Introduction
The oriental armyworm, Mythimna separata (Walker) (Lepidoptera: Noctuidae), is a major cereal crop pest in Asia and Australian countries (Sharma and Davies, Reference Sharma and Davies1983; Lee and Uhm, Reference Lee, Uhm, Drake and Gatehouse1995; He et al., Reference He, Bo, Guo and Du2017). Due to its typical seasonal behaviour of long-distance migration, generalist food habits, fast reproduction, high fecundity and strong adaptability (Li et al., Reference Li, Wang and Hu1964; Li, Reference Li1996; Wang et al., Reference Wang, Zhang, Ye and Luo2006; Jiang et al., Reference Jiang, Luo, Zhang, Sappington and Hu2011), M. separata has been considered a major biological challenge in China in recent decades (Zeng et al., Reference Zeng, Jiang and Liu2013; Jiang et al., Reference Jiang, Zhang, Cheng and Luo2014a). Additionally, due to global climate change, current crop structures and farming systems, and the adaptability of M. separata, the patterns of overwintering and outbreaks are changing, resulting in a significant decline in forecasting for prevention and control effects (Zhang et al., Reference Zhang, Zhang, Jiang, Zeng, Gao and Cheng2012; Jiang et al., Reference Jiang, Jiang, Zhang, Cheng and Luo2014b; Zhao and Cheng, Reference Zhao and Cheng2016). Therefore, it is essential to elucidate and understand the mechanisms of migration and outbreaks to improve the accuracy of forecasting for the prevention and sustainable management of M. separata in the future and reduce the use of pesticides to protect the environment.
From an evolutionary perspective, insect migration is considered a life-history strategy to leave a habitat incompatible with development and relocate to an adequate breeding environment (Drake et al., Reference Drake, Gatehouse, Farrow, Drake and Gatehouse1995; Drake and Reynolds, Reference Drake and Reynolds2012; Chapman et al., Reference Chapman, Reynolds and Wilson2015). Evidence of this theory is provided by previous studies on migratory pests of field crops, such as Mythimna separata and Agrotis ipsilon, both of which complete long-range migrations (Chen et al., Reference Chen, Bao, Drake, Farrow, Wang, Sun and Zhai1989; Johnson, Reference Johnson, Drake and Gatehouse1995; Jiang et al., Reference Jiang, Luo, Zhang, Sappington and Hu2011). Recently, Sappington (Reference Sappington2018) has demonstrated that migratory flight behaviour is characterized as non-appetitive and fundamentally differs from proximate or local search behaviours. However, after the termination of migratory flight by an internal circadian rhythm (the onset of dusk or dawn) or endogenous signals (Compton, Reference Compton, Bullock, Kenward and HailsDispersal2002), migratory insects rapidly search for potential resources such as food and suitable habitats for successful breeding (Dingle, Reference Dingle2014; Sappington, Reference Sappington2018).
It is generally thought that migrants in insects have greater reproductive potential that is passed on to the next generation than non-migrants; thus, it results in migrants investing more resources in reproduction than non-migrants (Chapman et al., Reference Chapman, Reynolds and Wilson2015), similar to migrating birds and mammals (Sibly et al., Reference Sibly, Witt, Wright, Venditti, Jetz and Brown2012; Stevens et al., Reference Stevens, Whitmee, Le Gaillard, Clobert, Böhning-Gaese, Bonte, Broandle, Dehling, Hof, Trochet and Baguette2014). Prior studies have shown that there exists a trade-off between migration and reproduction in migratory insects when considering their limited internal resources (Johnson, Reference Johnson1969; Rankin et al., Reference Rankin, McAnelly, Bodenhamer and Danthanarayana1986). The interaction relationship between migration and reproduction is not fixed, which differs from species to species, flight stage and other factors (Zhang et al., Reference Zhang, Pan, Sappington, Lu, Luo and Jiang2015). Some studies have verified that sex, age and juvenile hormone (JH) may affect the flight capacity and reproduction of adult insects, thereby altering the larval density of the offspring (Luo et al., Reference Luo, Li, Jiang and Hu2001; Cheng et al., Reference Cheng, Luo, Jiang and Sappington2012; Zhang et al., Reference Zhang, Cheng, Chapman, Sappington, Liu, Cheng and Jiang2020).
The onset of migratory behaviour is generally initiated by sexually immature adults for a variety of migratory noctuid species (Gatehouse and Zhang, Reference Gatehouse, Zhang, Drake and Gatehouse1995). The migration process usually involves long-range dispersal of sexually immature females, subsequent ovarian development and mating at the end of migration in the Noctuidae (Rhainds, Reference Rhainds2010). Similar patterns have been reported in redbacked cutworms, Euxoa ochrogaster and some other noctuid species (Gerber and Walkof, Reference Gerber and Walkof1992; Coombs et al., Reference Coombs, del Socorro, Fitt and Gregg1993), with a high proportion of mated females among migrant individuals with mature oocytes. Zhao et al. (Reference Zhao, Feng, Wu, Wu, Liu, Wu and McNeil2009) has also demonstrated both ovarian development and mating of M. separata females occur towards the end of the migratory phase, and the completion of migration takes a series of days rather than one continuous flight.
To date, many studies have been conducted in M. separata to explore the occurrence and regularity of migration, reconstruct migratory pathways, and develop monitoring and forecasting techniques using many tools, including radar observation, searchlight traps, flight mill experiments and trajectory analysis models (Chen et al., Reference Chen, Bao, Drake, Farrow, Wang, Sun and Zhai1989; Feng et al., Reference Feng, Zhao, Wu, Wu, Wu, Cheng and Guo2008; Jiang, Reference Jiang2018; Zhang et al., Reference Zhang, Zhang, Wang, Liu, Tang, Li, Cheng and Zhu2018). However, little is known about the effects of males on reproduction and flight initiation in M. separata. In this study, we investigated flight activity, flight capacity and other flight-related physiological results through a flight mill system (Kim et al., Reference Kim, Jones, Westbrook and Sappington2010; Rovnyak et al., Reference Rovnyak, Burks, Gassmann and Sappington2018). Our objectives were to evaluate the role of males participating in flight and reproductive activities and identify the most representative population reproduction and migration models for M. separata as well as provide a theoretical framework for determining the dynamics of populations to predict insect occurrence trends to prevent outbreaks of this pest.
Materials and methods
Insects
The M. separata samples in this experiment originated from field-collected individuals from Zhangjiakou, Hebei Province (41.85°N, 114.60°E); the population had been maintained for three generations in the laboratory at 23 ± 1 °C, 70% relative humidity (RH) and a photoperiod of 14:10 (light: dark). Larvae were reared with fresh corn seedlings as a food source in round glass bottles (9 cm × 13 cm, diameter × height) for pupation (10 larvae per bottle). After emergence, singe female and male were paired and transferred to cylindrical plastic cages (10 cm × 20 cm, diameter × height) for oviposition, and they were supplied with a 10% honey solution (vol:vol) until death.
Tethered flight
Flight tests of all treated moths (one male and one female unique pairs) at the same age were conducted in a flight mill system that automatically recorded the flight data (flight duration, flight distance and average flight velocity), as mentioned in Qin et al. (Reference Qin, Liu, Zhang, Cheng, Sappington and Jiang2018). Before the flight tests, all age-paired moths (initiated with 1-day-old adults) were tethered using a technique outlined in previous work (Jiang et al., Reference Jiang, Luo and Sappington2010; Cheng et al., Reference Cheng, Luo, Jiang and Sappington2012) and fed a honey solution.
Flight frequency treatments
To detect the effects of flying males on ovarian and reproductive development and the flight capacity of females according to their flight frequency, the males were divided into a flying group (males and females both flew) and a nonflying male group (only the females flew). All the treatments were initiated with 1-day-old adults and paired (one male and one female) by the same age. Flight durations of 0, 1, 2, 3, 4 and 5 nights were established under laboratory: the nonflying moths (0 flights) served as controls, moths that flew on the day 1 after emergence (one flight), days 1–2 (two flights), days 1–3 (three flights), days 1–4 (four flights), and days 1–5 (five flights), with a tethered flight for 12 h from 20:00 to 08:00, a period optimal for studying M. separata flight behaviour (Luo et al., Reference Luo, Jiang, Li and Hu1999; Lv et al., Reference Lv, Jiang, Zhang and Luo2014; Zhang et al., Reference Zhang, Cheng, Chapman, Sappington, Liu, Cheng and Jiang2020). Then, the effects of flying males on reproduction and flight frequency of adults were estimated by comparing the two male groups. To ensure the accuracy of the experimental results, all tether tests were conducted under dark conditions, and all the treatment moths were attached a short tether for 5 days. More than 25 pairs in each treatment were dissected for this experiment.
Reproductive parameters, ovarian development levels and flight capacity
After completion of the tethered flight, all treated females were paired with males of the same age (one male and one female) and placed in cylindrical plastic cages with fresh honey solution as a food source under the same laboratory conditions as before: 23 ± 1 °C, 70% RH, and a photoperiod of 14:10 (light: dark). Reproductive parameters including lifetime fecundity, preoviposition period (POP), period of first oviposition (PFO), oviposition period, female and male longevity, mating frequency and mating rate were recorded using the method of Zhang et al. (Reference Zhang, Pan, Sappington, Lu, Luo and Jiang2015, Reference Zhang, Cheng, Chapman, Sappington, Liu, Cheng and Jiang2020).The POP was used to calculate the duration between adult emergence and the first oviposition. The PFO was a major parameter to evaluate the synchronization of first laying-eggs, describing the time window from female's POP to the minimal POP.
The ovarian development grades and mating rates were determined after female death. The calculation of the ovarian development level followed the standard protocol, which has been described elsewhere. The ovarian development levels were divided into five grades according to the maturity of the ovary (Chen et al., Reference Chen, Zhang, Qi, Xu, Hou, Fan, Shen, Liu, Shi, Li, Du and Wu2019). The flight capacity (flight duration, flight distance and average flight velocity) of adults was calculated from the data recorded by the flight mill system. The ovarian development levels were divided into five grades as described
Data analysis
Prior to analysis, all data from this experiment were checked for homogeneity of variance. All reproductive parameters and flight capacities were analysed by a two-way analysis of variance (ANOVA) with flight frequency (six levels) and males (flew and not flew) as factors using Tukey's test (HSD) for mean comparisons. Pearson's correlation analysis was applied to examine the influences of flight capacity and flight frequency of males on reproductive development of age-paired females. All statistical analyses were carried out using SAS 9.20 software (SAS Institute, Cary, NC).
Results
Reproductive performances of M. separata adults treated with different flight frequency
POP was significantly affected by the flight frequency and the interaction between flight frequency and male groups, while the male groups had no effect on the POP (tables 1 and 2). Compared to that of the male-nonflying group, the POP of females in the male-flying group was significantly shortened in the one (F 1, 58 = 7.76, P = 0.007) and two night groups (F 1, 58 = 4.47, P = 0.039); however, it was significantly prolonged when the moths flew for three nights (F 1, 56 = 7.42, P = 0.009, table 2). There were significant differences in POP of females treated by different flight frequency (male-flying group: F 5, 164 = 28.75, P < 0.001; male-nonflying group: F 5, 164 = 10.88, P < 0.001). In the male-flying group, the POP of adults that flew for one and two nights was significantly shorter than the controls (0 nights), resulting in these females beginning to lay eggs significantly earlier than the controls; on the contrary, the POP was significantly increased in adults that flew for more than two nights, which showed a significant delay in reproductive development compared to the controls, one and two nights (table 2). In the male-nonflying group, the POP of females that flew for one and two nights was also significantly shorter than the controls; however, it was significantly prolonged in adults that flew for five nights (table 2).
Table 1. Results of two-way ANOVA analysis on effects of flight frequency and male groups (flying or nonflying males) on female reproductive parameters for M. separata
Table 2. Reproductive performances of M. separata adults between the two male groups treated by different flight frequency
Data in the table are mean ± SE. Different lowercase letters in the same row represent significant differences between different flight frequency by Tukey's HSD test at 5% level. Means in the same column followed by the same uppercase letter do not differ significantly (P < 0.05) by Tukey's HSD test.
Flight frequency and male groups had significant influences on the lifetime fecundity but there was no significant interaction (tables 1 and 2). Lifetime fecundity was significantly decreased with the increasing flight frequency (male-flying group: F 5, 164 = 22.91, P < 0.001; male-nonflying group: F 5, 164 = 10.19, P < 0.001), and it was significantly reduced in the three-, four- and five-night flight treatments compared to the controls, one- and two-nights in the male-flying group. Interesting, in the male-nonflying group, the moths in the four- and five-night flight treatments also produced fewer eggs than the females from the one- and two-night flight treatments (tables 1 and 2). Compared to the lifetime fecundity in the male-nonflying group at the same flight frequency, the lifetime fecundity in the male-flying group was significantly reduced in the four-night flight group (F 1, 52 = 3.99, P = 0.050, table 2).
The oviposition period was significantly affected by the flight frequency, male groups and their interaction (tables 1 and 2). Compared to the male-nonflying group, the oviposition period in the flying male-group was significantly increased in the one-, two- and four-night flight groups (one night: F 1, 58 = 4.23, P = 0.044, table 2; two nights: F 1, 56 = 9.90, P = 0.003; four nights: F 1, 52 = 5.20, P = 0.027). Flight frequency significantly affected the oviposition periods of moths (male-flying group: F 5, 164 = 4.84, P < 0.001; male-nonflying group: F 5, 164 = 8.18, P < 0.001). The one- and two-night flight treatments significantly increased the oviposition period compared to the five-night flight treatments in the male-flying group. However, the oviposition period of moths at each frequency was significantly less than the controls in the male-nonflying group (table 2).
Flight frequency significantly affected the PFO, mating frequency and mating rate (tables 1 and 2). The PFO of moths in the male-flying group was significantly reduced in the one-night group (F 1, 58 = 5.00, P = 0.029) and two-night group (F 1, 58 = 4.47, P = 0.039) compared to the male-nonflying group. There were significant differences in the PFO of moths that in the 0–5 flight nights groups between the two male groups (male-flying group: F 5, 164 = 4.77, P < 0.001; male-nonflying group: F 5, 164 = 2.77, P = 0.020). In the male-flying group, the PFO of adults that flew for two nights was significantly less than that of moths from the controls and flew for five nights. Likely, the oviposition time of females that flew for two nights was significantly more synchronous than those in the controls, four- and five-nights in the male-nonflying group, with PFO was significantly less 1.05 days and 1.38 days, respectively (table 2). Although no significant differences were observed in the mating frequency and mating rate between the two male groups at the same flight frequency, there were significant differences among the different flight frequency groups. In the male-flying group, the mating frequency (F 5, 164 = 5.53, P < 0.001) and mating rate (F 5, 164 = 3.79, P = 0.003) of adults that flew for four and five nights were significantly lower than those in the one and 1–2 night flight treatments.
The female longevity was also significantly affected by the flight frequency and male groups but not by their interaction (tables 1 and 2). The longevity of females that flew for 1–3 nights was significantly higher in the male-flying group than that in the male-nonflying group (one night: F 1, 58 = 5.22, P = 0.026; two nights: F 1, 56 = 8.21, P = 0.006; three nights: F 1, 56 = 12.23, P = 0.001, table 2). Flight frequency and the interaction between flight frequency and male groups had significant effects on the male longevity (tables 1 and 2). The longevity of males that flew for one and two nights was significantly higher than that of the nonflying males (one night: F 1, 58 = 4.76, P = 0.033; two nights: F 1, 56 = 6.20, P = 0.016), while it was significantly decreased in the five-nights group (F 1, 48 = 6.57, P = 0.003). In the male-flying group, the male longevity in moths that flew for one and two nights significantly exceeded that of those that flew for four and five nights (F 5, 164 = 8.61, P < 0.001). No significant differences were found in the nonflying group (F 5, 164 = 1.08, P = 0.375, table 2).
Ovarian development of M. separata adults treated with different flight frequency
Ovarian development was significantly affected by the flight frequency, male treatment and their interaction (tables 1 and 2). Comparisons of ovarian development between the two flying male groups at each flight frequency showed that ovarian development was significantly slower in the male-flying group than that in the male-nonflying group in moths that flew for three and four nights (three nights: F 1, 56 = 12.39, P = 0.001; four nights: F 1, 52 = 4.63, P = 0.036). The ovarian development grade in females that flew for one and two nights was significantly higher than that in the controls and the other flight frequency groups in the male-flying group (F 5, 164 = 21.97, P < 0.001). Likewise, the ovarian development grade of females was also significantly increased in the one- and two-night flight treatments compared to the controls, four- and five-night flight treatments in the male-nonflying group (F 5, 164 = 8.58, P < 0.001, table 2).
Comparison of female flight capacity between the two male groups
The flight duration and flight distance were significantly affected by male groups, flight frequency and the interaction between flight frequency and male treatment (tables 3 and 4). Although all flight parameters in the male-flying group were less than those in the male-nonflying group, the flight duration was significantly lower in only those that flew for four nights (F 1, 52 = 8.71, P = 0.005) and five nights (F 1, 48 = 9.34, P = 0.004). In addition, flight duration was prolonged with increasing flight frequency, which was significantly decreased in moths that flew for 1–2 night compared to those that flew for four and five nights in the male-flying group (F 4, 135 = 16.91, P < 0.001) and male-nonflying group (F 4, 135 = 40.51, P < 0.001), respectively (table 4).
Table 3. Results of two-way ANOVA analysis on effects of flight frequency and male groups (flying or nonflying males) on female flight parameters.
Table 4. Flight capacity of M. separata females between the two male groups treated by different flight frequency
Data in the table are mean ± SE. Different lowercase letters in the same row represent significant differences between different flight frequency by Tukey's HSD test at 5% level. The same uppercase letter in a same column indicates no significant difference between the two male groups by Tukey's HSD test at 5% level.
Table 5. Pearson's correlations of flight distance (km), flight velocity (km h−1), and flight duration (h) of males, with reproductive parameters of age-paired females
Compared to those in the male-nonflying group, the flight distances of females that flow for three, four and five nights were significantly decreased in the male-flying group (three nights: F 1, 56 = 5.51, P = 0.023; four nights: F 1, 52 = 8.84, P = 0.004; five nights: F 1, 48 = 10.18, P = 0.003). Similar results were obtained for the flight distance with flight frequency in the male-flying group (F 4, 135 = 15.57, P < 0.001) and male-nonflying group (F 4, 135 = 32.64, P < 0.001, table 4). Male groups and flight frequency had significant effects on the mean velocity; however, their interaction had no influence (tables 3 and 4). The average velocities of moths that flew for one, three and five nights in the male-flying group were significantly slower than those in the male-nonflying group (one night: F 1, 58 = 4.52, P = 0.038; three nights: F 1, 56 = 12.75, P = 0.001; five nights: F 1, 48 = 6.15, P = 0.017). Moths that flew for three nights had the lowest average velocity, with a velocity of less than 0.67 km h−1, compared to the highest those that flew for five nights in the male-flying group (F 4, 135 = 3.32, P < 0.013), while the lowest mean velocity of moths that flew for two nights was observed in the male-nonflying group (F 4, 135 = 3.99, P = 0.004, table 4).
Correlation of flight capacity of males, with reproduction of age-paired females
The correlation analysis (table 5) suggested a positive correlation between flight frequency and flight duration and distance of males with POP and PFO of age-paired females, but the relationship with the lifetime fecundity, oviposition period, mating frequency, ovarian development grade of age-paired females showed a significantly negative correlation. Collectively, these results suggested that flight capacity of males was negatively correlated with reproduction of age-paired females with the increase of flight frequency.
Discussion
Comparisons of M. separata reproductive performance and degree of ovarian development between the two male treatment groups at the same frequency revealed that males played a crucial role in reproductive and migratory regulation. Females that flew for 1–2 nights in the flying male group had a significantly shorter POP and PFO than those in the nonflying group, and the oviposition period and longevity of the females were significantly increased. In contrast, flight for more than two nights induced prolonged POP and PFO, as well as decreased lifetime fecundity, a decreased mating frequency, a decreased mating rate and a lower degree of ovarian development. In the current study, reproductive variations in response to different flight treatments were evaluated by assessing the POP, PFO and other reproductive parameters in a migratory insect (Jiang et al., Reference Jiang, Luo and Sappington2010; Cheng et al., Reference Cheng, Luo, Jiang and Sappington2012). A decrease in lifetime fecundity and an increase in POP are generally considered major reproductive costs of migration (Gunn et al., Reference Gunn, Gatehouse and Woodrow1989; Gibbs and Dyck, Reference Gibbs and Dyck2010). Overall, the results showed that flying males significantly promoted ovarian and reproductive development in M. separata that flew continuously for 1–2 nights compared to nonflying males; however, they could have the opposite effect for other flight frequencies.
To our knowledge, M. separata spends several nights flying long distances to reach breeding areas (Chen et al., Reference Chen, Bao, Drake, Farrow, Wang, Sun and Zhai1989). However, when M. separata migration begins and whether the first migratory period is consistent in males and females is still unknown. Prior studies found that some insects, such as Mythimna separata (Lin, Reference Lin, Lin, Chen, Shu, Hu and Cai1990), initiated their migration soon after eclosion when the ovaries of females were still immature (stage I or II). Luo et al. (Reference Luo, Jiang, Li and Hu1999) reported that in M. separata adults, flight at only 1 day after emergence significantly stimulated reproduction in different flight-age tests. Zhang et al. (Reference Zhang, Jiang and Luo2008) further observed that the first 24 h after emergence was a crucial time window for switching migrants into residents in M. separata. In addition, our previous study demonstrated that a positive effect of migration on reproduction occurred in only newly emerged (24 h) females that flew within 12 h (Lv et al., Reference Lv, Jiang, Zhang and Luo2014). Therefore, it is supposed that the 1st day after emergence may be the primary migratory time period for M. separata (Jiang et al., Reference Jiang, Jiang, Zhang, Cheng and Luo2014b). Similarly, recent research on Cnaphalocrocis medinalis proved that the first migration most likely occurred within the first 2 days after eclosion (Zhang et al., Reference Zhang, Pan, Sappington, Lu, Luo and Jiang2015).
The results of the flight frequency tests in the two male treatment groups showed that females in the male-flying male group that flew for 1–2 nights achieved faster ovarian and reproductive development because their POP was significantly shortened and their degree of ovarian development was significantly increased compared to those in the control group. In addition, their lifetime fecundity, mating frequency and mating rate were higher than those in the controls; their PFO was lower than that in the controls, and a significant decrease was found in adults that flew for two nights. By contrast, more than three nights of flight significantly inhibited reproductive and ovarian development, and the negative influence significantly intensified as flight frequency increased. Pearson's correlation analysis had also demonstrated that flight capacity of males had significantly negative correlations with reproduction of age-paired females with increasing the flight frequency.
In the male-nonflying group, the POP of adult females that flew for 1–2 nights was also significantly reduced compared to those in the controls. Furthermore, significant decreases in lifetime fecundity, mating frequency and the mating rate were observed in those that flew for 4–5 nights. Adults that flew for more than three nights had significantly decreased reproductive development compared to the controls and those that flew for one or two nights, suggesting that in females, migration for three nights resulted in significant reproductive costs. Hence, we supposed that males that flew for 1–2 nights significantly stimulated ovarian and reproductive development in females and weakened the flight willingness of females, with a decrease in flight frequency from 3 to 2. Insect migration occurs when the benefits of flight exceed the reproductive costs of migration (Chapman et al., Reference Chapman, Bell, Burgin, Reynolds, Pettersson, Hill, Bonsall and Thomas2012). Based on these results, we also hypothesized that M. separata individuals began their first migration within 2 days after eclosion and completed their migratory flight within two nights; otherwise, they would pay substantial reproductive costs. This speculation had been supported by radar observations (Chen et al., Reference Chen, Sun, Wang, Zhai, Bao, Drake and GatehouseInsect1995).
One possible reason might be their flight ability. Analysis of the flight capacity between the two male treatment groups revealed that the flight capacity of females in the flying male group was significantly weaker than that of females in the nonflying group because their flight duration and flight distance decreased with increasing flight frequency; significant decreases were observed in moths that flew for more than two nights and more than three nights, respectively. It is well known that a trade-off between migration and reproduction exists (Lorenz, Reference Lorenz2007). Limited internal resources are invested more in reproduction, thereby reducing the energy supply for flight, resulting in a decrease in flight capacity. Migratory and mating behaviours of M. separata occurred during the POP (Luo et al., Reference Luo, Jiang, Li and Hu1999; Zhao et al., Reference Zhao, Feng, Wu, Wu, Liu, Wu and McNeil2009; Jiang et al., Reference Jiang, Luo, Zhang, Sappington and Hu2011). Interestingly, the number of female M. separata was significantly higher than that of males during the first migratory stage (1–4 day after emergence) in light trapping experiments (Li et al., Reference Li, Wang and Su1987; Lin, Reference Lin, Lin, Chen, Shu, Hu and Cai1990; Zhang et al., Reference Zhang, Zhang, Wang, Liu, Tang, Li, Cheng and Zhu2018). It was also reported that the proportion of captured females gradually decreased following the age delay due to the maturation of the ovaries and impaired flight capacity in female M. separata (Wang and Zhang, Reference Wang and Zhang2001). Thus, we deduced that flying males significantly stimulated ovarian and reproductive development in females that flew for 1–2 nights at the expense of reducing subsequent flight capacity. Flying males impaired the flight capacity of age-paired females, which was conducive to reduce inconsistencies in the time of first migration between females and males and increase mating frequencies, thereby promoting ovarian development and reproductive development of M. separata. Our results are consistent with the previous findings that females emerging when and where male density is high tend to have a higher mating success, which results in increasing the proportion of mated females with mature oocytes among migrants (Gerber and Walkof, Reference Gerber and Walkof1992; Coombs et al., Reference Coombs, del Socorro, Fitt and Gregg1993; Rhainds, Reference Rhainds2010).
Another possible reason is related to JH, which regulates many essential physiological processes in insects, such as ovarian and flight muscle development (Luo et al., Reference Luo, Li, Jiang and Hu2001; Moon and Kim, Reference Moon and Kim2003; Saha et al., Reference Saha, Shin, Du, Roy, Zhao, Hou, Wang, Zou, Girke and Raikhel2016). Cusson et al. (Reference Cusson, Mcneil and Tobe1990) found that Pseudaletia unipuncta adults were transferred from 15 to 25 °C resulted in a marked increase in JH biosynthesis within 24 h. Heinrich (Reference Heinrich1993) reported that there was a significant increase in body temperature caused by active flight. Thus, the occurrence of sexual maturation and mating success within several days of initiating migratory flight is not a total surprise based on the above points (Zhao et al., Reference Zhao, Feng, Wu, Wu, Liu, Wu and McNeil2009). Migratory flight may increase fecundity under certain conditions (Rankin et al., Reference Rankin, McAnelly, Bodenhamer and Danthanarayana1986; Luo et al., Reference Luo, Jiang, Li and Hu1999). High JH levels are strongly associated with the rapid development of the ovaries, accelerating the degradation of flight muscles, resulting in the inhibition of migratory ability (Zera, Reference Zera2007; Sun et al., Reference Sun, Jing, Zhang, Stanley, Luo and Long2013). This onset of maturation is beneficial to mate and initiate oviposition as soon as possible after relocate suitable breeding habitats for migrant females (Wada et al., Reference Wada, Ogawa and Nakasuga1988).
In summary, our laboratory study demonstrated that M. separata individuals began their first migration within 2 days after eclosion and flew for two nights. The first migration period of females and males was not identical; female initiated their migration earlier than males due to a stronger flight capacity. Flying males significantly stimulated ovarian and reproductive development in females that flew for two nights, resulting in a subsequently weakened flight capacity and decreased flight propensity in females. These findings provide valuable information regarding the complex process of migration in M. separata. In the future, additional laboratory studies combined with field tests will be carried out to elucidate the mechanism of migration and outbreaks for the development of pest prevention and control strategies.
Acknowledgements
This research was supported by the Fundamental Research Funds of China West Normal University (Project No. 412/412834).
