4 to 24 h. (B) The first instars were exposed to constant photophases of 16, 14, 12, and 10 h, and varying scotophases of 4 to 24 h. Night-Interruption in 24-h Photoperiods An experiment was conducted in a diapause inducing condition at 18°C and 12:12(L:D) h to find the photosensitive phases to a light pulse in a 24-h light-dark cycle. The night was interrupted regularly by a single 1-h light pulse at 1-h interval. In this experiment, 11 groups of insects were used each with 100 individuals. At a fixed time after commencement of the scotophase, a 1-h light pulse was given to each group every 24 h. To scan the whole scotophase, the time of the light pulse was changed regularly in each group. The Incidence of Diapause in Field Conditions To find out the critical time for the incidence of diapause pupae, the first-instar larvae immediately after hatching were placed on the stock cultures on the field-grown sugar beets at 5-d intervals from early August to mid-October in 2015 and 2016. The most of full-grown larvae were pupated inside the soil covered by the cages (20 × 30 × 30 cm). Under field conditions, adult insects from nondiapause pupae usually emerge in less than 2 wk depending on the ambient temperature. However, pupae were considered in the stage of diapause, within a duration of at least twice as long as nondiapause. Diapause Termination in Field Condition Diapause pupae were collected from sugar beet fields from late October to late November in 2015 and 2016 and kept outdoor. Then, the date of adult emergence was recorded daily. Furthermore, the daily mean ambient temperature was obtained from a weather station located at ca. 1 km far from the sugar beet field. Photoperiod data were obtained from Iran Astronomy Observatory as a photophase between the sunrise and sunset. Statistical Analysis The critical day lengths, at which 50% of the individuals respond to diapause, were estimated by using logistic regression analysis. In this analysis, parameters of the model were estimated for prediction of critical day lengths at overall temperatures of 15 and 18°C. The incidence of diapause among treatments was compared by Pearson’s chi-square test to determine the sensitive stage for diapause induction. Then, pairwise comparisons were performed using Mantel–Haenszel test to compare the significant differences between treatments. In the generalized linear procedure, a binary logistic was used as the type of model to test the main effects of the photoperiod (photophase and scotophase) as a categorical predictor on diapause induction as a response variable. In this model, overall effects of 12-h photophase with different nondiel scotophases were compared with 12-h scotophase with different nondiel photophases (Fig. 1). A Pearson correlation test was used to investigate the relationship between mean daily temperature, and the incidence of diapause and the number of adult emergence under field conditions. All data were analyzed using IBM SPSS software version 22.0 (Landau and Everitt 2004). Results Photoperiodic Response Experiment for Diapause Induction Photoperiodic response for diapause induction was investigated under 24-h light-dark cycles at the constant temperatures of 15, 18, 20, and 25°C. The diapause was prevented at 20°C and higher temperature regardless of photoperiod (Fig. 2). Photoperiodic response curve showed that the incidence of diapause reached a peak at 11- and 12-h day lengths at both 15 and 18°C (Fig. 3). While the diapause was prevented under the day lengths of 14 h even at low temperatures. In this experiment, the critical day lengths for diapause induction were estimated to be 10.35 h (Constant = 15.964 ± 1.268, P(x) = −1.169 ± 0.093) at overall temperatures of 15 and 18°C. The critical temperature for 50% pupal diapause induction was 18.48°C (Constant = 10.645 ± 0.800, T(x) = −0.576 ± 0.043) at overall day lengths of 11 and 12 h. These results showed that the pupal diapause is extremely influenced by the photoperiod and temperature. Fig. 2. View largeDownload slide Frequency distribution of duration of the pupal stage of S. ocellatella in various rearing conditions at 15, 18, 20, and 25°C and photophase of 16, 14, 13, 12, 11, 10, and 8 h. Based on the photoperiodic control (16:8 [L:D] h), dashed line separates nondiapause (ND) from diapause (D) pupae. Fig. 2. View largeDownload slide Frequency distribution of duration of the pupal stage of S. ocellatella in various rearing conditions at 15, 18, 20, and 25°C and photophase of 16, 14, 13, 12, 11, 10, and 8 h. Based on the photoperiodic control (16:8 [L:D] h), dashed line separates nondiapause (ND) from diapause (D) pupae. Fig. 3. View largeDownload slide Photoperiodic response curves for the incidence of pupal diapause in S. ocellatella at 15, 18, 20, and 25°C (n = 100 for each point). The critical day lengths (incidence of 50% diapause) are indicated by the dotted vertical line. Fig. 3. View largeDownload slide Photoperiodic response curves for the incidence of pupal diapause in S. ocellatella at 15, 18, 20, and 25°C (n = 100 for each point). The critical day lengths (incidence of 50% diapause) are indicated by the dotted vertical line. The Most Sensitive Stage for Diapause Induction At 18°C, the incidence of pupal diapause was significantly interrupted when the late instars were moved from a long-day photoperiod (16:8 [L:D] h) to a short-day photoperiod (12:12 [L:D] h) (χ2 = 2.26, df = 4, P < 0.001). However, the diapause was induced in the first and second instars when they were exposed to a short-day photoperiod, and the incidence was about 60% in pupae (Fig. 4A). In contrast, the induction of diapause was significantly interrupted when early instars were transferred from 12:12 to 16:8 (L:D) h (χ2 = 2.12, df = 4, P < 0.001) (Fig. 4B). In the second instars, the incidence declined to ca. 50%, but the treatment of later instars did not cause a significant interruption in the induction of diapause (Fig. 4B). Fig. 4. View largeDownload slide The incidence of diapause in pupae of S. ocellatella at 18°C. (A) The rearing larvae (16:8 [L:D] h), at each stage (<12 h), were transferred to 12:12 (L:D) h and then continued until adult emergence. (B) The rearing larvae (12:12 [L:D] h) at each stage were moved to 16:8 (L:D) h and reared until adult emergence (n = 50 to 100 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Fig. 4. View largeDownload slide The incidence of diapause in pupae of S. ocellatella at 18°C. (A) The rearing larvae (16:8 [L:D] h), at each stage (<12 h), were transferred to 12:12 (L:D) h and then continued until adult emergence. (B) The rearing larvae (12:12 [L:D] h) at each stage were moved to 16:8 (L:D) h and reared until adult emergence (n = 50 to 100 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). The incidence of pupal diapause was significantly different (χ2 = 59.43, df = 4, P < 0.001) when each instar was moved from a long-day photoperiod (16:8 [L:D] h) to a short-day photoperiod (12:12 [L:D] h). The incidence was significantly increased to more than 80% only at the third instar; however, the incidence was near to 20% in the rest of the instars (Fig. 5A). Furthermore, the incidence of diapause was significantly altered (χ2 = 33.51, df = 4, P < 0.001) when each instar was moved from 12:12 to 16:8 (L:D) h. The incidence was high (more than 80%) except in the third instar (ca. 50%) (Fig. 5B). Fig. 5. View largeDownload slide Photosensitivity of diapause in pupae of S. ocellatella at 18°C. (A) Each instar larva at a long-day photoperiod (16:8 [L:D] h) was interrupted by a long-night photoperiod (12:12 [L:D] h). (B) Each instar at a long-night photoperiod was interrupted by a long-day photoperiod. The experiment was continued until adult emergence (n = 60 to 185 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Fig. 5. View largeDownload slide Photosensitivity of diapause in pupae of S. ocellatella at 18°C. (A) Each instar larva at a long-day photoperiod (16:8 [L:D] h) was interrupted by a long-night photoperiod (12:12 [L:D] h). (B) Each instar at a long-night photoperiod was interrupted by a long-day photoperiod. The experiment was continued until adult emergence (n = 60 to 185 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Photoperiodic Response Experiments Under Non-24-h Light-Dark Cycle The induction of diapause was prevented in the larvae of different photophases when the length of the scotophase was shorter or longer than 12 h (Fig. 6, left). The incidence of pupal diapause was 100% only at 12:12 (L:D) h and 18°C. Regardless of the different lengths of photophases (8, 10, 12, 14, and 16 h), the maximum incidence of diapause occurred at a 12-h scotophase (Fig. 6, right). Generally, the data showed that the scotophase was significantly more important than the photophase on diapause induction (Wald χ2 = 169.13, df = 1, P < 0.001). Therefore, the length of the scotophase played an essential role in the induction of pupal diapause in S. ocellatella. Fig. 6. View largeDownload slide The incidence of diapause in S. ocellatella under non-24-h light-dark cycle at 18°C. (Left) Scotophase held constant at 8, 10, 12, and 14 h and photophase varied. (Right) Photophase held constant at 16, 14, 12, and 10 h, and scotophase varied (n = 60 to 100 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Values at 12 h, with the same letters, are not significantly different (Mantel–Haenszel statistics, P < 0.05). (For the methods refer to Fig. 1.) Fig. 6. View largeDownload slide The incidence of diapause in S. ocellatella under non-24-h light-dark cycle at 18°C. (Left) Scotophase held constant at 8, 10, 12, and 14 h and photophase varied. (Right) Photophase held constant at 16, 14, 12, and 10 h, and scotophase varied (n = 60 to 100 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Values at 12 h, with the same letters, are not significantly different (Mantel–Haenszel statistics, P < 0.05). (For the methods refer to Fig. 1.) Night-Interruption Experiments in 24-h Photoperiods In this experiment, the scotophase at 12:12 (L:D) h and 18°C (a diapause inducing condition) was consistently interrupted by a 1-h light pulse at 1-h interval. Light pulse interrupted the induction of diapause significantly (χ2 = 48.67, df = 9, P < 0.001). A light pulse at 1 h after onset of scotophase reduced the incidence of diapause significantly to the lowest level (55%) (Fig. 7). Subsequent light pulses changed the incidence up to 65 to 75% alternatively when compared with the control (97%). Fig. 7. View largeDownload slide Night interruption experiment for the incidence of diapause in S. ocellatella at 12:12 (L:D) h and 18°C (n = 100). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Fig. 7. View largeDownload slide Night interruption experiment for the incidence of diapause in S. ocellatella at 12:12 (L:D) h and 18°C (n = 100). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). The Incidence of Diapause in the Field The incidence of pupal diapause was studied in field condition for 2 yr during the autumn and winter in 2015 and 2016 (Fig. 8). Diapause was observed in a few individuals hatched from the eggs, late in August. The incidence of diapause was increased with time and the critical time for diapause induction (the incidence of 50% diapause) was started from mid to late September. However, most of the insects entered diapause from October. Furthermore, there was a negative significant correlation between the incidence of diapause and the mean daily temperature (2015: r = −0.777, n = 16, P < 0.001; 2016: r = −0.892, n = 16, P < 0.001) and the photoperiod (2015: r = −0.959, n = 16, P < 0.001; 2016: r = −0.957, n = 16, P < 0.001). Therefore, as the degree of temperature and day length decreased, the incidence of diapause increased. Fig. 8. View largeDownload slide (A) Average temperature. (B) The incidence of pupal diapause of S. ocellatella that hatched on different dates under field conditions (n = 116 to 270 for each point). Fig. 8. View largeDownload slide (A) Average temperature. (B) The incidence of pupal diapause of S. ocellatella that hatched on different dates under field conditions (n = 116 to 270 for each point). Diapause Termination Under Field Conditions When the average daily temperature reached 13°C or above in late March 2015 and 2016 (Fig. 9A), the adult insects started to emerge from diapause pupae. A critical date for 50% emergence of population was found from mid-late April 2015 and 2016 (Fig. 9B). Furthermore, there was a positive significant correlation between temperature and cumulative adult emergence in 2016 (2015: r = 0.465, n = 9, P = 0.207; 2016: r = 0.818, n = 9, P = 0.007) and photoperiod (2015: r = 0.817, n = 9, P < 0.007; 2016: r = 0.774, n = 9, P = 0.014). The higher the temperature, the quicker the adult emergence; this shows the temperature-dependent nature of postdiapause emergence. Fig. 9. View largeDownload slide The cumulative rate of diapause termination under field conditions in overwintering pupae of S. ocellatella in Tehran, Iran in 2015 and 2016 (n = 100). Fig. 9. View largeDownload slide The cumulative rate of diapause termination under field conditions in overwintering pupae of S. ocellatella in Tehran, Iran in 2015 and 2016 (n = 100). Discussion Photoperiodic response curves in a population of S. ocellatella show that the diapause is induced within a narrow range of photoperiods. Studies in 12-h day length have shown that more than 90% of the individuals enter diapause. This response was found at 18°C and below, whereas higher temperature prevents the induction of diapause to less than 50%, for example, at 20°C ca. 37% and 25°C ca. 4%. Therefore, the photoperiod and temperature both play a critical role in the timing of diapause induction. The findings of the laboratory experiments are in agreement with the information obtained from the field conditions. As shown in the present study, it can be concluded that the critical date for the incidence of diapause may be found in eggs laid out from the last 10 d of September with a 12-h day length. Prolonged temperature increase may provide fast adult emergence and would thus even increase the risk of an additional generation more, whereas the occurrence of low temperatures increases the incidence of diapause pupae. Therefore, when the air temperature reaches below 18°C from late September, most of the late instars burrow into the soil and pupate in diapause state over winter. The larvae associated with an additional generation are cold tolerance and if they have available food, they may have the opportunity to feed and develop into a stage that enters the soil for pupation (Ganji and Moharramipour 2017). However, these larvae form a small population usually have little chance of survival. Larvae that do not pupate in the soil will die because the cut roots of sugar beet are destroyed by freezing. Some other insect species enter diapause from mid-summer when the condition is very favorable (Tauber et al. 1986, Danks 1987, Saunders 2010). This kind of diapause-regulating mechanism is reported in Chilo suppressalis (Walker) (Lepidoptera: Crambidae) when winter diapause is found in individuals that hatched on early August (Xiao et al. 2010). In this condition of insect, the high temperature (28°C) combined with the scotophases (11 to 16 h) may greatly enhance diapause. Even at a high temperature, for example, 31°C, diapause is induced in most larvae. This kind of mechanism is also found in Dendrolimus tabulaeformis (Tsai and Liu) (Lepidoptera: Lasiocampidae), Dyschirius punctatus (Dejean) (Han et al. 2005, Huang et al. 2005, Zeng et al. 2008), and Lobesia botrana (Denis and Schiffermüller) (Lepidoptera: Tortricidae) (Roditakis and Karandinos 2001). In the insects discussed earlier, the photoperiod is the dominant factor for diapause induction. Unlike these insects in S. ocellatella, the photoperiodic response curve is very narrow. As a result, in day length of 10 h or less, the diapause induction is prevented, even at low temperatures. Therefore, it is possible for the entering larvae to additional generation in autumn to possibly pass the winter without diapause. With this, it seems that the food is available and can support growth and development of the S. ocellatella until late autumn. Furthermore, winter sugar beets exist all through autumn and winter. In addition to photoperiod, the temperature may play a critical role in diapause induction. This phenomenon has also been found in Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae) (Hemmati et al. 2017). Potato tuber moth larvae can also burrow into tubers while the crop is still available in the field and later stored. As a matter of fact, the temperature is the most dependable environmental sign used by this insect to recognize the unfavorable condition and also to regulate the induction and maintenance of diapause. The expression of pupal diapause is normally most influenced by conditions experienced in the larval period (Danks 1987, Denlinger 2002). For example, Mamestra oleracea (L.) (Lepidoptera: Noctuidae) is sensitive to this factor only at the time of the last instar, whereas Hyalophora cecropia L. (Lepidoptera: Saturniidae) is sensitive in the last two instars before pupation. However, some species are sensitive early in the larval development. For example, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae) is the most sensitive in the first half of the larval period, but L. botrana is sensitive in the first instar (Danks 1987), whereas S. ocellatella, like Pieris melete (Menetries) (Lepidoptera: Pieridae) (Xue et al. 1997), is the most sensitive in the third instar. Furthermore, the third instar is the most sensitive in Ostrinia furnacalis Guenee (Lepidoptera: Crambidae) (Yang et al. 2014) and C. suppressalis (Xiao et al. 2010) that induce diapause in the last instar. The photoperiodic response curve for diapause induction under non-24-h light-dark cycles primarily depends on the length of the scotophase in S. ocellatella. The diapause induction was very low in cycles containing a short night, 4, 8, 20, and 24 h, but very high, more than 67%, in cycles containing a long night or 12 h, regardless of the various lengths of the photophase which are 8, 10, 12, 14, and 16 h. Therefore, duration of the scotophase is a critical factor for the induction of diapause in this moth. Almost in many insects, the length of the scotophase is the important phase for diapause induction (Beck 1980, Danks 1987, Spieth and Sauer 1991, Kimura and Masaki 1992, Wang et al. 2004, Xiao et al. 2010, Chen et al. 2014, Yang et al. 2014). On the contrary, in a number of insects such as Pyrrhocoris apterus (L.) (Heteroptera: Pyrrhocoridae) and Thyrassia penangae (Moore) (Lepidoptera: Zygaenidae), the length of the photophase is more important than the length of the scotophase for diapause induction (Saunders 1987, He et al. 2009). In this study, night-interruption experiments with a 1-h light pulse at 12:12 (L:D) h showed an important diapause interruption. As observed, it seems that the most photosensitive phase of this moth appears at the onset of scotophase; however, the sensitivity to the light pulse may vary between species (Masaki 1984). To investigate photoperiodic phenomena in insects, night-interruption experiments can be a helpful experimental instrument. All the studied cases of photoperiodic response were very sensitive to night interruption and the long night effect could be inversed by a light pulse (Saunders 2002, Xiao et al. 2009, Chen et al. 2014). Like S. ocellatella in some insects such as Colaphellus bowringi (Baly) (Coleoptera: Chrysomelidae) (Wang et al. 2004), L. botrana (Roditakis and Karandinos 2001), codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae) (Peterson and Hammer 1968), and Pieris brassicae (L.) (Lepidoptera: Pyralidae) (Spieth and Sauer 1991), a 1-h light pulse at the early of scotophase interrupts diapause induction. In D. tabulaeformis (Zeng et al. 2008) and Pseudopidorus fasciata Walker (Lepidoptera: Zygaenidae) (Wei et al. 2001), a 1-h light pulse at the middle of scotophase and in C. suppressalis (Xiao et al. 2010), T. penangae (He et al. 2009), and the rice stem borer, Mamestra brassicae (L.) (Lepidoptera: Noctuidae) (Kimura and Masaki 1993), the light at the late scotophase may interfere diapause. Furthermore, it has been shown that the highest photosensitivity is different under hibernation and aestivation conditions (Spieth et al. 2004, Xiao et al. 2009). The time of diapause induction and termination of S. ocellatella was studied under field conditions. According to the present findings, the winter diapause is induced in 95% of the individuals that hatched until mid-October. Meanwhile, the mean daily temperature experienced by individuals decreased to 19°C or less. This decrease shows photoperiod- and temperature-dependent nature of the main signs of diapause. The diapause pupae started to emerge on 26 or 30 March 2015, when the mean daily temperature increased to 13°C or above. Based on the laboratory and field experiments, these results further showed that both photoperiod and temperature were strongly related to the induction and termination in S. ocellatella. In general, predicting the time of diapause induction in autumn and adult emergence in spring for S. ocellatella can be possible by incorporating the results of the present study with weather information from S. ocellatella location. Moreover, these results confirmed the importance of understanding how changes in photoperiod and temperature cause insects to initiate and terminate diapause. There is need for detailed understanding of insect life cycles such as diapause because this information is useful in improving the computation and management of this pest. This result from the present study is worth giving the basic investigation on cognition of physiological and genetic mechanisms related to the effect of environmental cues (that is, temperature and photoperiod) on S. ocellatella diapause. It is suggested that the association of these factors should be investigated in future studies because this information can help in the establishment of management through interruption in regular development and/or diapause induction and termination. References Cited Ahmadi , F. , S. Moharramipour , and A. Mikani . 2017 . Changes of supercooling point and cold tolerance in diapausing pupae of sugar beet moth, Scrobipalpa ocellatella (Lepidoptera; Gelechiidae) . J. Entomol. Soc. Iran . 37 : 349 – 359 . Al-Keridis , L. A . 2016 . Biology, Ecology and control studies on sugar-beet mining moth, Scrobipalpa ocellatella . Der. Pharma. Chem . 8 : 166 – 171 . Amin , A. H. , A. Helmi , and S. A. El-Serwy . 2008 . 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The effect of temperature on the diapause and cold hardiness of Dendrolimus tabulaeformis (Lepidoptera: Lasiocampidae) . Eur. J. Entomol . 105 : 599 – 606 . Google Scholar Crossref Search ADS © The Author(s) 2018. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Environmental Entomology Oxford University Press

The Effect of Temperature and Photoperiod on Diapause Induction in Pupae of Scrobipalpa ocellatella (Lepidoptera: Gelechiidae)

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Abstract

Abstract Scrobipalpa ocellatella (Boyd) (Lepidoptera: Gelechiidae) is one of the most important pests of sugar beet that causes quantitative and qualitative yield loss in the late summer. To locate the position for diapause induction, combinations of constant temperatures at 15, 18, 20, and 25°C and day lengths of 8, 10, 12, 14, 16, and 24 h were studied from egg to adult emergence. The incidence of diapause peaked at 15 and 18°C, with the day lengths of 12 and 11 h, whereas low temperatures did not improve the effects of short photoperiods (day lengths of 8 and 10 h) in diapause induction. The results showed that the critical day length for diapause induction was 12.8 h at overall 15 and 18°C. It was observed that the third instar larvae were the most sensitive stage to the inductive photoperiod (12:12 [L:D] h). The non-24-h light-dark experiment showed that the nigh length is more important than the day lengths measurement. In a set of 24-h light-dark cycles at 2:12 (L:D) h, a 1-h light pulse declined diapause induction markedly 1 h after scotophase. Field monitoring of the S. ocellatella for 2 yr (2015 and 2016) showed that the 50% of larvae enter winter pupal diapause in early September and this proportion increases in response to a decrease in the day lengths and temperature. From this study, it was concluded that low temperature acts in conjunction with short-day photoperiod in diapause induction of S. ocellatella. photoperiod, temperature, Scrobipalpa ocellatella, diapause induction, sugar beet Scrobipalpa ocellatella (Boyd) (Lepidoptera: Gelechiidae) became one of the serious and destructive agro-economic pests in temperate regions in recent years (Amin et al. 2008). They are scattered across the Middle and South Europe, Northern Africa, Middle East, and some Asian countries such as Pakistan, Syria, China, Iraq, and Iran (Al-Keridis 2016). This species was first found in Karaj, Iran in 1936, and now it is widely distributed in sugar beet fields in Iran (Kheiri et al. 1980). Further information on the bioecology of this moth is still unknown, although some of its biological aspects have been investigated (Kheiri et al. 1980). In the field, the adult moths emerge early in the spring and lay their eggs under the young leaves or stems, which are sometimes laid in clusters (Esmaili et al. 1996). Early instars feed on leaves for a few days; thereafter, they tunnel into the roots, which results in damaging the roots. As a result of this damage, the yield and sugar content reduce (Evaristo 1983, Bassyouny et al. 1993, Rashidov and Khasanov 2003, Naseri et al. 2016). This pest is a multivoltine; meaning, each moth has three to six generations per year. They pass the winter as larvae with various instars and their habitat is cut head sugar beet Beta vulgaris L. residues (Kheiri 1991). The living larvae of different instars in residues have a broad range of supercooling point which is from −6 to −25°C during autumn and winter (Ganji and Moharramipour 2015). The present studies showed that full-grown larvae leave the food source and go into the soil for pupation. The cold hardiness, in freeze-intolerant insects, is usually increased as supercooling point decreased (Lee 2010). However, this relationship was not observed in overwintering larvae of S. ocellatella (Ganji and Moharramipour 2015). Since the sugar beet residues are destroyed in winter, it is possible for overwintering larvae to die at subzero temperatures. This hypothesis concludes that most larvae that complete their development pupating in the soil will have a chance to pass severe winter. It has been shown that S. ocellatella are in a state of diapause pupae over winter (Ahmadi et al. 2017). Diapause is a primary mechanism used by insects to adapt to their life cycles with local environmental changes (Denlinger 2002). In this case, three ecophysiological phases, including induction, maintenance, and termination, are mostly expressed in response to photoperiod and temperature (Taylor 1980, Danks 1987, Koštál 2006). It is important to study the overwintering features, since the density of the pest in the overwintering generation influences the following year’s population. Coming to the point of diapause initiation and termination in the field condition will help us to improve the prediction and management of this pest. Unfortunately, there is no information about the diapause induction of S. ocellatella with respect to temperature and photoperiod. First, the aim of this study was to clarify the role of photoperiod and temperature on diapause induction in overwintering pupae and also, to provide the information about how the moth initiates and terminates diapause. Moreover, the relationship between the duration of diapause and these environmental factors was also examined. To understand the complex life history of the moth, some challenges were encountered. Materials and Methods Insect Rearing Condition The laboratory colony of S. ocellatella originated from fields of the Sugar Beet Research Institute (35° 83ʹ 96″ N, 50° 86ʹ 63″E), Karaj, Iran, in late June, 2014. The adult insects, which emerged from field-collected larvae, were used to start the colony by ovipositing the eggs on the sugar beet var Aria at the sixth-leaf stage. Plants containing ca. 50 eggs (<48 h) were transferred into a ventilated transparent box (30 × 15 × 15 cm) with 3-cm sand in the bottom for pupation of wandering larvae. Insect culture was maintained in a growth chamber (3000 liters, Jal Tajhiz Co., Karaj, Iran) set at 25 ± 1°C, 65 ± 5% relative humidity, and 16:8 (L:D) h. But all laboratory experiments were conducted in double low temperature incubators (40 × 2 liters, Iran-Khodsaz Co., Tehran, Iran) equipped with controlled illumination by electric timers. In average, five larvae were reared on each plant. Diapause Induction Under 24-h Light–Dark Cycles The effect of photoperiod and temperature on diapause induction was investigated by rearing the newly oviposited eggs of the same age (300 to 400 eggs and <48 h) at temperatures 15, 18, 20, and 25°C in combination with 4:20, 8:16, 10:14, 11:13, 12:12, 13:11, 14:10, and 16:8 (L:D) h until adult emergence. To estimate the critical day lengths in 50% of the population, the incidence of diapause was determined. The time required for pupal development was 10 to 13 d at 18°C and 16:8 (L:D) h. Thus, the pupae with a development time of more than 22 d were considered to be in diapause. Sensitive Stage for Diapause Induction It has been shown that the larval stage is the most sensitive to the photoperiodic signal for diapause induction (Danks 1987). In this study, two experiments were used to locate the photosensitive stage as described by Spieth (1995). Each experiment was carried out at 18°C with cohorts of 50 to 100 newly hatched larvae (<12 holds). In the first experiment, larvae at the onset of each instar were switched from 12:12 to 16:8 (L:D) h and the rearing was continued until the emergence of adult insects or vice versa. In the second experiment, the larvae of each instar were exposed from 12:12 to 16:8 (L:D) h, and then after treatment they returned to their main condition where the rearing was continued until the adult emergence. Furthermore, the reverse experiment was conducted by exposing each instar from 16:8 to 12:12(L:D) h. Non-24-h Light-Dark Cycles Photoperiodic Responses An experiment was conducted in a 12:12 (L:D) h and temperature at 18°C to determine the preference of light or dark duration. In this experiment, the larvae were exposed to non-24-h light-dark cycles instar immediately after egg hatching with scotophases of 8, 10, 12, and 14 h and varying photophases of 4 to 24 h. On the contrary, the first instars were reared under constant photophases of 16, 14, 12, and 10 h and varying scotophases of 4 to 24 h (Fig. 1). At least cohorts of 60 to 100 individuals were used in each treatment. Fig. 1. View largeDownload slide Photoperiodic responses under non-24-h light-dark cycles. (A) The first-instar larvae (<12 h) were exposed to constant scotophases of 8, 10, 12, and 14 h, and varying photophases of 4 to 24 h. (B) The first instars were exposed to constant photophases of 16, 14, 12, and 10 h, and varying scotophases of 4 to 24 h. Fig. 1. View largeDownload slide Photoperiodic responses under non-24-h light-dark cycles. (A) The first-instar larvae (<12 h) were exposed to constant scotophases of 8, 10, 12, and 14 h, and varying photophases of 4 to 24 h. (B) The first instars were exposed to constant photophases of 16, 14, 12, and 10 h, and varying scotophases of 4 to 24 h. Night-Interruption in 24-h Photoperiods An experiment was conducted in a diapause inducing condition at 18°C and 12:12(L:D) h to find the photosensitive phases to a light pulse in a 24-h light-dark cycle. The night was interrupted regularly by a single 1-h light pulse at 1-h interval. In this experiment, 11 groups of insects were used each with 100 individuals. At a fixed time after commencement of the scotophase, a 1-h light pulse was given to each group every 24 h. To scan the whole scotophase, the time of the light pulse was changed regularly in each group. The Incidence of Diapause in Field Conditions To find out the critical time for the incidence of diapause pupae, the first-instar larvae immediately after hatching were placed on the stock cultures on the field-grown sugar beets at 5-d intervals from early August to mid-October in 2015 and 2016. The most of full-grown larvae were pupated inside the soil covered by the cages (20 × 30 × 30 cm). Under field conditions, adult insects from nondiapause pupae usually emerge in less than 2 wk depending on the ambient temperature. However, pupae were considered in the stage of diapause, within a duration of at least twice as long as nondiapause. Diapause Termination in Field Condition Diapause pupae were collected from sugar beet fields from late October to late November in 2015 and 2016 and kept outdoor. Then, the date of adult emergence was recorded daily. Furthermore, the daily mean ambient temperature was obtained from a weather station located at ca. 1 km far from the sugar beet field. Photoperiod data were obtained from Iran Astronomy Observatory as a photophase between the sunrise and sunset. Statistical Analysis The critical day lengths, at which 50% of the individuals respond to diapause, were estimated by using logistic regression analysis. In this analysis, parameters of the model were estimated for prediction of critical day lengths at overall temperatures of 15 and 18°C. The incidence of diapause among treatments was compared by Pearson’s chi-square test to determine the sensitive stage for diapause induction. Then, pairwise comparisons were performed using Mantel–Haenszel test to compare the significant differences between treatments. In the generalized linear procedure, a binary logistic was used as the type of model to test the main effects of the photoperiod (photophase and scotophase) as a categorical predictor on diapause induction as a response variable. In this model, overall effects of 12-h photophase with different nondiel scotophases were compared with 12-h scotophase with different nondiel photophases (Fig. 1). A Pearson correlation test was used to investigate the relationship between mean daily temperature, and the incidence of diapause and the number of adult emergence under field conditions. All data were analyzed using IBM SPSS software version 22.0 (Landau and Everitt 2004). Results Photoperiodic Response Experiment for Diapause Induction Photoperiodic response for diapause induction was investigated under 24-h light-dark cycles at the constant temperatures of 15, 18, 20, and 25°C. The diapause was prevented at 20°C and higher temperature regardless of photoperiod (Fig. 2). Photoperiodic response curve showed that the incidence of diapause reached a peak at 11- and 12-h day lengths at both 15 and 18°C (Fig. 3). While the diapause was prevented under the day lengths of 14 h even at low temperatures. In this experiment, the critical day lengths for diapause induction were estimated to be 10.35 h (Constant = 15.964 ± 1.268, P(x) = −1.169 ± 0.093) at overall temperatures of 15 and 18°C. The critical temperature for 50% pupal diapause induction was 18.48°C (Constant = 10.645 ± 0.800, T(x) = −0.576 ± 0.043) at overall day lengths of 11 and 12 h. These results showed that the pupal diapause is extremely influenced by the photoperiod and temperature. Fig. 2. View largeDownload slide Frequency distribution of duration of the pupal stage of S. ocellatella in various rearing conditions at 15, 18, 20, and 25°C and photophase of 16, 14, 13, 12, 11, 10, and 8 h. Based on the photoperiodic control (16:8 [L:D] h), dashed line separates nondiapause (ND) from diapause (D) pupae. Fig. 2. View largeDownload slide Frequency distribution of duration of the pupal stage of S. ocellatella in various rearing conditions at 15, 18, 20, and 25°C and photophase of 16, 14, 13, 12, 11, 10, and 8 h. Based on the photoperiodic control (16:8 [L:D] h), dashed line separates nondiapause (ND) from diapause (D) pupae. Fig. 3. View largeDownload slide Photoperiodic response curves for the incidence of pupal diapause in S. ocellatella at 15, 18, 20, and 25°C (n = 100 for each point). The critical day lengths (incidence of 50% diapause) are indicated by the dotted vertical line. Fig. 3. View largeDownload slide Photoperiodic response curves for the incidence of pupal diapause in S. ocellatella at 15, 18, 20, and 25°C (n = 100 for each point). The critical day lengths (incidence of 50% diapause) are indicated by the dotted vertical line. The Most Sensitive Stage for Diapause Induction At 18°C, the incidence of pupal diapause was significantly interrupted when the late instars were moved from a long-day photoperiod (16:8 [L:D] h) to a short-day photoperiod (12:12 [L:D] h) (χ2 = 2.26, df = 4, P < 0.001). However, the diapause was induced in the first and second instars when they were exposed to a short-day photoperiod, and the incidence was about 60% in pupae (Fig. 4A). In contrast, the induction of diapause was significantly interrupted when early instars were transferred from 12:12 to 16:8 (L:D) h (χ2 = 2.12, df = 4, P < 0.001) (Fig. 4B). In the second instars, the incidence declined to ca. 50%, but the treatment of later instars did not cause a significant interruption in the induction of diapause (Fig. 4B). Fig. 4. View largeDownload slide The incidence of diapause in pupae of S. ocellatella at 18°C. (A) The rearing larvae (16:8 [L:D] h), at each stage (<12 h), were transferred to 12:12 (L:D) h and then continued until adult emergence. (B) The rearing larvae (12:12 [L:D] h) at each stage were moved to 16:8 (L:D) h and reared until adult emergence (n = 50 to 100 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Fig. 4. View largeDownload slide The incidence of diapause in pupae of S. ocellatella at 18°C. (A) The rearing larvae (16:8 [L:D] h), at each stage (<12 h), were transferred to 12:12 (L:D) h and then continued until adult emergence. (B) The rearing larvae (12:12 [L:D] h) at each stage were moved to 16:8 (L:D) h and reared until adult emergence (n = 50 to 100 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). The incidence of pupal diapause was significantly different (χ2 = 59.43, df = 4, P < 0.001) when each instar was moved from a long-day photoperiod (16:8 [L:D] h) to a short-day photoperiod (12:12 [L:D] h). The incidence was significantly increased to more than 80% only at the third instar; however, the incidence was near to 20% in the rest of the instars (Fig. 5A). Furthermore, the incidence of diapause was significantly altered (χ2 = 33.51, df = 4, P < 0.001) when each instar was moved from 12:12 to 16:8 (L:D) h. The incidence was high (more than 80%) except in the third instar (ca. 50%) (Fig. 5B). Fig. 5. View largeDownload slide Photosensitivity of diapause in pupae of S. ocellatella at 18°C. (A) Each instar larva at a long-day photoperiod (16:8 [L:D] h) was interrupted by a long-night photoperiod (12:12 [L:D] h). (B) Each instar at a long-night photoperiod was interrupted by a long-day photoperiod. The experiment was continued until adult emergence (n = 60 to 185 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Fig. 5. View largeDownload slide Photosensitivity of diapause in pupae of S. ocellatella at 18°C. (A) Each instar larva at a long-day photoperiod (16:8 [L:D] h) was interrupted by a long-night photoperiod (12:12 [L:D] h). (B) Each instar at a long-night photoperiod was interrupted by a long-day photoperiod. The experiment was continued until adult emergence (n = 60 to 185 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Photoperiodic Response Experiments Under Non-24-h Light-Dark Cycle The induction of diapause was prevented in the larvae of different photophases when the length of the scotophase was shorter or longer than 12 h (Fig. 6, left). The incidence of pupal diapause was 100% only at 12:12 (L:D) h and 18°C. Regardless of the different lengths of photophases (8, 10, 12, 14, and 16 h), the maximum incidence of diapause occurred at a 12-h scotophase (Fig. 6, right). Generally, the data showed that the scotophase was significantly more important than the photophase on diapause induction (Wald χ2 = 169.13, df = 1, P < 0.001). Therefore, the length of the scotophase played an essential role in the induction of pupal diapause in S. ocellatella. Fig. 6. View largeDownload slide The incidence of diapause in S. ocellatella under non-24-h light-dark cycle at 18°C. (Left) Scotophase held constant at 8, 10, 12, and 14 h and photophase varied. (Right) Photophase held constant at 16, 14, 12, and 10 h, and scotophase varied (n = 60 to 100 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Values at 12 h, with the same letters, are not significantly different (Mantel–Haenszel statistics, P < 0.05). (For the methods refer to Fig. 1.) Fig. 6. View largeDownload slide The incidence of diapause in S. ocellatella under non-24-h light-dark cycle at 18°C. (Left) Scotophase held constant at 8, 10, 12, and 14 h and photophase varied. (Right) Photophase held constant at 16, 14, 12, and 10 h, and scotophase varied (n = 60 to 100 for each point). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Values at 12 h, with the same letters, are not significantly different (Mantel–Haenszel statistics, P < 0.05). (For the methods refer to Fig. 1.) Night-Interruption Experiments in 24-h Photoperiods In this experiment, the scotophase at 12:12 (L:D) h and 18°C (a diapause inducing condition) was consistently interrupted by a 1-h light pulse at 1-h interval. Light pulse interrupted the induction of diapause significantly (χ2 = 48.67, df = 9, P < 0.001). A light pulse at 1 h after onset of scotophase reduced the incidence of diapause significantly to the lowest level (55%) (Fig. 7). Subsequent light pulses changed the incidence up to 65 to 75% alternatively when compared with the control (97%). Fig. 7. View largeDownload slide Night interruption experiment for the incidence of diapause in S. ocellatella at 12:12 (L:D) h and 18°C (n = 100). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). Fig. 7. View largeDownload slide Night interruption experiment for the incidence of diapause in S. ocellatella at 12:12 (L:D) h and 18°C (n = 100). Values with the same letters are not significantly different (Mantel–Haenszel statistics, P < 0.05). The Incidence of Diapause in the Field The incidence of pupal diapause was studied in field condition for 2 yr during the autumn and winter in 2015 and 2016 (Fig. 8). Diapause was observed in a few individuals hatched from the eggs, late in August. The incidence of diapause was increased with time and the critical time for diapause induction (the incidence of 50% diapause) was started from mid to late September. However, most of the insects entered diapause from October. Furthermore, there was a negative significant correlation between the incidence of diapause and the mean daily temperature (2015: r = −0.777, n = 16, P < 0.001; 2016: r = −0.892, n = 16, P < 0.001) and the photoperiod (2015: r = −0.959, n = 16, P < 0.001; 2016: r = −0.957, n = 16, P < 0.001). Therefore, as the degree of temperature and day length decreased, the incidence of diapause increased. Fig. 8. View largeDownload slide (A) Average temperature. (B) The incidence of pupal diapause of S. ocellatella that hatched on different dates under field conditions (n = 116 to 270 for each point). Fig. 8. View largeDownload slide (A) Average temperature. (B) The incidence of pupal diapause of S. ocellatella that hatched on different dates under field conditions (n = 116 to 270 for each point). Diapause Termination Under Field Conditions When the average daily temperature reached 13°C or above in late March 2015 and 2016 (Fig. 9A), the adult insects started to emerge from diapause pupae. A critical date for 50% emergence of population was found from mid-late April 2015 and 2016 (Fig. 9B). Furthermore, there was a positive significant correlation between temperature and cumulative adult emergence in 2016 (2015: r = 0.465, n = 9, P = 0.207; 2016: r = 0.818, n = 9, P = 0.007) and photoperiod (2015: r = 0.817, n = 9, P < 0.007; 2016: r = 0.774, n = 9, P = 0.014). The higher the temperature, the quicker the adult emergence; this shows the temperature-dependent nature of postdiapause emergence. Fig. 9. View largeDownload slide The cumulative rate of diapause termination under field conditions in overwintering pupae of S. ocellatella in Tehran, Iran in 2015 and 2016 (n = 100). Fig. 9. View largeDownload slide The cumulative rate of diapause termination under field conditions in overwintering pupae of S. ocellatella in Tehran, Iran in 2015 and 2016 (n = 100). Discussion Photoperiodic response curves in a population of S. ocellatella show that the diapause is induced within a narrow range of photoperiods. Studies in 12-h day length have shown that more than 90% of the individuals enter diapause. This response was found at 18°C and below, whereas higher temperature prevents the induction of diapause to less than 50%, for example, at 20°C ca. 37% and 25°C ca. 4%. Therefore, the photoperiod and temperature both play a critical role in the timing of diapause induction. The findings of the laboratory experiments are in agreement with the information obtained from the field conditions. As shown in the present study, it can be concluded that the critical date for the incidence of diapause may be found in eggs laid out from the last 10 d of September with a 12-h day length. Prolonged temperature increase may provide fast adult emergence and would thus even increase the risk of an additional generation more, whereas the occurrence of low temperatures increases the incidence of diapause pupae. Therefore, when the air temperature reaches below 18°C from late September, most of the late instars burrow into the soil and pupate in diapause state over winter. The larvae associated with an additional generation are cold tolerance and if they have available food, they may have the opportunity to feed and develop into a stage that enters the soil for pupation (Ganji and Moharramipour 2017). However, these larvae form a small population usually have little chance of survival. Larvae that do not pupate in the soil will die because the cut roots of sugar beet are destroyed by freezing. Some other insect species enter diapause from mid-summer when the condition is very favorable (Tauber et al. 1986, Danks 1987, Saunders 2010). This kind of diapause-regulating mechanism is reported in Chilo suppressalis (Walker) (Lepidoptera: Crambidae) when winter diapause is found in individuals that hatched on early August (Xiao et al. 2010). In this condition of insect, the high temperature (28°C) combined with the scotophases (11 to 16 h) may greatly enhance diapause. Even at a high temperature, for example, 31°C, diapause is induced in most larvae. This kind of mechanism is also found in Dendrolimus tabulaeformis (Tsai and Liu) (Lepidoptera: Lasiocampidae), Dyschirius punctatus (Dejean) (Han et al. 2005, Huang et al. 2005, Zeng et al. 2008), and Lobesia botrana (Denis and Schiffermüller) (Lepidoptera: Tortricidae) (Roditakis and Karandinos 2001). In the insects discussed earlier, the photoperiod is the dominant factor for diapause induction. Unlike these insects in S. ocellatella, the photoperiodic response curve is very narrow. As a result, in day length of 10 h or less, the diapause induction is prevented, even at low temperatures. Therefore, it is possible for the entering larvae to additional generation in autumn to possibly pass the winter without diapause. With this, it seems that the food is available and can support growth and development of the S. ocellatella until late autumn. Furthermore, winter sugar beets exist all through autumn and winter. In addition to photoperiod, the temperature may play a critical role in diapause induction. This phenomenon has also been found in Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae) (Hemmati et al. 2017). Potato tuber moth larvae can also burrow into tubers while the crop is still available in the field and later stored. As a matter of fact, the temperature is the most dependable environmental sign used by this insect to recognize the unfavorable condition and also to regulate the induction and maintenance of diapause. The expression of pupal diapause is normally most influenced by conditions experienced in the larval period (Danks 1987, Denlinger 2002). For example, Mamestra oleracea (L.) (Lepidoptera: Noctuidae) is sensitive to this factor only at the time of the last instar, whereas Hyalophora cecropia L. (Lepidoptera: Saturniidae) is sensitive in the last two instars before pupation. However, some species are sensitive early in the larval development. For example, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae) is the most sensitive in the first half of the larval period, but L. botrana is sensitive in the first instar (Danks 1987), whereas S. ocellatella, like Pieris melete (Menetries) (Lepidoptera: Pieridae) (Xue et al. 1997), is the most sensitive in the third instar. Furthermore, the third instar is the most sensitive in Ostrinia furnacalis Guenee (Lepidoptera: Crambidae) (Yang et al. 2014) and C. suppressalis (Xiao et al. 2010) that induce diapause in the last instar. The photoperiodic response curve for diapause induction under non-24-h light-dark cycles primarily depends on the length of the scotophase in S. ocellatella. The diapause induction was very low in cycles containing a short night, 4, 8, 20, and 24 h, but very high, more than 67%, in cycles containing a long night or 12 h, regardless of the various lengths of the photophase which are 8, 10, 12, 14, and 16 h. Therefore, duration of the scotophase is a critical factor for the induction of diapause in this moth. Almost in many insects, the length of the scotophase is the important phase for diapause induction (Beck 1980, Danks 1987, Spieth and Sauer 1991, Kimura and Masaki 1992, Wang et al. 2004, Xiao et al. 2010, Chen et al. 2014, Yang et al. 2014). On the contrary, in a number of insects such as Pyrrhocoris apterus (L.) (Heteroptera: Pyrrhocoridae) and Thyrassia penangae (Moore) (Lepidoptera: Zygaenidae), the length of the photophase is more important than the length of the scotophase for diapause induction (Saunders 1987, He et al. 2009). In this study, night-interruption experiments with a 1-h light pulse at 12:12 (L:D) h showed an important diapause interruption. As observed, it seems that the most photosensitive phase of this moth appears at the onset of scotophase; however, the sensitivity to the light pulse may vary between species (Masaki 1984). To investigate photoperiodic phenomena in insects, night-interruption experiments can be a helpful experimental instrument. All the studied cases of photoperiodic response were very sensitive to night interruption and the long night effect could be inversed by a light pulse (Saunders 2002, Xiao et al. 2009, Chen et al. 2014). Like S. ocellatella in some insects such as Colaphellus bowringi (Baly) (Coleoptera: Chrysomelidae) (Wang et al. 2004), L. botrana (Roditakis and Karandinos 2001), codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae) (Peterson and Hammer 1968), and Pieris brassicae (L.) (Lepidoptera: Pyralidae) (Spieth and Sauer 1991), a 1-h light pulse at the early of scotophase interrupts diapause induction. In D. tabulaeformis (Zeng et al. 2008) and Pseudopidorus fasciata Walker (Lepidoptera: Zygaenidae) (Wei et al. 2001), a 1-h light pulse at the middle of scotophase and in C. suppressalis (Xiao et al. 2010), T. penangae (He et al. 2009), and the rice stem borer, Mamestra brassicae (L.) (Lepidoptera: Noctuidae) (Kimura and Masaki 1993), the light at the late scotophase may interfere diapause. Furthermore, it has been shown that the highest photosensitivity is different under hibernation and aestivation conditions (Spieth et al. 2004, Xiao et al. 2009). The time of diapause induction and termination of S. ocellatella was studied under field conditions. According to the present findings, the winter diapause is induced in 95% of the individuals that hatched until mid-October. Meanwhile, the mean daily temperature experienced by individuals decreased to 19°C or less. This decrease shows photoperiod- and temperature-dependent nature of the main signs of diapause. The diapause pupae started to emerge on 26 or 30 March 2015, when the mean daily temperature increased to 13°C or above. Based on the laboratory and field experiments, these results further showed that both photoperiod and temperature were strongly related to the induction and termination in S. ocellatella. In general, predicting the time of diapause induction in autumn and adult emergence in spring for S. ocellatella can be possible by incorporating the results of the present study with weather information from S. ocellatella location. Moreover, these results confirmed the importance of understanding how changes in photoperiod and temperature cause insects to initiate and terminate diapause. There is need for detailed understanding of insect life cycles such as diapause because this information is useful in improving the computation and management of this pest. 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The effect of temperature on the diapause and cold hardiness of Dendrolimus tabulaeformis (Lepidoptera: Lasiocampidae) . Eur. J. Entomol . 105 : 599 – 606 . Google Scholar Crossref Search ADS © The Author(s) 2018. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Journal

Environmental EntomologyOxford University Press

Published: Oct 3, 2018

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